fix(storage): restore multipart disk compression and make the legacy decompressor resumable

Multipart uploads have bypassed disk compression since #5169 removed the session marker as a stopgap for mid-stream GET failures. The actual root cause was never the multipart layout: the legacy DecompressReader reset its payload consumption state on every poll re-entry, so a Poll::Pending in the middle of a block payload (routine under the erasure duplex) desynchronized the block framing and surfaced as LZ4 frameType errors. This rewrites the decoder as a resumable state machine, restores the multipart session compression marker, reports logical part sizes in ListParts, and makes the rebalance migration read raw stored bytes so compressed and encrypted objects survive migration verbatim.

Fixes #5957. Internal tracking: backlog#1848, backlog#1850.

.config/nextest.toml

@@ -252,10 +252,16 @@ test-group = 'ecstore-serial-flaky'
 # cluster, so it keeps the lane's parallel-safe / no-external-dependency
 # properties. The RustFS warm backend has no loopback guard (that guard is
 # replication-only), so it needs no opt-in env for its 127.0.0.1 tier target.
+#
+# Disk compression (backlog#1848): the `compression` module joins the smoke
+# lane so the multipart disk-compression roundtrips (restored after
+# rustfs/rustfs#5169 disabled them) have PR-lane signal, not just merge-gate.
+# Single-node servers on random ports with isolated temp dirs — meets the
+# admission criteria unchanged.
 [profile.e2e-smoke]
 default-filter = """
     package(e2e_test) & (
-        test(/^(delete_marker_migration_semantics|version_id_regression|list_objects_v2_pagination|list_object_versions_regression|list_objects_duplicates|list_buckets_double_slash|list_buckets_auth|list_buckets_iam_filter|leading_slash_key|special_chars|create_bucket_region|delete_objects_versioning|head_object_consistency|head_object_range|copy_object_metadata|copy_object_tagging|copy_source_invalid_date|content_encoding|multipart_storage_class|storage_class_capability|ssec_copy|anonymous_access|bucket_policy_check|presigned_negative|negative_sigv4|admin_auth|notification_webhook|tls_hot_reload|console_smoke|admin_iam_crud|admin_pools|sts_query_compat)_test::|^fake_s3_target::/)
+        test(/^(delete_marker_migration_semantics|version_id_regression|list_objects_v2_pagination|list_object_versions_regression|list_objects_duplicates|list_buckets_double_slash|list_buckets_auth|list_buckets_iam_filter|leading_slash_key|special_chars|create_bucket_region|delete_objects_versioning|head_object_consistency|head_object_range|copy_object_metadata|copy_object_tagging|copy_source_invalid_date|content_encoding|compression|multipart_storage_class|storage_class_capability|ssec_copy|anonymous_access|bucket_policy_check|presigned_negative|negative_sigv4|admin_auth|notification_webhook|tls_hot_reload|console_smoke|admin_iam_crud|admin_pools|sts_query_compat)_test::|^fake_s3_target::/)
         | test(/^replication_extension_test::(test_replication_check_succeeds_with_remote_target|test_replication_check_rejects_target_without_object_lock|test_set_remote_target_rejects_unversioned_source_bucket|test_replication_check_rejects_unversioned_source_bucket|test_replication_check_rejects_missing_replication_config|test_replication_check_rejects_invalid_bucket|test_set_remote_target_rejects_same_bucket_on_same_deployment|test_set_remote_target_rejects_unversioned_target_bucket|test_set_remote_target_update_requires_arn|test_set_remote_target_update_rejects_missing_target|test_set_remote_target_rejects_invalid_target_url|test_set_remote_target_rejects_self_signed_https_target_without_skip_tls_verify|test_set_remote_target_rejects_private_ca_https_target_without_ca_cert_pem|test_list_remote_targets_rejects_empty_bucket|test_list_remote_targets_rejects_invalid_bucket|test_remove_remote_target_rejects_missing_target|test_remove_remote_target_rejects_missing_arn|test_remove_remote_target_rejects_invalid_bucket|test_remove_remote_target_rejects_target_used_by_replication|test_delete_bucket_replication_removes_remote_target)$/)
         | test(/^reliant::lifecycle::/)
         | test(/^reliant::tiering::/)

crates/e2e_test/src/common.rs

@@ -67,7 +67,10 @@ fn configured_capture_log_path(temp_dir: &str) -> Option<String> {
     capture_log_path(Path::new(&log_dir), temp_dir).map(|path| path.to_string_lossy().into_owned())
 }
 
-fn capture_command_logs(command: &mut Command, log_path: Option<&str>) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+pub(crate) fn capture_command_logs(
+    command: &mut Command,
+    log_path: Option<&str>,
+) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
     let Some(log_path) = log_path else {
         return Ok(());
     };

crates/e2e_test/src/compression_test.rs

@@ -2,6 +2,7 @@
 
 use crate::common::{RustFSTestEnvironment, init_logging, rustfs_binary_path};
 use aws_sdk_s3::primitives::ByteStream;
+use aws_sdk_s3::types::{CompletedMultipartUpload, CompletedPart};
 use serial_test::serial;
 use std::fs;
 use std::path::PathBuf;
@@ -25,6 +26,15 @@ fn generate_compressible_data(size: usize) -> Vec<u8> {
     data
 }
 
+/// Deterministic 2048-byte-period binary pattern that compresses extremely well: every part
+/// yields many compressed blocks, which is exactly the shape that reproduced the mid-payload
+/// Pending truncation (rustfs/rustfs#5957).
+fn generate_high_ratio_binary_data(size: usize, seed: u8) -> Vec<u8> {
+    (0..size)
+        .map(|i| ((i as u64).wrapping_mul(2_654_435_761).wrapping_add(seed as u64) >> 3) as u8)
+        .collect()
+}
+
 fn find_part_files(temp_dir: &str, bucket: &str, object_key: &str) -> Vec<PathBuf> {
     let bucket_path = PathBuf::from(temp_dir).join(bucket);
     let mut part_files = Vec::new();
@@ -55,7 +65,11 @@ async fn start_rustfs_with_compression(env: &mut RustFSTestEnvironment) -> Resul
     env.cleanup_existing_processes().await?;
 
     let binary_path = rustfs_binary_path();
-    let process = Command::new(&binary_path)
+    // Route the child's stdout/stderr through the shared RUSTFS_E2E_LOG_DIR
+    // capture (survives the temp-dir cleanup on Drop and is uploaded as a CI
+    // artifact); without the env var the child inherits stdio as before.
+    let mut command = Command::new(&binary_path);
+    command
         .env("RUSTFS_CONSOLE_ENABLE", "false")
         .env("RUSTFS_COMPRESSION_ENABLED", "true")
         .args([
@@ -66,8 +80,9 @@ async fn start_rustfs_with_compression(env: &mut RustFSTestEnvironment) -> Resul
             "--secret-key",
             &env.secret_key,
             &env.temp_dir,
-        ])
-        .spawn()?;
+        ]);
+    crate::common::capture_command_logs(&mut command, env.capture_log_path.as_deref())?;
+    let process = command.spawn()?;
 
     env.process = Some(process);
 
