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.

docs/architecture/workload-admission-contracts.md

@@ -12,7 +12,7 @@
 | Class | Provider (`impl WorkloadAdmissionSnapshotProvider`) | `active` / `queued` / `limit` source | Reports `Unknown` when |
 |---|---|---|---|
 | `ForegroundRead` | `ConcurrencyManager` in `rustfs/src/storage/concurrency/manager.rs` (source of truth); re-exposed unchanged by the RustFS runtime provider | disk-read permits in use / `None` (the semaphore exposes no waiter count) / configured max concurrent disk reads | the storage registry has no entry |
-| `ForegroundWrite` | `ConcurrencyManager` in `rustfs/src/storage/concurrency/manager.rs` (source of truth); re-exposed unchanged by the RustFS runtime provider | foreground-write permits in use or legacy active-write counter / `None` / configured or derived write-admission limit | the storage registry has no entry |
+| `ForegroundWrite` | `ConcurrencyManager` in `rustfs/src/storage/concurrency/manager.rs` (source of truth); re-exposed unchanged by the RustFS runtime provider | foreground-write permits in use or legacy active-write counter / multipart parts waiting in the bounded admission queue (`None` for the strict and legacy policies) / configured or derived write-admission limit | the storage registry has no entry |
 | `Metadata` | `RustFsWorkloadAdmissionSnapshotProvider` in `rustfs/src/workload_admission.rs` | `Open` once the bucket metadata runtime handle exists; no counts | bucket metadata runtime not initialized |
 | `Scanner` | same | scanner active work-unit counter / none / configured set-scan limit when nonzero | scanner runtime not initialized |
 | `Repair` | same | heal active tasks / heal queue length / `None` (limits live behind the async heal manager state) | heal manager not initialized |

rustfs/src/storage/concurrency/manager.rs

@@ -29,12 +29,17 @@ use rustfs_io_core::BytesPool;
 use rustfs_io_core::io_profile::{AccessPattern, IoPatternDetector, StorageMedia, detect_storage_media};
 use rustfs_io_metrics::bandwidth::{BandwidthMonitor, BandwidthSnapshot};
 use rustfs_io_metrics::{MetricsCollector, PerformanceMetrics};
+use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, LazyLock, Mutex};
 use std::time::Duration;
 use tokio::sync::{OwnedSemaphorePermit, Semaphore};
 use tracing::debug;
 
 const DERIVED_LARGE_PUT_ADMISSION_LIMIT_MAX: usize = 32;
+// A queued multipart part holds a connection but no body, so the queue can be
+// several times deeper than the permit pool. Sixteen uploads sending sixteen
+// parts each through one node fits inside the derived depth of 32 * 16.
+const DERIVED_MULTIPART_ADMISSION_MAX_PENDING_FACTOR: usize = 16;
 // Framed S2 alone can retain one encoded and one decoded block of roughly
 // 4 MiB each, while other codecs have their own larger windows. Four keeps
 // useful request parallelism without scaling codec memory and CPU with clients.
@@ -149,6 +154,28 @@ struct ForegroundWriteAdmissionGate {
     semaphore: Arc<Semaphore>,
     limit: usize,
     wait_timeout: Duration,
+    /// Requests currently waiting in the bounded multipart queue.
+    pending: Arc<AtomicUsize>,
+}
+
+/// Reservation of one slot in the bounded multipart wait queue; released on
+/// drop so a cancelled or timed-out waiter never leaks queue depth.
+struct PendingSlot(Arc<AtomicUsize>);
+
+impl PendingSlot {
+    fn reserve(pending: &Arc<AtomicUsize>, max_pending: usize) -> Option<Self> {
+        if pending.fetch_add(1, Ordering::AcqRel) >= max_pending {
+            pending.fetch_sub(1, Ordering::AcqRel);
+            return None;
+        }
+        Some(Self(pending.clone()))
+    }
+}
+
+impl Drop for PendingSlot {
+    fn drop(&mut self) {
+        self.0.fetch_sub(1, Ordering::AcqRel);
+    }
 }
 
 impl ForegroundWriteAdmissionGate {
@@ -157,6 +184,7 @@ impl ForegroundWriteAdmissionGate {
             semaphore: Arc::new(Semaphore::new(limit)),
             limit,
             wait_timeout,
+            pending: Arc::new(AtomicUsize::new(0)),
         }
     }
 
