fix(storage): queue multipart parts for foreground write permits (#7337)

Multipart parts shared the 250 ms direct-PutObject wait on the foreground
write permit pool, so SDK-default concurrency (many parts per upload in
flight at once) was rejected wholesale with SlowDown at stock settings.
Keep the pool that bounds in-flight bodies, but let parts wait in a
bounded queue with their own timeout before body ingest, report the
queue depth in the ForegroundWrite admission snapshot, and document the
foreground write admission environment variables.

crates/config/README.md

@@ -130,6 +130,37 @@ Scanner cycle budget controls:
   - timeout returns S3 `SlowDown`, so clients should use normal SDK retry handling.
   - this is not a fdatasync or group-commit switch. Track fdatasync batching separately with `rustfs_s3_put_object_rename_fdatasync_batch_files`.
 
+## Foreground write admission environment variables
+
+Large direct `PutObject` requests and multipart `UploadPart` requests share one
+per-process permit pool that bounds how many bodies are ingested and written
+concurrently. Small direct PUTs stay on the legacy path.
+
+- `RUSTFS_PUT_LARGE_FOREGROUND_ADMISSION_ENABLE`
+  - enables the default-on pool; `false` keeps only the soft request counter.
+  - default is `true`.
+- `RUSTFS_PUT_LARGE_FOREGROUND_ADMISSION_LIMIT`
+  - permits in the pool; `0` derives half of `RUSTFS_OBJECT_MAX_CONCURRENT_DISK_READS`, clamped to `32`.
+  - default is `0` (32 permits at stock settings).
+- `RUSTFS_PUT_LARGE_FOREGROUND_ADMISSION_MIN_SIZE_BYTES`
+  - smallest direct `PutObject` that takes a permit; unknown-size requests always do.
+  - default is `33554432` (32 MiB).
+- `RUSTFS_PUT_LARGE_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS`
+  - how long a direct `PutObject` waits for a permit before returning S3 `SlowDown`.
+  - default is `250`.
+- `RUSTFS_PUT_MULTIPART_FOREGROUND_ADMISSION_MIN_SIZE_BYTES`
+  - smallest `UploadPart` that takes a permit; `0` gates every part.
+  - default is `0`.
+- `RUSTFS_PUT_MULTIPART_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS`
+  - how long an `UploadPart` waits in the bounded queue for a permit before returning S3 `SlowDown`; `0` rejects immediately when the pool is full.
+  - default is `30000`. Parts wait before body ingest, so SDK-default clients that send every part of an upload concurrently drain through the pool instead of failing.
+- `RUSTFS_PUT_MULTIPART_FOREGROUND_ADMISSION_MAX_PENDING`
+  - maximum `UploadPart` requests waiting for a permit at once; parts beyond it return `SlowDown` without waiting.
+  - default is `0`, which derives 16 times the permit limit (512 at stock settings).
+- `RUSTFS_PUT_FOREGROUND_ADMISSION_ENABLE`, `RUSTFS_PUT_FOREGROUND_ADMISSION_LIMIT`, `RUSTFS_PUT_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS`
+  - experimental strict gate that applies to every foreground write regardless of size and replaces the pool above when enabled.
+  - default is disabled; enabling it with limit `0` disables foreground write admission entirely.
+
 ## Remote tier timeout environment variables
 
 - `RUSTFS_TIER_REMOTE_CONNECT_TIMEOUT_SECS`

crates/config/src/constants/object.rs

@@ -365,13 +365,36 @@ pub const ENV_PUT_MULTIPART_FOREGROUND_ADMISSION_MIN_SIZE_BYTES: &str =
     "RUSTFS_PUT_MULTIPART_FOREGROUND_ADMISSION_MIN_SIZE_BYTES";
 pub const DEFAULT_PUT_MULTIPART_FOREGROUND_ADMISSION_MIN_SIZE_BYTES: usize = 0;
 
-/// Time in milliseconds an automatic foreground write waits for a permit.
+/// Time in milliseconds an automatic foreground direct PutObject waits for a permit.
 ///
 /// A short wait smooths transient bursts while still returning S3
 /// `SlowDown`/503 before body ingest when the node is already saturated.
 pub const ENV_PUT_LARGE_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS: &str = "RUSTFS_PUT_LARGE_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS";
 pub const DEFAULT_PUT_LARGE_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS: u64 = 250;
 
+/// Time in milliseconds a multipart UploadPart waits for a foreground write permit.
+///
+/// SDK-default multipart clients send every part of an upload concurrently, so
+/// a single node routinely sees several times more parts in flight than the
+/// permit pool allows. Those parts have not ingested a body yet, so queueing
+/// them costs a connection rather than memory or internode streams; the pool
+/// still bounds the number of parts being written. The wait is long enough for
+/// an ordinary queue to drain on modest hardware, and a part that cannot get a
+/// permit within it fails with S3 `SlowDown`/503 for the client to retry.
+/// `0` rejects immediately when the pool is full.
+pub const ENV_PUT_MULTIPART_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS: &str =
+    "RUSTFS_PUT_MULTIPART_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS";
+pub const DEFAULT_PUT_MULTIPART_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS: u64 = 30_000;
+
+/// Maximum multipart UploadPart requests waiting for a foreground write permit per process.
+///
+/// Parts beyond this queue depth are rejected with S3 `SlowDown`/503 without
+/// waiting, so a genuinely saturated node still fails fast instead of holding
+/// an unbounded set of connections open for the whole wait timeout.
+/// `0` derives the depth from the permit limit.
+pub const ENV_PUT_MULTIPART_FOREGROUND_ADMISSION_MAX_PENDING: &str = "RUSTFS_PUT_MULTIPART_FOREGROUND_ADMISSION_MAX_PENDING";
+pub const DEFAULT_PUT_MULTIPART_FOREGROUND_ADMISSION_MAX_PENDING: usize = 0;
+
 const _: () = assert!(DEFAULT_PUT_LARGE_FOREGROUND_ADMISSION_ENABLE);
 
 /// Environment variable for minimum GetObject timeout in seconds.