@@ -154,3 +169,646 @@ async fn test_compression_roundtrip() -> Result<(), Box<dyn std::error::Error +
     env.stop_server();
     Ok(())
 }
+
+const MULTIPART_COMPRESSION_BUCKET: &str = "compression-multipart-bucket";
+const MPU_PART1_SIZE: usize = 5 * 1024 * 1024;
+const MPU_PART2_SIZE: usize = 1024 * 1024;
+
+async fn multipart_upload(
+    client: &aws_sdk_s3::Client,
+    bucket: &str,
+    key: &str,
+    parts: &[&[u8]],
+) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    let create = client.create_multipart_upload().bucket(bucket).key(key).send().await?;
+    let upload_id = create.upload_id().ok_or("missing upload id")?.to_string();
+
+    let mut completed_parts = Vec::with_capacity(parts.len());
+    for (i, part) in parts.iter().enumerate() {
+        let part_number = (i + 1) as i32;
+        let upload = client
+            .upload_part()
+            .bucket(bucket)
+            .key(key)
+            .upload_id(&upload_id)
+            .part_number(part_number)
+            .body(ByteStream::from(part.to_vec()))
+            .send()
+            .await?;
+        completed_parts.push(
+            CompletedPart::builder()
+                .part_number(part_number)
+                .e_tag(upload.e_tag().unwrap_or_default())
+                .build(),
+        );
+    }
+
+    client
+        .complete_multipart_upload()
+        .bucket(bucket)
+        .key(key)
+        .upload_id(&upload_id)
+        .multipart_upload(CompletedMultipartUpload::builder().set_parts(Some(completed_parts)).build())
+        .send()
+        .await?;
+    Ok(())
+}
+
+async fn fetch_range(
+    client: &aws_sdk_s3::Client,
+    bucket: &str,
+    key: &str,
+    range: &str,
+) -> Result<Vec<u8>, Box<dyn std::error::Error + Send + Sync>> {
+    let response = client.get_object().bucket(bucket).key(key).range(range).send().await?;
+    Ok(response.body.collect().await?.into_bytes().to_vec())
+}
+
+/// Multipart disk compression roundtrip: parts are written as independent
+/// compressed streams and every GET shape must reassemble the original bytes
+/// (rustfs/rustfs#5957: multipart uploads previously bypassed disk compression
+/// entirely).
+#[tokio::test]
+#[serial]
+async fn test_compression_multipart_roundtrip() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    init_logging();
+    info!("Starting multipart compression roundtrip test");
+
+    let mut env = RustFSTestEnvironment::new().await?;
+    start_rustfs_with_compression(&mut env).await?;
+
+    let client = env.create_s3_client();
+    env.create_test_bucket(MULTIPART_COMPRESSION_BUCKET).await?;
+
+    let object_key = "multipart-compressible.txt";
+    let part1 = generate_compressible_data(MPU_PART1_SIZE);
+    let part2 = generate_compressible_data(MPU_PART2_SIZE);
+    let mut original_data = part1.clone();
+    original_data.extend_from_slice(&part2);
+    let total_size = original_data.len();
+
+    multipart_upload(&client, MULTIPART_COMPRESSION_BUCKET, object_key, &[&part1, &part2]).await?;
+
+    let head_response = client
+        .head_object()
+        .bucket(MULTIPART_COMPRESSION_BUCKET)
+        .key(object_key)
+        .send()
+        .await?;
+    assert_eq!(
+        head_response.content_length().unwrap_or(0) as usize,
+        total_size,
+        "Content-Length should be the logical object size"
+    );
+
+    let part_files = find_part_files(&env.temp_dir, MULTIPART_COMPRESSION_BUCKET, object_key);
+    assert!(!part_files.is_empty(), "expected on-disk part files for the multipart object");
+    let total_physical_size: u64 = part_files.iter().filter_map(|p| fs::metadata(p).ok()).map(|m| m.len()).sum();
+    assert!(
+        total_physical_size < (total_size / 2) as u64,
+        "Physical size {total_physical_size} should be well below original size {total_size} (multipart compression applied)"
+    );
+    info!("Multipart physical storage size: {total_physical_size} bytes (compressed from {total_size} bytes)");
+
+    // Full GET must reassemble both independently compressed parts.
+    let get_response = client
+        .get_object()
+        .bucket(MULTIPART_COMPRESSION_BUCKET)
+        .key(object_key)
+        .send()
+        .await?;
+    let downloaded = get_response.body.collect().await?.into_bytes();
+    assert_eq!(downloaded.len(), total_size);
+    assert_eq!(&downloaded[..], &original_data[..], "full GET data mismatch");
+
+    // Range fully inside part 1.
+    let range_inside_part1 = fetch_range(&client, MULTIPART_COMPRESSION_BUCKET, object_key, "bytes=1024-999423").await?;
+    assert_eq!(&range_inside_part1[..], &original_data[1024..999424], "part-1 range mismatch");
+
+    // Range crossing the part boundary.
+    let boundary_start = MPU_PART1_SIZE - 128 * 1024;
+    let boundary_end = MPU_PART1_SIZE + 128 * 1024 - 1;
+    let range_crossing = fetch_range(
+        &client,
+        MULTIPART_COMPRESSION_BUCKET,
+        object_key,
+        &format!("bytes={boundary_start}-{boundary_end}"),
+    )
+    .await?;
+    assert_eq!(
+        &range_crossing[..],
+        &original_data[boundary_start..boundary_end + 1],
+        "boundary-crossing range mismatch"
+    );
+
+    // Range fully inside part 2.
+    let part2_start = MPU_PART1_SIZE + 4096;
+    let part2_end = MPU_PART1_SIZE + 256 * 1024 - 1;
+    let range_inside_part2 = fetch_range(
+        &client,
+        MULTIPART_COMPRESSION_BUCKET,
+        object_key,
+        &format!("bytes={part2_start}-{part2_end}"),
+    )
+    .await?;
+    assert_eq!(
+        &range_inside_part2[..],
+        &original_data[part2_start..part2_end + 1],
+        "part-2 range mismatch"
+    );
+
+    // Suffix range (last 128 KiB, entirely in part 2).
+    let suffix_len = 128 * 1024;
+    let suffix = fetch_range(&client, MULTIPART_COMPRESSION_BUCKET, object_key, &format!("bytes=-{suffix_len}")).await?;
+    assert_eq!(&suffix[..], &original_data[total_size - suffix_len..], "suffix range mismatch");
+
+    // partNumber GETs must return each original part.
+    for (part_number, expected) in [(1, &part1), (2, &part2)] {
+        let response = client
+            .get_object()
+            .bucket(MULTIPART_COMPRESSION_BUCKET)
+            .key(object_key)
+            .part_number(part_number)
+            .send()
+            .await?;
+        let body = response.body.collect().await?.into_bytes();
+        assert_eq!(&body[..], &expected[..], "partNumber={part_number} GET mismatch");
+    }
+
+    info!("Multipart compression roundtrip test passed");
+    env.delete_test_bucket(MULTIPART_COMPRESSION_BUCKET).await?;
+    env.stop_server();
+    Ok(())
+}
+
+const MPU_HIGH_RATIO_BUCKET: &str = "compression-mpu-high-ratio-bucket";
+
+/// High-ratio binary multipart payload: the object key is on the compression allow-list, so the
+/// disk-compression path runs and each part is stored as many compressed blocks — the shape that
+/// reproduced the mid-payload Pending truncation (rustfs/rustfs#5957). Every GET shape must return
+/// the exact original bytes, and the stored size must show the data really was compressed.
+#[tokio::test]
+#[serial]
+async fn test_compression_multipart_high_ratio_binary_roundtrip() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    init_logging();
+    info!("Starting multipart high-ratio binary compression roundtrip test");
+
+    let mut env = RustFSTestEnvironment::new().await?;
+    start_rustfs_with_compression(&mut env).await?;
+
+    let client = env.create_s3_client();
+    env.create_test_bucket(MPU_HIGH_RATIO_BUCKET).await?;
+
+    let object_key = "multipart-high-ratio.txt";
+    let part1 = generate_high_ratio_binary_data(MPU_PART1_SIZE, 7);
+    let part2 = generate_high_ratio_binary_data(MPU_PART2_SIZE, 61);
+    let mut original_data = part1.clone();
+    original_data.extend_from_slice(&part2);
+    let total_size = original_data.len();
+
+    multipart_upload(&client, MPU_HIGH_RATIO_BUCKET, object_key, &[&part1, &part2]).await?;
+
+    let head_response = client
+        .head_object()
+        .bucket(MPU_HIGH_RATIO_BUCKET)
+        .key(object_key)
+        .send()
+        .await?;
+    assert_eq!(
+        head_response.content_length().unwrap_or(0) as usize,
+        total_size,
+        "Content-Length should be the logical object size"
+    );
+
+    // This pattern compresses to roughly 1/50 of its logical size, so a comfortably loose 2x
+    // margin still proves the parts were stored compressed rather than raw or double-encoded.
+    let part_files = find_part_files(&env.temp_dir, MPU_HIGH_RATIO_BUCKET, object_key);
+    assert!(!part_files.is_empty(), "expected on-disk part files for the multipart object");
+    let total_physical_size: u64 = part_files.iter().filter_map(|p| fs::metadata(p).ok()).map(|m| m.len()).sum();
+    assert!(
+        total_physical_size < (total_size as u64) / 2,
+        "Physical size {total_physical_size} should be far below the logical size {total_size} for high-ratio data"
+    );
+    info!("High-ratio multipart physical storage size: {total_physical_size} bytes (logical {total_size} bytes)");
+
+    info!("step: full GET");
+    let get_response = client
+        .get_object()
+        .bucket(MPU_HIGH_RATIO_BUCKET)
+        .key(object_key)
+        .send()
+        .await?;
+    let downloaded = get_response.body.collect().await?.into_bytes();
+    assert_eq!(downloaded.len(), total_size);
+    assert_eq!(&downloaded[..], &original_data[..], "full GET data mismatch");
+
+    // Range crossing the part boundary.
+    info!("step: boundary range GET");
+    let boundary_start = MPU_PART1_SIZE - 128 * 1024;
+    let boundary_end = MPU_PART1_SIZE + 128 * 1024 - 1;
+    let range_crossing = fetch_range(
+        &client,
+        MPU_HIGH_RATIO_BUCKET,
+        object_key,
+        &format!("bytes={boundary_start}-{boundary_end}"),
+    )
+    .await?;
+    assert_eq!(
+        &range_crossing[..],
+        &original_data[boundary_start..boundary_end + 1],
+        "boundary-crossing range mismatch"
+    );
+
+    // partNumber GET for the trailing part.
+    info!("step: partNumber GET");
+    let part2_response = client
+        .get_object()
+        .bucket(MPU_HIGH_RATIO_BUCKET)
+        .key(object_key)
+        .part_number(2)
+        .send()
+        .await?;
+    let part2_body = part2_response.body.collect().await?.into_bytes();
+    assert_eq!(&part2_body[..], &part2[..], "partNumber=2 GET mismatch");
+
+    info!("Multipart high-ratio binary compression roundtrip test passed");
+    env.delete_test_bucket(MPU_HIGH_RATIO_BUCKET).await?;
+    env.stop_server();
+    Ok(())
+}
+
+const MPU_COPY_COMPRESSION_BUCKET: &str = "compression-mpu-copy-bucket";
+const MPU_COPY_SOURCE_SIZE: usize = 6 * 1024 * 1024;
+const MPU_COPY_RANGE_LEN: usize = 5 * 1024 * 1024;
+
+/// UploadPartCopy feeds a part from an already stored (and already compressed) object. The copied
+/// range must be decompressed on read and re-compressed into the destination part, so the final
+/// object has to match "source prefix + uploaded tail" byte for byte.
+#[tokio::test]
+#[serial]
+async fn test_compression_multipart_upload_part_copy_roundtrip() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    init_logging();
+    info!("Starting multipart upload-part-copy compression roundtrip test");
+
+    let mut env = RustFSTestEnvironment::new().await?;
+    start_rustfs_with_compression(&mut env).await?;
+
+    let client = env.create_s3_client();
+    env.create_test_bucket(MPU_COPY_COMPRESSION_BUCKET).await?;
+
+    // Source object: a plain PUT that goes through the single-stream compression path.
+    let source_key = "copy-source.txt";
+    let source_data = generate_compressible_data(MPU_COPY_SOURCE_SIZE);
+    client
+        .put_object()
+        .bucket(MPU_COPY_COMPRESSION_BUCKET)
+        .key(source_key)
+        .body(ByteStream::from(source_data.clone()))
+        .send()
+        .await?;
+
+    // Destination object: part 1 copied from the source, part 2 uploaded directly.
+    let target_key = "copy-target.txt";
+    let part2 = generate_compressible_data(MPU_PART2_SIZE);
+    let mut expected_data = source_data[..MPU_COPY_RANGE_LEN].to_vec();
+    expected_data.extend_from_slice(&part2);
+    let total_size = expected_data.len();
+
+    let create = client
+        .create_multipart_upload()
+        .bucket(MPU_COPY_COMPRESSION_BUCKET)
+        .key(target_key)
+        .send()
+        .await?;
+    let upload_id = create.upload_id().ok_or("missing upload id")?.to_string();
+
+    let copy_part = client
+        .upload_part_copy()
