feat(storage): stage multipart compression behind RUSTFS_COMPRESSION_MULTIPART_ENABLED

Review follow-up: a rolling-upgrade window must not create new compressed multipart objects while pre-fix nodes (whose decompressor is not resumable) may still serve reads. The session marker is now additionally gated on RUSTFS_COMPRESSION_MULTIPART_ENABLED, default off, so the restored capability stays dark until the operator confirms fleet convergence. The default flips per the multipart-compression-default-off-window entry in docs/architecture/compat-cleanup-register.md once the minimum supported direct-upgrade release ships the resumable decoder.

crates/ecstore/src/api/mod.rs

@@ -276,7 +276,9 @@ pub mod cluster {
 }
 
 pub mod compression {
-    pub use crate::io_support::compress::{MIN_DISK_COMPRESSIBLE_SIZE, is_disk_compressible, is_disk_compression_enabled};
+    pub use crate::io_support::compress::{
+        MIN_DISK_COMPRESSIBLE_SIZE, is_disk_compressible, is_disk_compression_enabled, is_multipart_disk_compression_enabled,
+    };
 }
 
 pub mod config {

crates/ecstore/src/io_support/compress.rs

@@ -31,6 +31,13 @@ pub const ENV_DISK_COMPRESSION_MIME_TYPES: &str = "RUSTFS_COMPRESSION_MIME_TYPES
 // Environment variable for additional extensions to exclude from compression (comma-separated, e.g. ".foo,.bar")
 pub const ENV_ADDED_EXCLUDE_COMPRESS_EXTENSIONS: &str = "RUSTFS_ADDED_EXCLUDE_COMPRESS_EXTENSIONS";
 
+// Environment variable to additionally enable disk compression for multipart uploads.
+// Default off: nodes from before the resumable decompressor fix fail transient reads of
+// compressed objects, so multipart compression stays dark until the operator confirms the
+// fleet has converged on a fixed build (see docs/architecture/compat-cleanup-register.md,
+// `multipart-compression-default-off-window`, for the default-flip condition).
+pub const ENV_DISK_COMPRESSION_MULTIPART_ENABLED: &str = "RUSTFS_COMPRESSION_MULTIPART_ENABLED";
+
 pub const DEFAULT_DISK_COMPRESS_EXTENSIONS: &str = ".txt,.log,.csv,.json,.tar,.xml,.bin";
 pub const DEFAULT_DISK_COMPRESS_MIME_TYPES: &str = "text/*,application/json,application/xml,binary/octet-stream";
 
@@ -171,6 +178,21 @@ pub fn is_disk_compression_enabled() -> bool {
     DISK_COMPRESSION_CONFIG.get_or_init(parse_disk_compression_config).enabled
 }
 
+// Parsed once at first use, mirroring DISK_COMPRESSION_CONFIG.
+static MULTIPART_DISK_COMPRESSION_ENABLED: OnceLock<bool> = OnceLock::new();
+
+/// Whether multipart uploads may advertise disk compression. Requires the
+/// regular disk-compression gates to pass as well; this is the staged-rollout
+/// switch that keeps multipart compression dark during rolling upgrades from
+/// builds whose decompressor was not yet resumable.
+pub fn is_multipart_disk_compression_enabled() -> bool {
+    *MULTIPART_DISK_COMPRESSION_ENABLED.get_or_init(|| {
+        env::var(ENV_DISK_COMPRESSION_MULTIPART_ENABLED)
+            .map(|s| matches!(s.to_ascii_lowercase().as_str(), "true" | "on" | "1"))
+            .unwrap_or(false)
+    })
+}
+
 fn is_disk_compressible_with_config(headers: &http::HeaderMap, object_name: &str, config: &DiskCompressionConfig) -> bool {
     // Check if disk compression is enabled (read once at first use, then fixed for process lifetime)
     if !config.enabled {