@@ -164,6 +192,35 @@ impl ForegroundWriteAdmissionGate {
         self.limit.saturating_sub(self.semaphore.available_permits())
     }
 
+    fn pending(&self) -> usize {
+        self.pending.load(Ordering::Acquire)
+    }
+
+    /// Admit through the same permit pool as [`Self::admit`], but let the
+    /// request wait in a bounded queue for `wait_timeout` instead of failing
+    /// on the gate's own short wait. A full queue rejects immediately.
+    async fn admit_queued(
+        &self,
+        wait_timeout: Duration,
+        max_pending: usize,
+    ) -> Result<ForegroundWriteAdmission, tokio::sync::AcquireError> {
+        match self.semaphore.clone().try_acquire_owned() {
+            Ok(permit) => return Ok(ForegroundWriteAdmission::Admitted(permit)),
+            Err(tokio::sync::TryAcquireError::Closed) => return Ok(ForegroundWriteAdmission::Rejected),
+            Err(tokio::sync::TryAcquireError::NoPermits) => {}
+        }
+        if wait_timeout.is_zero() {
+            return Ok(ForegroundWriteAdmission::Rejected);
+        }
+        let Some(_slot) = PendingSlot::reserve(&self.pending, max_pending) else {
+            return Ok(ForegroundWriteAdmission::Rejected);
+        };
+        match tokio::time::timeout(wait_timeout, self.semaphore.clone().acquire_owned()).await {
+            Ok(permit) => Ok(ForegroundWriteAdmission::Admitted(permit?)),
+            Err(_) => Ok(ForegroundWriteAdmission::Rejected),
+        }
+    }
+
     async fn admit(&self) -> Result<ForegroundWriteAdmission, tokio::sync::AcquireError> {
         if self.wait_timeout.is_zero() {
             return Ok(match self.semaphore.clone().try_acquire_owned() {
@@ -194,6 +251,8 @@ enum ForegroundWriteAdmissionPolicy {
         gate: ForegroundWriteAdmissionGate,
         put_object_min_size_bytes: usize,
         multipart_part_min_size_bytes: usize,
+        multipart_wait_timeout: Duration,
+        multipart_max_pending: usize,
     },
 }
 
@@ -252,11 +311,24 @@ impl ForegroundWriteAdmissionPolicy {
             rustfs_config::ENV_PUT_LARGE_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS,
             rustfs_config::DEFAULT_PUT_LARGE_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS,
         ));
+        let multipart_wait_timeout = Duration::from_millis(rustfs_utils::get_env_u64(
+            rustfs_config::ENV_PUT_MULTIPART_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS,
+            rustfs_config::DEFAULT_PUT_MULTIPART_FOREGROUND_ADMISSION_WAIT_TIMEOUT_MS,
+        ));
+        let multipart_max_pending = derive_multipart_admission_max_pending(
+            rustfs_utils::get_env_usize(
+                rustfs_config::ENV_PUT_MULTIPART_FOREGROUND_ADMISSION_MAX_PENDING,
+                rustfs_config::DEFAULT_PUT_MULTIPART_FOREGROUND_ADMISSION_MAX_PENDING,
+            ),
+            large_limit,
+        );
 
         Self::Large {
             gate: ForegroundWriteAdmissionGate::new(large_limit, wait_timeout),
             put_object_min_size_bytes,
             multipart_part_min_size_bytes,
+            multipart_wait_timeout,
+            multipart_max_pending,
         }
     }
 