+        .bucket(MPU_COPY_COMPRESSION_BUCKET)
+        .key(target_key)
+        .upload_id(&upload_id)
+        .part_number(1)
+        .copy_source(format!("{MPU_COPY_COMPRESSION_BUCKET}/{source_key}"))
+        .copy_source_range(format!("bytes=0-{}", MPU_COPY_RANGE_LEN - 1))
+        .send()
+        .await?;
+    let copy_etag = copy_part
+        .copy_part_result()
+        .and_then(|r| r.e_tag())
+        .ok_or("missing copy part etag")?
+        .to_string();
+
+    let uploaded_part = client
+        .upload_part()
+        .bucket(MPU_COPY_COMPRESSION_BUCKET)
+        .key(target_key)
+        .upload_id(&upload_id)
+        .part_number(2)
+        .body(ByteStream::from(part2.clone()))
+        .send()
+        .await?;
+
+    client
+        .complete_multipart_upload()
+        .bucket(MPU_COPY_COMPRESSION_BUCKET)
+        .key(target_key)
+        .upload_id(&upload_id)
+        .multipart_upload(
+            CompletedMultipartUpload::builder()
+                .parts(CompletedPart::builder().part_number(1).e_tag(copy_etag).build())
+                .parts(
+                    CompletedPart::builder()
+                        .part_number(2)
+                        .e_tag(uploaded_part.e_tag().unwrap_or_default())
+                        .build(),
+                )
+                .build(),
+        )
+        .send()
+        .await?;
+
+    let head_response = client
+        .head_object()
+        .bucket(MPU_COPY_COMPRESSION_BUCKET)
+        .key(target_key)
+        .send()
+        .await?;
+    assert_eq!(
+        head_response.content_length().unwrap_or(0) as usize,
+        total_size,
+        "Content-Length should be the logical object size"
+    );
+
+    let part_files = find_part_files(&env.temp_dir, MPU_COPY_COMPRESSION_BUCKET, target_key);
+    assert!(!part_files.is_empty(), "expected on-disk part files for the copied object");
+    let total_physical_size: u64 = part_files.iter().filter_map(|p| fs::metadata(p).ok()).map(|m| m.len()).sum();
+    assert!(
+        total_physical_size < (total_size / 2) as u64,
+        "Physical size {total_physical_size} should be well below original size {total_size} (copied part compression applied)"
+    );
+
+    let get_response = client
+        .get_object()
+        .bucket(MPU_COPY_COMPRESSION_BUCKET)
+        .key(target_key)
+        .send()
+        .await?;
+    let downloaded = get_response.body.collect().await?.into_bytes();
+    assert_eq!(downloaded.len(), total_size);
+    assert_eq!(&downloaded[..], &expected_data[..], "copied multipart GET data mismatch");
+
+    info!("Multipart upload-part-copy compression roundtrip test passed");
+    env.delete_test_bucket(MPU_COPY_COMPRESSION_BUCKET).await?;
+    env.stop_server();
+    Ok(())
+}
+
+const MPU_THREE_PARTS_BUCKET: &str = "compression-mpu-three-parts-bucket";
+const MPU_THREE_PARTS_TAIL_SIZE: usize = 512 * 1024;
+
+/// Three-part upload with uneven part sizes: each partNumber GET must map back to exactly one
+/// compressed part stream, and a suffix range must resolve inside the trailing part.
+#[tokio::test]
+#[serial]
+async fn test_compression_multipart_three_parts_part_number_gets() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    init_logging();
+    info!("Starting three-part multipart compression partNumber test");
+
+    let mut env = RustFSTestEnvironment::new().await?;
+    start_rustfs_with_compression(&mut env).await?;
+
+    let client = env.create_s3_client();
+    env.create_test_bucket(MPU_THREE_PARTS_BUCKET).await?;
+
+    let object_key = "multipart-three-parts.txt";
+    let part1 = generate_compressible_data(MPU_PART1_SIZE);
+    let part2 = generate_compressible_data(MPU_PART1_SIZE);
+    let part3 = generate_compressible_data(MPU_THREE_PARTS_TAIL_SIZE);
+    let mut original_data = part1.clone();
+    original_data.extend_from_slice(&part2);
+    original_data.extend_from_slice(&part3);
+    let total_size = original_data.len();
+
+    multipart_upload(&client, MPU_THREE_PARTS_BUCKET, object_key, &[&part1, &part2, &part3]).await?;
+
+    let head_response = client
+        .head_object()
+        .bucket(MPU_THREE_PARTS_BUCKET)
+        .key(object_key)
+        .send()
+        .await?;
+    assert_eq!(
+        head_response.content_length().unwrap_or(0) as usize,
+        total_size,
+        "Content-Length should be the logical object size"
+    );
+
+    let part_files = find_part_files(&env.temp_dir, MPU_THREE_PARTS_BUCKET, object_key);
+    assert!(!part_files.is_empty(), "expected on-disk part files for the multipart object");
+    let total_physical_size: u64 = part_files.iter().filter_map(|p| fs::metadata(p).ok()).map(|m| m.len()).sum();
+    assert!(
+        total_physical_size < (total_size / 2) as u64,
+        "Physical size {total_physical_size} should be well below original size {total_size} (multipart compression applied)"
+    );
+
+    // Every partNumber GET must return exactly the bytes of the corresponding uploaded part.
+    for (part_number, expected) in [(1, &part1), (2, &part2), (3, &part3)] {
+        let response = client
+            .get_object()
+            .bucket(MPU_THREE_PARTS_BUCKET)
+            .key(object_key)
+            .part_number(part_number)
+            .send()
+            .await?;
+        let body = response.body.collect().await?.into_bytes();
+        assert_eq!(&body[..], &expected[..], "partNumber={part_number} GET mismatch");
+    }
+
+    // Suffix range (last 64 KiB) resolves inside the trailing part.
+    let suffix_len = 64 * 1024;
+    let suffix = fetch_range(&client, MPU_THREE_PARTS_BUCKET, object_key, &format!("bytes=-{suffix_len}")).await?;
+    assert_eq!(&suffix[..], &original_data[total_size - suffix_len..], "suffix range mismatch");
+
+    info!("Three-part multipart compression partNumber test passed");
+    env.delete_test_bucket(MPU_THREE_PARTS_BUCKET).await?;
+    env.stop_server();
+    Ok(())
+}
+
+const MPU_SSE_COMPRESSION_BUCKET: &str = "compression-mpu-sse-bucket";
+
+async fn start_rustfs_with_compression_and_sse(
+    env: &mut RustFSTestEnvironment,
+) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    use base64::Engine;
+    env.cleanup_existing_processes().await?;
+
+    let binary_path = rustfs_binary_path();
+    let master_key = base64::engine::general_purpose::STANDARD.encode([0x42u8; 32]);
+    // Server output goes to a file inside the per-test temp dir so a failing
+    // run can be diagnosed from the child's logs.
+    let server_log = std::fs::File::create(format!("{}/server.log", env.temp_dir))?;
+    let server_log_err = server_log.try_clone()?;
+    let process = Command::new(&binary_path)
+        .env("RUSTFS_CONSOLE_ENABLE", "false")
+        .env("RUSTFS_COMPRESSION_ENABLED", "true")
+        .env("RUSTFS_SSE_S3_MASTER_KEY", master_key)
+        .env("RUST_LOG", "rustfs=info,rustfs_ecstore=info")
+        .stdout(std::process::Stdio::from(server_log))
+        .stderr(std::process::Stdio::from(server_log_err))
+        .args([
+            "--address",
+            &env.address,
+            "--access-key",
+            &env.access_key,
+            "--secret-key",
+            &env.secret_key,
+            &env.temp_dir,
+        ])
+        .spawn()?;
+
+    env.process = Some(process);
+
+    info!("Waiting for RustFS server with compression + SSE-S3 enabled on {}", env.address);
+    for i in 0..30 {
+        if TcpStream::connect(&env.address).await.is_ok() {
+            info!("RustFS server is ready after {} attempts", i + 1);
+            return Ok(());
+        }
+        if i == 29 {
+            return Err("RustFS server failed to become ready".into());
+        }
+        sleep(Duration::from_secs(1)).await;
+    }
+    Ok(())
+}
+
+/// SSE-S3 + disk compression multipart: each part is compressed and then encrypted, and every GET
+/// shape must still return the original plaintext bytes. Physical size must shrink because the
+/// compression runs before encryption.
+#[tokio::test]
+#[serial]
+async fn test_compression_multipart_sse_s3_roundtrip() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    use aws_sdk_s3::types::ServerSideEncryption;
+
+    init_logging();
+    info!("Starting SSE-S3 multipart compression roundtrip test");
+
+    let mut env = RustFSTestEnvironment::new().await?;
+    start_rustfs_with_compression_and_sse(&mut env).await?;
+
+    let client = env.create_s3_client();
+    env.create_test_bucket(MPU_SSE_COMPRESSION_BUCKET).await?;
+
+    let object_key = "multipart-sse-compressible.txt";
+    let part1 = generate_compressible_data(MPU_PART1_SIZE);
+    let part2 = generate_compressible_data(MPU_PART2_SIZE);
+    let mut original_data = part1.clone();
+    original_data.extend_from_slice(&part2);
+    let total_size = original_data.len();
+
+    let create = client
+        .create_multipart_upload()
+        .bucket(MPU_SSE_COMPRESSION_BUCKET)
+        .key(object_key)
+        .server_side_encryption(ServerSideEncryption::Aes256)
+        .send()
+        .await?;
+    let upload_id = create.upload_id().ok_or("missing upload id")?.to_string();
+
+    let mut completed_parts = Vec::new();
+    for (i, part) in [&part1, &part2].into_iter().enumerate() {
+        let part_number = (i + 1) as i32;
+        let upload = client
+            .upload_part()
+            .bucket(MPU_SSE_COMPRESSION_BUCKET)
+            .key(object_key)
+            .upload_id(&upload_id)
+            .part_number(part_number)
+            .body(ByteStream::from(part.clone()))
+            .send()
+            .await?;
+        completed_parts.push(
+            CompletedPart::builder()
+                .part_number(part_number)
+                .e_tag(upload.e_tag().unwrap_or_default())
+                .build(),
+        );
+    }
+
+    client
+        .complete_multipart_upload()
+        .bucket(MPU_SSE_COMPRESSION_BUCKET)
+        .key(object_key)
+        .upload_id(&upload_id)
+        .multipart_upload(CompletedMultipartUpload::builder().set_parts(Some(completed_parts)).build())
+        .send()
+        .await?;
+
+    let head_response = client
+        .head_object()
+        .bucket(MPU_SSE_COMPRESSION_BUCKET)
+        .key(object_key)
+        .send()
+        .await?;
+    assert_eq!(
+        head_response.content_length().unwrap_or(0) as usize,
+        total_size,
+        "Content-Length should be the logical object size"
+    );
+    assert_eq!(
+        head_response.server_side_encryption(),
+        Some(&ServerSideEncryption::Aes256),
+        "HEAD must report SSE-S3"
+    );
+
+    let part_files = find_part_files(&env.temp_dir, MPU_SSE_COMPRESSION_BUCKET, object_key);
+    assert!(!part_files.is_empty(), "expected on-disk part files for the multipart object");
+    let total_physical_size: u64 = part_files.iter().filter_map(|p| fs::metadata(p).ok()).map(|m| m.len()).sum();
+    assert!(
+        total_physical_size < (total_size / 2) as u64,
+        "Physical size {total_physical_size} should be well below original size {total_size} (compress-then-encrypt applied)"
+    );
+
+    let get_response = client
+        .get_object()
+        .bucket(MPU_SSE_COMPRESSION_BUCKET)
+        .key(object_key)
+        .send()
+        .await?;
+    let downloaded = get_response.body.collect().await?.into_bytes();
+    assert_eq!(downloaded.len(), total_size);
+    assert_eq!(&downloaded[..], &original_data[..], "SSE-S3 multipart full GET data mismatch");
+
+    // Range crossing the part boundary must decrypt and decompress across parts.
+    let boundary_start = MPU_PART1_SIZE - 64 * 1024;
+    let boundary_end = MPU_PART1_SIZE + 64 * 1024 - 1;
+    let range_crossing = fetch_range(
+        &client,
+        MPU_SSE_COMPRESSION_BUCKET,
+        object_key,
+        &format!("bytes={boundary_start}-{boundary_end}"),
+    )
+    .await?;
+    assert_eq!(
+        &range_crossing[..],
+        &original_data[boundary_start..boundary_end + 1],
+        "SSE-S3 boundary-crossing range mismatch"
+    );
+
+    // partNumber GET for the trailing part.
+    let part2_response = client
+        .get_object()
+        .bucket(MPU_SSE_COMPRESSION_BUCKET)
+        .key(object_key)
+        .part_number(2)
+        .send()
+        .await?;
+    let part2_body = part2_response.body.collect().await?.into_bytes();
+    assert_eq!(&part2_body[..], &part2[..], "SSE-S3 partNumber=2 GET mismatch");
+
+    info!("SSE-S3 multipart compression roundtrip test passed");
+    env.delete_test_bucket(MPU_SSE_COMPRESSION_BUCKET).await?;
+    env.stop_server();
+    Ok(())
+}

crates/e2e_test/src/inline_fast_path_cluster_test.rs

@@ -1828,9 +1828,11 @@ async fn four_node_compressed_inline_fallback() -> TestResult {
     Ok(())
 }
 
+/// Multipart disk compression is live again, so a compression-enabled cluster classifies multipart objects as compressed and the roundtrip (full GET plus partNumber GET) must still return the original bytes.
+/// Reverting the multipart compression fix must fail this test.
 #[tokio::test]
 #[serial]
-async fn four_node_multipart_ignores_disk_compression_fallback() -> TestResult {
+async fn four_node_multipart_disk_compression_roundtrip() -> TestResult {
     init_logging();
 
     let collector = OtlpMetricCollector::start().await?;
@@ -1839,22 +1841,22 @@ async fn four_node_multipart_ignores_disk_compression_fallback() -> TestResult {
     cluster.set_env("RUSTFS_COMPRESSION_ENABLED", "true");
     cluster.start().await?;
 
-    let bucket = "inline-multipart-compression-fallback";
+    let bucket = "inline-multipart-compression-roundtrip";
     cluster.create_test_bucket(bucket).await?;
     let client = cluster.create_s3_client(0)?;
-    let key = "multipart/compression-disabled.txt";
+    let key = "multipart/compressed.txt";
     let (body, second_part, etag) = put_two_part_multipart(&client, bucket, key).await?;
 
     assert_reader_path(
         &collector,
         &client,
-        ReaderPathExpectation::for_class(ReaderObject::new(bucket, key, &body, etag.as_deref(), None), LEGACY_DUPLEX, MULTIPART),
+        ReaderPathExpectation::for_class(ReaderObject::new(bucket, key, &body, etag.as_deref(), None), LEGACY_DUPLEX, COMPRESSED),
     )
     .await?;
     assert_part_number_reader_path(
         &collector,
         &client,
-        PartNumberReaderPathExpectation::new(bucket, key, &second_part, body.len(), MULTIPART, LEGACY_DUPLEX),
+        PartNumberReaderPathExpectation::new(bucket, key, &second_part, body.len(), COMPRESSED, LEGACY_DUPLEX),
     )
     .await?;
 
@@ -1890,14 +1892,21 @@ async fn four_node_mixed_msgpack_compat_mode_preserves_fallback_controls() -> Te
         ReaderPathExpectation::for_class(
             ReaderObject::new(bucket, multipart_key, &multipart_body, multipart_etag.as_deref(), None),
             LEGACY_DUPLEX,
-            MULTIPART,
+            COMPRESSED,
         ),
     )
     .await?;
     assert_part_number_reader_path(
         &collector,
         &client,
-        PartNumberReaderPathExpectation::new(bucket, multipart_key, &second_part, multipart_body.len(), MULTIPART, LEGACY_DUPLEX),
+        PartNumberReaderPathExpectation::new(
+            bucket,
+            multipart_key,
+            &second_part,
+            multipart_body.len(),
+            COMPRESSED,
+            LEGACY_DUPLEX,
+        ),
     )
     .await?;
     assert_msgpack_decode_observed(&collector, &decode_before).await?;
@@ -2353,7 +2362,11 @@ async fn four_node_mixed_msgpack_compat_mode_preserves_fallback_controls_during_
     hot_client.create_bucket().bucket(bucket).send().await?;
     put_lifecycle_with_transition_retry(&hot_client, bucket, &tier_name).await?;
 
-    let key = "transition/mixed-multipart.bin";
+    // `.zip` sits on the disk-compression exclusion list: this test pins
+    // msgpack compat controls across ILM transition, and a compressed object
+    // would classify as `compressed` instead of `remote` (and the warm-tier
+    // read path does not decode compression — tracked separately).
+    let key = "transition/mixed-multipart.zip";
     let (body, second_part, etag) = put_two_part_multipart(&hot_client, bucket, key).await?;
     wait_for_transition(&hot_client, bucket, key, &tier_name).await?;
     assert!(

crates/ecstore/src/object_api/readers.rs

@@ -1727,6 +1727,423 @@ mod tests {
         assert_eq!(actual, b"fghijkl");
     }
 