@@ -275,11 +347,25 @@ impl ForegroundWriteAdmissionPolicy {
 
     #[cfg(test)]
     fn large_for_test(enabled: bool, limit: usize, min_size_bytes: usize, wait_timeout: Duration) -> Self {
+        Self::large_with_multipart_queue_for_test(enabled, limit, min_size_bytes, wait_timeout, wait_timeout, 0)
+    }
+
+    #[cfg(test)]
+    fn large_with_multipart_queue_for_test(
+        enabled: bool,
+        limit: usize,
+        min_size_bytes: usize,
+        wait_timeout: Duration,
+        multipart_wait_timeout: Duration,
+        multipart_max_pending: usize,
+    ) -> Self {
         if enabled && limit > 0 {
             Self::Large {
                 gate: ForegroundWriteAdmissionGate::new(limit, wait_timeout),
                 put_object_min_size_bytes: min_size_bytes,
                 multipart_part_min_size_bytes: 0,
+                multipart_wait_timeout,
+                multipart_max_pending: derive_multipart_admission_max_pending(multipart_max_pending, limit),
             }
         } else {
             Self::LegacyCounterOnly
@@ -298,35 +384,45 @@ impl ForegroundWriteAdmissionPolicy {
                 gate,
                 put_object_min_size_bytes,
                 multipart_part_min_size_bytes,
-            } => {
-                let min_size_bytes = match kind {
-                    ForegroundWriteAdmissionKind::PutObject => *put_object_min_size_bytes,
-                    ForegroundWriteAdmissionKind::MultipartPart => *multipart_part_min_size_bytes,
-                };
-                if should_gate_foreground_write(size, min_size_bytes) {
+                multipart_wait_timeout,
+                multipart_max_pending,
+            } => match kind {
+                ForegroundWriteAdmissionKind::PutObject if should_gate_foreground_write(size, *put_object_min_size_bytes) => {
                     gate.admit().await
-                } else {
-                    Ok(ForegroundWriteAdmission::Disabled)
                 }
-            }
+                ForegroundWriteAdmissionKind::MultipartPart
+                    if should_gate_foreground_write(size, *multipart_part_min_size_bytes) =>
+                {
+                    gate.admit_queued(*multipart_wait_timeout, *multipart_max_pending).await
+                }
+                _ => Ok(ForegroundWriteAdmission::Disabled),
+            },
         }
     }
 
     fn snapshot(&self, legacy_limit: usize) -> WorkloadAdmissionSnapshot {
         match self {
-            Self::Disabled => put_admission_snapshot(0, 0, None),
-            Self::LegacyCounterOnly => put_admission_snapshot(PutObjectGuard::concurrent_count(), legacy_limit, None),
+            Self::Disabled => put_admission_snapshot(0, None, 0, None),
+            Self::LegacyCounterOnly => put_admission_snapshot(PutObjectGuard::concurrent_count(), None, legacy_limit, None),
             Self::Strict(gate) => {
-                put_admission_snapshot(gate.active(), gate.limit, Some("foreground write admission permits exhausted"))
-            }
-            Self::Large { gate, .. } => {
-                put_admission_snapshot(gate.active(), gate.limit, Some("large foreground write admission permits exhausted"))
+                put_admission_snapshot(gate.active(), None, gate.limit, Some("foreground write admission permits exhausted"))
             }
+            Self::Large { gate, .. } => put_admission_snapshot(
+                gate.active(),
+                Some(gate.pending()),
+                gate.limit,
+                Some("large foreground write admission permits exhausted"),
+            ),
         }
     }
 }
 