+    /// Compresses one multipart part exactly like the write path does
+    /// (`WritePlan::with_compression` wraps each part in its own
+    /// `compression_reader`), returning the on-disk bytes and the storage-format
+    /// compression index.
+    async fn compressed_part_fixture(data: &[u8]) -> (Vec<u8>, Option<Bytes>) {
+        use crate::io_support::rio::TryGetIndex as _;
+        let mut compressor =
+            crate::io_support::rio::compression_reader(Cursor::new(data.to_vec()), CompressionAlgorithm::default(), false);
+        let mut compressed = Vec::new();
+        compressor.read_to_end(&mut compressed).await.expect("compress part stream");
+        let index = compressor
+            .try_get_index()
+            .map(crate::io_support::rio::compression_index_storage_bytes);
+        (compressed, index)
+    }
+
+    struct CompressedMultipartFixture {
+        object_info: ObjectInfo,
+        stored: Vec<u8>,
+        plaintext: Vec<u8>,
+    }
+
+    /// Builds the on-disk representation of a compressed multipart object: each
+    /// part is an independent compressed stream and the storage layer serves
+    /// their concatenation.
+    async fn compressed_multipart_fixture(part_sizes: &[usize]) -> CompressedMultipartFixture {
+        let pattern = b"compressed multipart read path fixture data ";
+        let mut plaintext = Vec::new();
+        let mut stored = Vec::new();
+        let mut parts = Vec::with_capacity(part_sizes.len());
+
+        for (i, part_size) in part_sizes.iter().enumerate() {
+            let mut part_plaintext = Vec::with_capacity(*part_size);
+            while part_plaintext.len() < *part_size {
+                part_plaintext.extend_from_slice(pattern);
+                part_plaintext.push(i as u8);
+            }
+            part_plaintext.truncate(*part_size);
+
+            let (compressed, index) = compressed_part_fixture(&part_plaintext).await;
+            parts.push(ObjectPartInfo {
+                number: i + 1,
+                size: compressed.len(),
+                actual_size: *part_size as i64,
+                index,
+                ..Default::default()
+            });
+            stored.extend_from_slice(&compressed);
+            plaintext.extend_from_slice(&part_plaintext);
+        }
+
+        let mut user_defined = HashMap::new();
+        rustfs_utils::http::insert_str(
+            &mut user_defined,
+            rustfs_utils::http::SUFFIX_COMPRESSION,
+            crate::io_support::rio::compression_metadata_value(CompressionAlgorithm::default()),
+        );
+        rustfs_utils::http::insert_str(&mut user_defined, rustfs_utils::http::SUFFIX_ACTUAL_SIZE, plaintext.len().to_string());
+
+        let object_info = ObjectInfo {
+            bucket: "test-bucket".to_string(),
+            name: "compressed-multipart".to_string(),
+            size: stored.len() as i64,
+            etag: Some(format!("6bcf86bed8807b8e78f0fc6e0a53079d-{}", part_sizes.len())),
+            parts: Arc::new(parts),
+            user_defined: Arc::new(user_defined),
+            ..Default::default()
+        };
+
+        CompressedMultipartFixture {
+            object_info,
+            stored,
+            plaintext,
+        }
+    }
+
+    /// Plans the read once to learn the storage window, then serves exactly that
+    /// window — mirroring how `set_disk` feeds the erasure read into the
+    /// returned reader.
+    async fn read_compressed_multipart(
+        fixture: &CompressedMultipartFixture,
+        rs: Option<HTTPRangeSpec>,
+        opts: &ObjectOptions,
+    ) -> Vec<u8> {
+        let headers = HeaderMap::new();
+        let (_, offset, length) =
+            GetObjectReader::new(Box::new(Cursor::new(Vec::new())), rs.clone(), &fixture.object_info, opts, &headers)
+                .await
+                .expect("plan compressed multipart read");
+
+        let end = offset + usize::try_from(length).expect("storage window length must be non-negative");
+        assert!(
+            end <= fixture.stored.len(),
+            "planned storage window {offset}..{end} exceeds stored stream of {} bytes",
+            fixture.stored.len()
+        );
+        let window = fixture.stored[offset..end].to_vec();
+
+        let (mut reader, replay_offset, replay_length) =
+            GetObjectReader::new(Box::new(Cursor::new(window)), rs, &fixture.object_info, opts, &headers)
+                .await
+                .expect("build compressed multipart reader");
+        assert_eq!((replay_offset, replay_length), (offset, length), "read plan must be deterministic");
+
+        reader.read_all().await.expect("read compressed multipart stream")
+    }
+
+    /// Byte pattern with a 2 KiB period: it compresses extremely well while
+    /// looking nothing like ASCII fixtures. Mirrors the e2e generator that
+    /// exposed a truncated full GET on high-ratio multipart payloads.
+    fn high_ratio_binary_payload(size: usize, seed: u8) -> Vec<u8> {
+        (0..size)
+            .map(|i| ((i as u64).wrapping_mul(2_654_435_761).wrapping_add(seed as u64) >> 3) as u8)
+            .collect()
+    }
+
+    #[tokio::test]
+    async fn compressed_multipart_full_get_handles_high_ratio_binary_payload() {
+        let part_sizes = [5 * 1024 * 1024_usize, 1024 * 1024];
+        let mut plaintext = Vec::new();
+        let mut stored = Vec::new();
+        let mut parts = Vec::with_capacity(part_sizes.len());
+
+        for (i, part_size) in part_sizes.iter().enumerate() {
+            let part_plaintext = high_ratio_binary_payload(*part_size, if i == 0 { 7 } else { 61 });
+            let (compressed, index) = compressed_part_fixture(&part_plaintext).await;
+            parts.push(ObjectPartInfo {
+                number: i + 1,
+                size: compressed.len(),
+                actual_size: *part_size as i64,
+                index,
+                ..Default::default()
+            });
+            stored.extend_from_slice(&compressed);
+            plaintext.extend_from_slice(&part_plaintext);
+        }
+
+        let mut user_defined = HashMap::new();
+        rustfs_utils::http::insert_str(
+            &mut user_defined,
+            rustfs_utils::http::SUFFIX_COMPRESSION,
+            crate::io_support::rio::compression_metadata_value(CompressionAlgorithm::default()),
+        );
+        rustfs_utils::http::insert_str(&mut user_defined, rustfs_utils::http::SUFFIX_ACTUAL_SIZE, plaintext.len().to_string());
+        let fixture = CompressedMultipartFixture {
+            object_info: ObjectInfo {
+                bucket: "test-bucket".to_string(),
+                name: "high-ratio-multipart".to_string(),
+                size: stored.len() as i64,
+                etag: Some("6bcf86bed8807b8e78f0fc6e0a53079d-2".to_string()),
+                parts: Arc::new(parts),
+                user_defined: Arc::new(user_defined),
+                ..Default::default()
+            },
+            stored,
+            plaintext,
+        };
+
+        let read = read_compressed_multipart(&fixture, None, &ObjectOptions::default()).await;
+
+        assert_eq!(read.len(), fixture.plaintext.len(), "full GET must return the logical size");
+        assert_eq!(read, fixture.plaintext, "high-ratio multipart payload must survive the roundtrip");
+    }
+
+    /// Full GET over a compressed multipart object must decode across part
+    /// boundaries: every part is an independent compressed stream (this is also
+    /// the on-disk shape written by builds before rustfs/rustfs#5169 disabled
+    /// multipart compression, so this pins legacy-object readability).
+    #[tokio::test]
+    async fn compressed_multipart_full_get_decodes_across_part_boundaries() {
+        let fixture = compressed_multipart_fixture(&[3 * 1024 * 1024, 2 * 1024 * 1024, 512 * 1024]).await;
+
+        let read = read_compressed_multipart(&fixture, None, &ObjectOptions::default()).await;
+
+        assert_eq!(read.len(), fixture.plaintext.len(), "full GET must return the logical size");
+        assert_eq!(read, fixture.plaintext, "full GET must reassemble all parts");
+    }
+
+    #[tokio::test]
+    async fn compressed_multipart_range_get_crosses_part_boundary() {
+        let fixture = compressed_multipart_fixture(&[3 * 1024 * 1024, 2 * 1024 * 1024]).await;
+        let boundary = 3 * 1024 * 1024_i64;
+        let rs = HTTPRangeSpec {
+            is_suffix_length: false,
+            start: boundary - 100_000,
+            end: boundary + 100_000 - 1,
+        };
+
+        let read = read_compressed_multipart(&fixture, Some(rs), &ObjectOptions::default()).await;
+
+        let expected = &fixture.plaintext[(boundary - 100_000) as usize..(boundary + 100_000) as usize];
+        assert_eq!(read, expected, "boundary-crossing range must splice both parts");
+    }
+
+    #[tokio::test]
+    async fn compressed_multipart_range_get_seeks_into_later_part() {
+        let fixture = compressed_multipart_fixture(&[3 * 1024 * 1024, 4 * 1024 * 1024]).await;
+        // Deep inside part 2 so the plan skips part 1 entirely and (when the
+        // part carries an index) seeks within part 2.
+        let start = 3 * 1024 * 1024_i64 + 2 * 1024 * 1024_i64 + 137;
+        let rs = HTTPRangeSpec {
+            is_suffix_length: false,
+            start,
+            end: start + 64 * 1024 - 1,
+        };
+
+        let read = read_compressed_multipart(&fixture, Some(rs), &ObjectOptions::default()).await;
+
+        let expected = &fixture.plaintext[start as usize..(start + 64 * 1024) as usize];
+        assert_eq!(read, expected, "range inside a later part must decode from that part");
+    }
+
+    /// Parts written without a compression index (small parts skip the index in
+    /// the rio-v2 backend) must still be rangeable: the plan starts at the part
+    /// boundary and skips decompressed bytes.
+    #[tokio::test]
+    async fn compressed_multipart_range_get_works_without_part_indexes() {
+        let mut fixture = compressed_multipart_fixture(&[1024 * 1024, 1024 * 1024]).await;
+        let parts = fixture
+            .object_info
+            .parts
+            .iter()
+            .map(|part| ObjectPartInfo {
+                index: None,
+                ..part.clone()
+            })
+            .collect::<Vec<_>>();
+        fixture.object_info.parts = Arc::new(parts);
+
+        let start = 1024 * 1024_i64 + 4096;
+        let rs = HTTPRangeSpec {
+            is_suffix_length: false,
+            start,
+            end: start + 32 * 1024 - 1,
+        };
+
+        let read = read_compressed_multipart(&fixture, Some(rs), &ObjectOptions::default()).await;
+
+        let expected = &fixture.plaintext[start as usize..(start + 32 * 1024) as usize];
+        assert_eq!(read, expected, "index-less parts must fall back to part-boundary skip");
+    }
+
+    #[tokio::test]
+    async fn compressed_multipart_part_number_get_returns_single_part() {
+        let part_sizes = [3 * 1024 * 1024, 2 * 1024 * 1024, 512 * 1024];
+        let fixture = compressed_multipart_fixture(&part_sizes).await;
+
+        let mut logical_offset = 0_usize;
+        for (i, part_size) in part_sizes.iter().enumerate() {
+            let opts = ObjectOptions {
+                part_number: Some(i + 1),
+                ..Default::default()
+            };
+
+            let read = read_compressed_multipart(&fixture, None, &opts).await;
+
+            let expected = &fixture.plaintext[logical_offset..logical_offset + part_size];
+            assert_eq!(read.len(), *part_size, "partNumber={} GET must return the part's logical size", i + 1);
+            assert_eq!(read, expected, "partNumber={} GET must return the original part bytes", i + 1);
+            logical_offset += part_size;
+        }
+    }
+
+    #[tokio::test]
+    async fn compressed_multipart_suffix_range_reads_tail() {
+        let fixture = compressed_multipart_fixture(&[3 * 1024 * 1024, 1024 * 1024]).await;
+        let suffix_len = 128 * 1024_i64;
+        let rs = HTTPRangeSpec {
+            is_suffix_length: true,
+            start: suffix_len,
+            end: -1,
+        };
+
+        let read = read_compressed_multipart(&fixture, Some(rs), &ObjectOptions::default()).await;
+
+        let expected = &fixture.plaintext[fixture.plaintext.len() - suffix_len as usize..];
+        assert_eq!(read, expected, "suffix range must return the tail of the last part");
+    }
+
+    /// Builds an SSE-C + disk-compression multipart object exactly like the
+    /// write path: each part is compressed into its own stream and then
+    /// encrypted with the per-part key schedule. The fixture is
+    /// legacy-encryption-specific (`rustfs_rio::EncryptReader`), matching the
+    /// pre-existing `build_legacy_ssec_multipart_fixture` shape, while the
+    /// compression layer follows the active backend feature.
+    async fn compressed_encrypted_multipart_fixture(key_bytes: [u8; 32], part_sizes: &[usize]) -> CompressedMultipartFixture {
+        let pattern = b"compressed encrypted multipart fixture data ";
+        let mut plaintext = Vec::new();
+        let mut stored = Vec::new();
+        let mut parts = Vec::with_capacity(part_sizes.len());
+
+        for (i, part_size) in part_sizes.iter().enumerate() {
+            let part_number = i + 1;
+            let mut part_plaintext = Vec::with_capacity(*part_size);
+            while part_plaintext.len() < *part_size {
+                part_plaintext.extend_from_slice(pattern);
+                part_plaintext.push(part_number as u8);
+            }
+            part_plaintext.truncate(*part_size);
+
+            let (compressed, index) = compressed_part_fixture(&part_plaintext).await;
+            let mut part_cipher = Vec::new();
+            rustfs_rio::EncryptReader::new_multipart(Cursor::new(compressed), key_bytes, LEGACY_FIXTURE_BASE_NONCE, part_number)
+                .read_to_end(&mut part_cipher)
+                .await
+                .expect("encrypt compressed fixture part");
+
+            parts.push(ObjectPartInfo {
+                number: part_number,
+                size: part_cipher.len(),
+                actual_size: *part_size as i64,
+                index,
+                ..Default::default()
+            });
+            stored.extend_from_slice(&part_cipher);
+            plaintext.extend_from_slice(&part_plaintext);
+        }
+
+        let mut user_defined = legacy_ssec_multipart_metadata(key_bytes, plaintext.len());
+        rustfs_utils::http::insert_str(
+            &mut user_defined,
+            rustfs_utils::http::SUFFIX_COMPRESSION,
+            crate::io_support::rio::compression_metadata_value(CompressionAlgorithm::default()),
+        );
+        rustfs_utils::http::insert_str(&mut user_defined, rustfs_utils::http::SUFFIX_ACTUAL_SIZE, plaintext.len().to_string());
+
+        let object_info = ObjectInfo {
+            bucket: "test-bucket".to_string(),
+            name: "compressed-encrypted-multipart".to_string(),
+            size: stored.len() as i64,
+            etag: Some(format!("6bcf86bed8807b8e78f0fc6e0a53079d-{}", part_sizes.len())),
+            parts: Arc::new(parts),
+            user_defined: Arc::new(user_defined),
+            ..Default::default()
+        };
+
+        CompressedMultipartFixture {
+            object_info,
+            stored,
+            plaintext,
+        }
+    }
+
+    async fn read_compressed_encrypted_multipart(
+        fixture: &CompressedMultipartFixture,
+        key_bytes: [u8; 32],
+        rs: Option<HTTPRangeSpec>,
+        opts: &ObjectOptions,
+    ) -> Vec<u8> {
+        let headers = ssec_headers_from_key(key_bytes);
+        let (_, offset, length) =
+            GetObjectReader::new(Box::new(Cursor::new(Vec::new())), rs.clone(), &fixture.object_info, opts, &headers)
+                .await
+                .expect("plan compressed encrypted multipart read");
+
+        let end = offset + usize::try_from(length).expect("storage window length must be non-negative");
+        assert!(
+            end <= fixture.stored.len(),
+            "planned storage window {offset}..{end} exceeds stored stream of {} bytes",
+            fixture.stored.len()
+        );
+        let window = fixture.stored[offset..end].to_vec();
+
+        let (mut reader, replay_offset, replay_length) =
+            GetObjectReader::new(Box::new(Cursor::new(window)), rs, &fixture.object_info, opts, &headers)
+                .await
+                .expect("build compressed encrypted multipart reader");
+        assert_eq!((replay_offset, replay_length), (offset, length), "read plan must be deterministic");
+
+        reader.read_all().await.expect("read compressed encrypted multipart stream")
+    }
+
+    #[tokio::test]
+    async fn compressed_encrypted_multipart_full_get_roundtrip() {
+        let key_bytes = [0x6Eu8; 32];
+        let fixture = compressed_encrypted_multipart_fixture(key_bytes, &[3 * 1024 * 1024, 1024 * 1024]).await;
+
+        let read = read_compressed_encrypted_multipart(&fixture, key_bytes, None, &ObjectOptions::default()).await;
+
+        assert_eq!(read.len(), fixture.plaintext.len(), "full GET must return the logical size");
+        assert_eq!(read, fixture.plaintext, "SSE-C + compression full GET must reassemble all parts");
+    }
+
+    #[tokio::test]
+    async fn compressed_encrypted_multipart_range_crosses_part_boundary() {
+        let key_bytes = [0x6Eu8; 32];
+        let fixture = compressed_encrypted_multipart_fixture(key_bytes, &[3 * 1024 * 1024, 1024 * 1024]).await;
+        let boundary = 3 * 1024 * 1024_i64;
+        let rs = HTTPRangeSpec {
+            is_suffix_length: false,
+            start: boundary - 65_536,
+            end: boundary + 65_536 - 1,
+        };
+
+        let read = read_compressed_encrypted_multipart(&fixture, key_bytes, Some(rs), &ObjectOptions::default()).await;
+
+        let expected = &fixture.plaintext[(boundary - 65_536) as usize..(boundary + 65_536) as usize];
+        assert_eq!(read, expected, "SSE-C + compression boundary-crossing range must splice both parts");
+    }
+
+    #[tokio::test]
+    async fn compressed_encrypted_multipart_part_number_get_returns_single_part() {
+        let key_bytes = [0x6Eu8; 32];
+        let part_sizes = [3 * 1024 * 1024, 1024 * 1024];
+        let fixture = compressed_encrypted_multipart_fixture(key_bytes, &part_sizes).await;
+
+        let opts = ObjectOptions {
+            part_number: Some(2),
+            ..Default::default()
+        };
+        let read = read_compressed_encrypted_multipart(&fixture, key_bytes, None, &opts).await;
+
+        let expected = &fixture.plaintext[part_sizes[0]..];
+        assert_eq!(read.len(), part_sizes[1], "partNumber=2 GET must return the part's logical size");
+        assert_eq!(read, expected, "partNumber=2 GET must return the original part bytes");
+    }
+
     #[tokio::test]
     async fn test_get_object_reader_rejects_ssec_read_without_headers() {
         let object_info = ObjectInfo {

crates/rio/src/compress_reader.rs

@@ -71,6 +71,7 @@ where
 
     /// Optional: allow users to customize block_size
     pub fn with_block_size(inner: R, block_size: usize, compression_algorithm: CompressionAlgorithm) -> Self {
+        debug_assert!(block_size > 0, "CompressReader block_size must be non-zero");
         Self {
             inner,
             buffer: Vec::new(),
@@ -183,11 +184,21 @@ pin_project! {
         buffer: Vec<u8>,
         buffer_pos: usize,
         finished: bool,
+        // A previously surfaced stream error is sticky: without this, a caller
+        // that polls again after an error would restart at the header phase and
+        // read a truncated tail as a clean EOF, converting the error into a
+        // silently short body.
+        poisoned: bool,
         // Fields for saving header read progress across polls
         header_buf: [u8; 8],
         header_read: usize,
-        header_done: bool,
-        // Fields for saving compressed block read progress across polls
+        // Fields for saving compressed block read progress across polls.
+        // `compressed_len > 0` means a block payload is in flight: the header has
+        // been fully parsed and `compressed_read` bytes of the payload are already
+        // consumed from the inner stream. The header phase must not run again (and
+        // must not reset `compressed_read`) until this block completes, or a
+        // `Poll::Pending` in the middle of a payload would silently drop the bytes
+        // read so far and desynchronize the block framing.
         compressed_buf: Vec<u8>,
         compressed_read: usize,
         compressed_len: usize,
@@ -205,9 +216,9 @@ where
             buffer: Vec::new(),
             buffer_pos: 0,
             finished: false,
+            poisoned: false,
             header_buf: [0u8; 8],
             header_read: 0,
-            header_done: false,
             compressed_buf: Vec::new(),
             compressed_read: 0,
             compressed_len: 0,
@@ -236,54 +247,72 @@ where
         if *this.finished {
             return Poll::Ready(Ok(()));
         }
-        // Read header
-        while !*this.header_done && *this.header_read < HEADER_LEN {
-            let mut temp = [0u8; HEADER_LEN];
-            let mut temp_buf = ReadBuf::new(&mut temp[0..HEADER_LEN - *this.header_read]);
-            match this.inner.as_mut().poll_read(cx, &mut temp_buf) {
-                Poll::Pending => return Poll::Pending,
-                Poll::Ready(Ok(())) => {
-                    let n = temp_buf.filled().len();
-                    if n == 0 {
-                        break;
+        if *this.poisoned {
+            *this.poisoned = true;
+            return Poll::Ready(Err(io::Error::new(io::ErrorKind::InvalidData, "decompress reader previously failed")));
+        }
+
+        if *this.compressed_len == 0 {
+            // Read the 8-byte block header, resuming across polls via `header_read`.
+            while *this.header_read < HEADER_LEN {
+                let mut temp = [0u8; HEADER_LEN];
+                let mut temp_buf = ReadBuf::new(&mut temp[0..HEADER_LEN - *this.header_read]);
+                match this.inner.as_mut().poll_read(cx, &mut temp_buf) {
+                    Poll::Pending => return Poll::Pending,
+                    Poll::Ready(Ok(())) => {
+                        let n = temp_buf.filled().len();
+                        if n == 0 {
+                            if *this.header_read == 0 {
+                                // Clean EOF on a block boundary.
+                                *this.finished = true;
+                                return Poll::Ready(Ok(()));
+                            }
+                            *this.poisoned = true;
+                            return Poll::Ready(Err(io::Error::new(
+                                io::ErrorKind::UnexpectedEof,
+                                "unexpected EOF while reading compressed block header",
+                            )));
+                        }
+                        this.header_buf[*this.header_read..*this.header_read + n].copy_from_slice(&temp_buf.filled()[..n]);
+                        *this.header_read += n;
+                    }
+                    Poll::Ready(Err(e)) => {
+                        // error!("DecompressReader poll_read: read header error: {e}");
+                        *this.poisoned = true;
+                        return Poll::Ready(Err(e));
                     }
-                    this.header_buf[*this.header_read..*this.header_read + n].copy_from_slice(&temp_buf.filled()[..n]);
-                    *this.header_read += n;
-                }
-                Poll::Ready(Err(e)) => {
-                    // error!("DecompressReader poll_read: read header error: {e}");
-                    return Poll::Ready(Err(e));
                 }
             }
-            if *this.header_read < HEADER_LEN {
-                return Poll::Pending;
-            }
-        }
-        if !*this.header_done && *this.header_read == 0 {
-            return Poll::Ready(Ok(()));
-        }
-        let typ = this.header_buf[0];
-        let len = (this.header_buf[1] as usize) | ((this.header_buf[2] as usize) << 8) | ((this.header_buf[3] as usize) << 16);
-        let crc = (this.header_buf[4] as u32)
-            | ((this.header_buf[5] as u32) << 8)
-            | ((this.header_buf[6] as u32) << 16)
-            | ((this.header_buf[7] as u32) << 24);
-        *this.header_read = 0;
-        *this.header_done = true;
 
-        if typ == COMPRESS_TYPE_END {
+            let typ = this.header_buf[0];
+            let len =
+                (this.header_buf[1] as usize) | ((this.header_buf[2] as usize) << 8) | ((this.header_buf[3] as usize) << 16);
+            *this.header_read = 0;
+
+            if typ == COMPRESS_TYPE_END {
+                *this.compressed_read = 0;
+                *this.compressed_len = 0;
+                *this.finished = true;
+                return Poll::Ready(Ok(()));
+            }
+            if typ != COMPRESS_TYPE_COMPRESSED && typ != COMPRESS_TYPE_UNCOMPRESSED {
+                // error!("DecompressReader unknown compression type: {typ}");
+                *this.poisoned = true;
+                return Poll::Ready(Err(io::Error::new(io::ErrorKind::InvalidData, "Unknown compression type")));
+            }
+            if len == 0 {
+                *this.poisoned = true;
+                return Poll::Ready(Err(io::Error::new(io::ErrorKind::InvalidData, "Invalid compressed block length")));
+            }
+
+            if this.compressed_buf.len() < len {
+                this.compressed_buf.resize(len, 0);
+            }
+            *this.compressed_len = len;
             *this.compressed_read = 0;
-            *this.compressed_len = 0;
-            *this.finished = true;
-            return Poll::Ready(Ok(()));
         }
 
-        if this.compressed_buf.len() < len {
-            this.compressed_buf.resize(len, 0);
-        }
-        *this.compressed_len = len;
-        *this.compressed_read = 0;
-
+        // Fill the in-flight block payload, resuming across polls via `compressed_read`.
         while *this.compressed_read < *this.compressed_len {
             let mut temp_buf = ReadBuf::new(&mut this.compressed_buf[*this.compressed_read..*this.compressed_len]);
             match this.inner.as_mut().poll_read(cx, &mut temp_buf) {
@@ -291,7 +320,13 @@ where
                 Poll::Ready(Ok(())) => {
                     let n = temp_buf.filled().len();
                     if n == 0 {
-                        break;
+                        *this.compressed_read = 0;
+                        *this.compressed_len = 0;
+                        *this.poisoned = true;
+                        return Poll::Ready(Err(io::Error::new(
+                            io::ErrorKind::UnexpectedEof,
+                            "unexpected EOF while reading compressed block payload",
+                        )));
                     }
                     *this.compressed_read += n;
                 }
@@ -299,10 +334,17 @@ where
                     // error!("DecompressReader poll_read: read compressed block error: {e}");
                     *this.compressed_read = 0;
                     *this.compressed_len = 0;
+                    *this.poisoned = true;
                     return Poll::Ready(Err(e));
                 }
             }
         }
+
+        let typ = this.header_buf[0];
+        let crc = (this.header_buf[4] as u32)
+            | ((this.header_buf[5] as u32) << 8)
+            | ((this.header_buf[6] as u32) << 16)
+            | ((this.header_buf[7] as u32) << 24);
         let compressed_buf = &this.compressed_buf[..*this.compressed_len];
         // `compressed_buf`'s length comes from the untrusted 24-bit header length field, so it
         // can be shorter than 16 bytes. `uvarint` is safe on any slice length (reads at most 10
@@ -316,6 +358,7 @@ where
         if uvarint <= 0 || uvarint as usize > compressed_buf.len() {
             *this.compressed_read = 0;
             *this.compressed_len = 0;
+            *this.poisoned = true;
             return Poll::Ready(Err(io::Error::new(io::ErrorKind::InvalidData, "Invalid compressed block length prefix")));
         }
         let compressed_data = &compressed_buf[uvarint as usize..];
@@ -326,21 +369,29 @@ where
                     // error!("DecompressReader decompress_block error: {e}");
                     *this.compressed_read = 0;
                     *this.compressed_len = 0;
+                    *this.poisoned = true;
                     return Poll::Ready(Err(e));
                 }
             }
-        } else if typ == COMPRESS_TYPE_UNCOMPRESSED {
-            compressed_data.to_vec()
         } else {
-            // error!("DecompressReader unknown compression type: {typ}");
+            // The header phase already rejected every type other than
+            // COMPRESS_TYPE_COMPRESSED / COMPRESS_TYPE_UNCOMPRESSED.
+            compressed_data.to_vec()
+        };
+        if decompressed.is_empty() {
+            // The writer never emits zero-length plaintext blocks; an empty
+            // decode surfacing as Ready(Ok) with no bytes would read as EOF and
+            // silently truncate the stream.
+            *this.poisoned = true;
             *this.compressed_read = 0;
             *this.compressed_len = 0;
-            return Poll::Ready(Err(io::Error::new(io::ErrorKind::InvalidData, "Unknown compression type")));
-        };
+            return Poll::Ready(Err(io::Error::new(io::ErrorKind::InvalidData, "Empty compressed block")));
+        }
         if decompressed.len() != uncompress_len as usize {
             // error!("DecompressReader decompressed length mismatch: {} != {}", decompressed.len(), uncompress_len);
             *this.compressed_read = 0;
             *this.compressed_len = 0;
+            *this.poisoned = true;
             return Poll::Ready(Err(io::Error::new(io::ErrorKind::InvalidData, "Decompressed length mismatch")));
         }
         let actual_crc = {
@@ -352,13 +403,13 @@ where
             // error!("DecompressReader CRC32 mismatch: actual {actual_crc} != expected {crc}");
             *this.compressed_read = 0;
             *this.compressed_len = 0;
+            *this.poisoned = true;
             return Poll::Ready(Err(io::Error::new(io::ErrorKind::InvalidData, "CRC32 mismatch")));
         }
         *this.buffer = decompressed;
         *this.buffer_pos = 0;
         *this.compressed_read = 0;
         *this.compressed_len = 0;
-        *this.header_done = false;
         let to_copy = min(buf.remaining(), this.buffer.len());
         buf.put_slice(&this.buffer[..to_copy]);
         *this.buffer_pos += to_copy;
@@ -493,6 +544,184 @@ mod tests {
         assert_eq!(&decompressed, &data);
     }
 
+    /// Wraps a reader so every other poll returns `Poll::Pending` and every
+    /// `Ready` poll serves at most `chunk` bytes. This is the shape a duplex
+    /// pipe produces when the erasure writer is slower than the decoder, which
+    /// is exactly what desynchronized the block framing before the resumable
+    /// payload state was added (rustfs/rustfs#5957 multipart GET truncation).
+    struct PendingChunkReader<R> {
+        inner: R,
+        chunk: usize,
+        pending_next: bool,
+    }
+
+    impl<R> PendingChunkReader<R> {
+        fn new(inner: R, chunk: usize) -> Self {
+            Self {
+                inner,
+                chunk,
+                pending_next: true,
+            }
+        }
+    }
+
+    impl<R: AsyncRead + Unpin> AsyncRead for PendingChunkReader<R> {
+        fn poll_read(
+            mut self: std::pin::Pin<&mut Self>,
+            cx: &mut std::task::Context<'_>,
+            buf: &mut tokio::io::ReadBuf<'_>,
+        ) -> std::task::Poll<std::io::Result<()>> {
+            if self.pending_next {
+                self.pending_next = false;
+                cx.waker().wake_by_ref();
+                return std::task::Poll::Pending;
+            }
+            self.pending_next = true;
+            let cap = self.chunk.min(buf.remaining());
+            let mut scratch = vec![0u8; cap];
+            let mut inner_buf = tokio::io::ReadBuf::new(&mut scratch);
+            match std::pin::Pin::new(&mut self.inner).poll_read(cx, &mut inner_buf) {
+                std::task::Poll::Ready(Ok(())) => {
+                    buf.put_slice(inner_buf.filled());
+                    std::task::Poll::Ready(Ok(()))
+                }
+                other => other,
+            }
+        }
+    }
+
+    fn patterned_payload(size: usize, seed: u8) -> Vec<u8> {
+        (0..size)
+            .map(|i| ((i as u64).wrapping_mul(2_654_435_761).wrapping_add(seed as u64) >> 3) as u8)
+            .collect()
+    }
+
+    /// Root-cause regression for the multipart compressed GET truncation: a
+    /// `Poll::Pending` in the middle of a block payload must not drop the bytes
+    /// already consumed. Before the resumable payload state, the decoder reset
+    /// `compressed_read` on every re-poll and surfaced
+    /// `LZ4 error: ERROR_frameType_unknown` mid-stream.
+    #[tokio::test]
+    async fn test_decompress_reader_survives_pending_mid_payload() {
+        let data = patterned_payload(100 * 1024, 7);
+        let mut compress_reader =
+            CompressReader::with_block_size(Cursor::new(data.clone()), 8192, CompressionAlgorithm::default());
+        let mut compressed = Vec::new();
+        compress_reader.read_to_end(&mut compressed).await.unwrap();
+
+        for chunk in [1usize, 3, 7, 8, 17, 1000, 8192] {
+            let inner = PendingChunkReader::new(Cursor::new(compressed.clone()), chunk);
+            let mut decompress_reader = DecompressReader::new(inner, CompressionAlgorithm::default());
+            let mut decompressed = Vec::new();
+            decompress_reader.read_to_end(&mut decompressed).await.unwrap();
+            assert_eq!(decompressed, data, "pending-chunked decode must be byte-exact for chunk={chunk}");
+        }
+    }
+
+    /// Two independently compressed streams concatenated back to back — the
+    /// on-disk shape of a compressed multipart object — must decode across the
+    /// stream boundary even when every poll can suspend mid-block.
+    #[tokio::test]
+    async fn test_decompress_reader_survives_pending_across_concatenated_streams() {
+        let part1 = patterned_payload(64 * 1024, 7);
+        let part2 = patterned_payload(24 * 1024, 61);
+
+        let mut stored = Vec::new();
+        for part in [&part1, &part2] {
+            let mut compress_reader =
+                CompressReader::with_block_size(Cursor::new(part.clone()), 8192, CompressionAlgorithm::default());
+            let mut compressed = Vec::new();
+            compress_reader.read_to_end(&mut compressed).await.unwrap();
+            stored.extend_from_slice(&compressed);
+        }
+
+        let mut expected = part1;
+        expected.extend_from_slice(&part2);
+
+        for chunk in [1usize, 5, 8, 13, 4096] {
+            let inner = PendingChunkReader::new(Cursor::new(stored.clone()), chunk);
+            let mut decompress_reader = DecompressReader::new(inner, CompressionAlgorithm::default());
+            let mut decompressed = Vec::new();
+            decompress_reader.read_to_end(&mut decompressed).await.unwrap();
+            assert_eq!(
+                decompressed, expected,
+                "concatenated part streams must decode byte-exact for chunk={chunk}"
+            );
+        }
+    }
+
+    /// After the first stream error, every further poll must keep failing.
+    /// Without the sticky poison a retrying caller would restart at the header
+    /// phase and read the truncated tail as a clean EOF — converting a hard
+    /// error into a silently short body.
+    #[tokio::test]
+    async fn test_decompress_reader_error_is_sticky() {
+        let data = patterned_payload(32 * 1024, 7);
+        let mut compress_reader = CompressReader::with_block_size(Cursor::new(data), 8192, CompressionAlgorithm::default());
+        let mut compressed = Vec::new();
+        compress_reader.read_to_end(&mut compressed).await.unwrap();
+        compressed.truncate(compressed.len() - 3);
+
+        let mut decompress_reader = DecompressReader::new(Cursor::new(compressed), CompressionAlgorithm::default());
+        let mut out = Vec::new();
+        let first = decompress_reader
+            .read_to_end(&mut out)
+            .await
+            .expect_err("truncated payload must error");
+        assert_eq!(first.kind(), std::io::ErrorKind::UnexpectedEof);
+
+        let mut retry = Vec::new();
+        let second = decompress_reader
+            .read_to_end(&mut retry)
+            .await
+            .expect_err("a poll after the first error must not turn into a clean EOF");
+        assert_eq!(second.kind(), std::io::ErrorKind::InvalidData);
+        assert!(retry.is_empty(), "no bytes may be produced after the stream failed");
+    }
+
+    /// A stream cut off in the middle of a block payload must fail with a clean
+    /// UnexpectedEof instead of decoding a short buffer.
+    #[tokio::test]
+    async fn test_decompress_reader_truncated_payload_is_unexpected_eof() {
+        let data = patterned_payload(32 * 1024, 7);
+        let mut compress_reader = CompressReader::with_block_size(Cursor::new(data), 8192, CompressionAlgorithm::default());
+        let mut compressed = Vec::new();
+        compress_reader.read_to_end(&mut compressed).await.unwrap();
+
+        compressed.truncate(compressed.len() - 3);
+        let mut decompress_reader = DecompressReader::new(Cursor::new(compressed), CompressionAlgorithm::default());
+        let mut out = Vec::new();
+        let err = decompress_reader
+            .read_to_end(&mut out)
+            .await
+            .expect_err("truncated payload must error");
+        assert_eq!(err.kind(), std::io::ErrorKind::UnexpectedEof);
+    }
+
+    /// A stream cut off in the middle of a block header must fail with a clean
+    /// UnexpectedEof instead of parsing a garbage header.
+    #[tokio::test]
+    async fn test_decompress_reader_truncated_header_is_unexpected_eof() {
+        let data = patterned_payload(12 * 1024, 7);
+        let mut compress_reader = CompressReader::with_block_size(Cursor::new(data), 8192, CompressionAlgorithm::default());
+        let mut compressed = Vec::new();
+        compress_reader.read_to_end(&mut compressed).await.unwrap();
+
+        // Keep the first full block plus 3 bytes of the next header.
+        let ln = (compressed[1] as usize) | ((compressed[2] as usize) << 8) | ((compressed[3] as usize) << 16);
+        let first_block_end = 8 + ln;
+        assert!(compressed.len() > first_block_end, "fixture must contain more than one block");
+        compressed.truncate(first_block_end + 3);
+
+        let mut decompress_reader = DecompressReader::new(Cursor::new(compressed), CompressionAlgorithm::default());
+        let mut out = Vec::new();
+        let err = decompress_reader
+            .read_to_end(&mut out)
+            .await
+            .expect_err("truncated header must error");
+        assert_eq!(err.kind(), std::io::ErrorKind::UnexpectedEof);
+    }
+
     // Regression: a corrupted block whose 24-bit length field is < 16 must not panic.
     // Header layout (HEADER_LEN = 8): [type, len_lo, len_mid, len_hi, crc0..crc3], then `len`
     // bytes of block body. Pre-fix, poll_read sliced `compressed_buf[0..16]` unconditionally,
@@ -518,6 +747,85 @@ mod tests {
         assert_eq!(res.unwrap_err().kind(), std::io::ErrorKind::InvalidData);
     }
 
+    // Header-level fail-closed matrix, built by hand so the decoder is exercised against bytes no
+    // encoder in this crate can produce. Header layout (HEADER_LEN = 8):
+    // [type, len_lo, len_mid, len_hi, crc0..crc3], then `len` body bytes = uvarint(plain_len) + data.
+    #[tokio::test]
+    async fn test_decompress_reader_header_validation_matrix() {
+        // Build a block whose body is `uvarint(plain.len()) + plain` (i.e. the
+        // COMPRESS_TYPE_UNCOMPRESSED shape), with the header CRC taken over the plaintext exactly
+        // like the production writer does.
+        fn build_raw_block(typ: u8, plain: &[u8], len_override: Option<usize>) -> Vec<u8> {
+            let crc = {
+                let mut hasher = crc_fast::Digest::new(crc_fast::CrcAlgorithm::Crc32IsoHdlc);
+                hasher.update(plain);
+                hasher.finalize() as u32
+            };
+            let mut uvarint_buf = [0u8; 10];
+            let int_len = put_uvarint(&mut uvarint_buf[..], plain.len() as u64);
+            let body_len = int_len + plain.len();
+            let len = len_override.unwrap_or(body_len);
+
+            let mut out = Vec::with_capacity(HEADER_LEN + body_len);
+            out.push(typ);
+            out.push((len & 0xFF) as u8);
+            out.push(((len >> 8) & 0xFF) as u8);
+            out.push(((len >> 16) & 0xFF) as u8);
+            out.extend_from_slice(&crc.to_le_bytes());
+            out.extend_from_slice(&uvarint_buf[..int_len]);
+            out.extend_from_slice(plain);
+            out
+        }
+
+        let plain = b"uncompressed passthrough payload";
+
+        // (a) A well-formed uncompressed block decodes to the plaintext verbatim.
+        let mut out = Vec::new();
+        DecompressReader::new(
+            Cursor::new(build_raw_block(COMPRESS_TYPE_UNCOMPRESSED, plain, None)),
+            CompressionAlgorithm::default(),
+        )
+        .read_to_end(&mut out)
+        .await
+        .expect("a well-formed uncompressed block must decode");
+        assert_eq!(out.as_slice(), plain.as_slice());
+
+        // (b) An unknown block type must be rejected instead of being treated as passthrough.
+        let mut out = Vec::new();
+        let err = DecompressReader::new(Cursor::new(build_raw_block(0x7E, plain, None)), CompressionAlgorithm::default())
+            .read_to_end(&mut out)
+            .await
+            .expect_err("unknown compression type must error");
+        assert_eq!(err.kind(), std::io::ErrorKind::InvalidData);
+        assert!(err.to_string().contains("Unknown compression type"), "got: {err}");
+
+        // (c) A zero-length block would stall the decoder, so it must be rejected up front.
+        let mut out = Vec::new();
+        let err = DecompressReader::new(
+            Cursor::new(build_raw_block(COMPRESS_TYPE_UNCOMPRESSED, plain, Some(0))),
+            CompressionAlgorithm::default(),
+        )
+        .read_to_end(&mut out)
+        .await
+        .expect_err("zero-length block must error");
+        assert_eq!(err.kind(), std::io::ErrorKind::InvalidData);
+        assert!(err.to_string().contains("Invalid compressed block length"), "got: {err}");
+
+        // (d) A block that decodes to zero plaintext bytes must be rejected: the
+        // writer never emits empty blocks, and an empty decode surfacing as
+        // Ready(Ok) with no bytes would read as EOF and silently truncate.
+        let mut out = Vec::new();
+        let err = DecompressReader::new(
+            Cursor::new(build_raw_block(COMPRESS_TYPE_UNCOMPRESSED, b"", None)),
+            CompressionAlgorithm::default(),
+        )
+        .read_to_end(&mut out)
+        .await
+        .expect_err("empty block must error");
+        assert_eq!(err.kind(), std::io::ErrorKind::InvalidData);
+        assert!(err.to_string().contains("Empty compressed block"), "got: {err}");
+    }
+
     // Directly exercises the length-prefix guard: an unterminated varint (all continuation bytes)
     // makes `uvarint` return 0, which must be rejected as an invalid length prefix.
     #[tokio::test]

rustfs/src/app/multipart_usecase.rs

@@ -27,6 +27,7 @@ use super::storage_api::multipart_usecase::bucket::{
     replication::{must_replicate_object, schedule_object_replication},
     versioning_sys::BucketVersioningSys,
 };
+use super::storage_api::multipart_usecase::compression::is_disk_compressible;
 #[cfg(test)]
 use super::storage_api::multipart_usecase::contract::http::HTTPPreconditions;
 use super::storage_api::multipart_usecase::contract::multipart::{CompletePart, MultipartOperations as _, MultipartUploadResult};
@@ -39,7 +40,7 @@ use super::storage_api::multipart_usecase::error::{StorageError, is_err_object_n
 use super::storage_api::multipart_usecase::helper::OperationHelper;
 #[cfg(test)]
 use super::storage_api::multipart_usecase::io::{DecryptReader, EncryptReader, HardLimitReader, boxed_reader, wrap_reader};
-use super::storage_api::multipart_usecase::io::{HashReader, WriteEncryption, WritePlan};
+use super::storage_api::multipart_usecase::io::{HashReader, WriteEncryption, WritePlan, compression_metadata_value};
 use super::storage_api::multipart_usecase::object_utils::to_s3s_etag;
 use super::storage_api::multipart_usecase::options::{
     copy_src_opts, extract_metadata_from_mime, get_complete_multipart_upload_opts_with_replication_authorization,
@@ -210,6 +211,21 @@ fn create_multipart_upload_metadata(
     metadata
 }
 
+/// A multipart session advertises disk compression only when the object key/headers
+/// qualify AND the session is not an SSE-C ciphertext-passthrough replication session,
+/// which must preserve source bytes verbatim.
+///
+/// Each part is compressed as an independent stream; the GET path decodes across part
+/// boundaries (see `ReadTransform::Compressed`), so the session may advertise
+/// object-level compression again.
+///
+/// Unlike single PUT there is no `MIN_DISK_COMPRESSIBLE_SIZE` floor here: the total
+/// object size is unknown at CreateMultipartUpload time, so tiny multipart objects pay
+/// the (harmless) framing overhead. This is a deliberate trade-off, not a bug.
+fn should_advertise_session_compression(ciphertext_passthrough: bool, disk_compressible: bool) -> bool {
+    !ciphertext_passthrough && disk_compressible
+}
+
 async fn validate_table_catalog_object_mutation(bucket: &str, key: &str) -> S3Result<()> {
     table_catalog::validate_bucket_object_mutation(bucket, key)
         .await
@@ -837,8 +853,13 @@ impl DefaultMultipartUsecase {
             None => (None, None),
         };
 
-        // Multipart parts are independent physical streams. Advertising object-level
-        // compression here would make GET decode the completed object as one stream.
+        if should_advertise_session_compression(ciphertext_passthrough, is_disk_compressible(&req.headers, &key)) {
+            rustfs_utils::http::insert_str(
+                &mut metadata,
+                rustfs_utils::http::SUFFIX_COMPRESSION,
+                compression_metadata_value(CompressionAlgorithm::default()),
+            );
+        }
 
         let mt2 = metadata.clone();
         let mut opts: ObjectOptions =
@@ -1632,6 +1653,25 @@ mod tests {
         DefaultMultipartUsecase::without_context()
     }
 
+    #[test]
+    fn session_compression_is_advertised_only_for_non_passthrough_compressible_uploads() {
+        // (ciphertext_passthrough, disk_compressible, expected)
+        let cases = [
+            (false, false, false),
+            (false, true, true),
+            (true, false, false),
+            (true, true, false),
+        ];
+
+        for (ciphertext_passthrough, disk_compressible, expected) in cases {
+            assert_eq!(
+                should_advertise_session_compression(ciphertext_passthrough, disk_compressible),
+                expected,
+                "ciphertext_passthrough={ciphertext_passthrough} disk_compressible={disk_compressible}"
+            );
+        }
+    }
+
     #[test]
     fn quota_accounting_uses_logical_size_when_available() {
         let mut metadata = HashMap::new();

rustfs/src/app/storage_api.rs

@@ -1153,7 +1153,8 @@ pub(crate) mod multipart_usecase {
     }
 
     pub(crate) use super::{
-        access, bucket, data_usage, error, helper, io, object_utils, options, request_context, s3_api, set_disk, sse,
+        access, bucket, compression, data_usage, error, helper, io, object_utils, options, request_context, s3_api,
+        set_disk, sse,
     };
     pub(crate) use crate::storage::storage_api::{ECStore, StorageObjectInfo, StorageObjectOptions, StoragePutObjReader};
 }

rustfs/src/storage/s3_api/multipart.rs

@@ -51,7 +51,14 @@ pub(crate) fn build_list_parts_output(res: ListPartsInfo) -> ListPartsOutput {
                     e_tag: p.etag.map(|etag| to_s3s_etag(&etag)),
                     last_modified: p.last_mod.map(Timestamp::from),
                     part_number: p.part_num.try_into().ok(),
-                    size: p.size.try_into().ok(),
+                    // Compressed parts store fewer bytes than the client sent; S3
+                    // semantics report the uploaded (logical) size, matching
+                    // GetObjectAttributes ObjectParts.
+                    size: if p.actual_size > 0 {
+                        Some(p.actual_size)
+                    } else {
+                        p.size.try_into().ok()
+                    },
                     ..Default::default()
                 })
                 .collect(),
@@ -247,6 +254,63 @@ mod tests {
         assert_eq!(output.initiator, Some(rustfs_initiator()));
     }
 
+    #[test]
+    fn test_list_parts_output_reports_logical_size_for_compressed_parts() {
+        let input = ListPartsInfo {
+            bucket: "bucket-a".to_string(),
+            object: "obj-a".to_string(),
+            upload_id: "upload-a".to_string(),
+            parts: vec![PartInfo {
+                part_num: 1,
+                // Stored (compressed) bytes on disk vs. the logical size the client uploaded.
+                size: 1_024,
+                actual_size: 8_388_608,
+                ..Default::default()
+            }],
+            ..Default::default()
+        };
+
+        let output = build_list_parts_output(input);
+        let parts = output.parts.as_ref().expect("parts should be present");
+
+        assert_eq!(parts.len(), 1);
+        assert_eq!(
+            parts[0].size,
+            Some(8_388_608),
+            "compressed parts must report the uploaded logical size, not the stored size"
+        );
+    }
+
+    #[test]
+    fn test_list_parts_output_falls_back_to_stored_size_when_actual_size_unknown() {
+        let input = ListPartsInfo {
+            parts: vec![
+                PartInfo {
+                    part_num: 1,
+                    size: 1_024,
+                    // Uncompressed parts leave actual_size unset.
+                    actual_size: 0,
+                    ..Default::default()
+                },
+                PartInfo {
+                    part_num: 2,
+                    size: 1_024,
+                    // Legacy/unknown sentinel must not leak a negative size to clients.
+                    actual_size: -1,
+                    ..Default::default()
+                },
+            ],
+            ..Default::default()
+        };
+
+        let output = build_list_parts_output(input);
+        let parts = output.parts.as_ref().expect("parts should be present");
+
+        assert_eq!(parts.len(), 2);
+        assert_eq!(parts[0].size, Some(1024));
+        assert_eq!(parts[1].size, Some(1024));
+    }
+
     #[test]
     fn test_list_parts_output_normalizes_legacy_storage_class_and_handles_overflow_markers() {
         let input = ListPartsInfo {