-fn put_admission_snapshot(active: usize, limit: usize, hard_gate_reason: Option<&'static str>) -> WorkloadAdmissionSnapshot {
+fn put_admission_snapshot(
+    active: usize,
+    queued: Option<usize>,
+    limit: usize,
+    hard_gate_reason: Option<&'static str>,
+) -> WorkloadAdmissionSnapshot {
     let state = if limit == 0 {
         AdmissionState::Disabled
     } else if active >= limit {
@@ -336,7 +432,7 @@ fn put_admission_snapshot(active: usize, limit: usize, hard_gate_reason: Option<
     };
 
     let admission =
-        WorkloadAdmissionSnapshot::new(WorkloadClass::ForegroundWrite, state).with_counts(Some(active), None, Some(limit));
+        WorkloadAdmissionSnapshot::new(WorkloadClass::ForegroundWrite, state).with_counts(Some(active), queued, Some(limit));
 
     match state {
         AdmissionState::Disabled => admission.with_reason("foreground write admission disabled"),
@@ -347,6 +443,13 @@ fn put_admission_snapshot(active: usize, limit: usize, hard_gate_reason: Option<
     }
 }
 
+fn derive_multipart_admission_max_pending(configured_max_pending: usize, limit: usize) -> usize {
+    if configured_max_pending > 0 {
+        return configured_max_pending;
+    }
+    limit.saturating_mul(DERIVED_MULTIPART_ADMISSION_MAX_PENDING_FACTOR)
+}
+
 fn derive_large_put_admission_limit(configured_limit: usize, max_disk_reads: usize) -> usize {
     if configured_limit > 0 {
         return configured_limit;
@@ -464,6 +567,24 @@ impl ConcurrencyManager {
         manager
     }
 
+    #[cfg(test)]
+    pub(crate) fn with_multipart_admission_queue_for_test(
+        limit: usize,
+        multipart_wait_timeout: Duration,
+        multipart_max_pending: usize,
+    ) -> Self {
+        let mut manager = Self::new();
+        manager.foreground_write_admission_policy = ForegroundWriteAdmissionPolicy::large_with_multipart_queue_for_test(
+            true,
+            limit,
+            rustfs_config::DEFAULT_PUT_LARGE_FOREGROUND_ADMISSION_MIN_SIZE_BYTES,
+            Duration::ZERO,
+            multipart_wait_timeout,
+            multipart_max_pending,
+        );
+        manager
+    }
+
     #[cfg(test)]
     pub(crate) fn with_large_put_admission_for_test(
         enabled: bool,
@@ -1108,7 +1229,7 @@ mod integration_tests {
     use super::super::request_guard::GetObjectGuard;
     use super::{
         ConcurrencyManager, ForegroundWriteAdmission, SNOWBALL_ARCHIVE_DECODER_LIMIT, SNOWBALL_MEMBER_COMMIT_LIMIT,
-        SNOWBALL_STAGING_BYTES_LIMIT, derive_large_put_admission_limit,
+        SNOWBALL_STAGING_BYTES_LIMIT, derive_large_put_admission_limit, derive_multipart_admission_max_pending,
     };
     use crate::storage::storage_api::concurrency_consumer::PutObjectGuard;
     use rustfs_concurrency::{AdmissionState, WorkloadAdmissionSnapshotProvider, WorkloadClass};
@@ -1524,6 +1645,110 @@ mod integration_tests {
         drop((first, second));
     }
 
+    #[tokio::test(start_paused = true)]
+    #[serial]
+    async fn test_concurrency_manager_multipart_part_waits_for_released_permit() {
+        let manager = ConcurrencyManager::with_multipart_admission_queue_for_test(1, Duration::from_secs(30), 0);
+        let held = manager
+            .admit_multipart_part(8 * 1024 * 1024)
+            .await
+            .expect("first multipart part admission should acquire");
+        assert!(matches!(held, ForegroundWriteAdmission::Admitted(_)));
+
+        let waiter_manager = manager.clone();
+        let waiter = tokio::spawn(async move { waiter_manager.admit_multipart_part(8 * 1024 * 1024).await });
+        tokio::task::yield_now().await;
+        assert_eq!(manager.put_object_admission_snapshot().queued, Some(1));
+
+        tokio::time::advance(Duration::from_secs(5)).await;
+        drop(held);
+
+        let admission = waiter
+            .await
+            .expect("multipart admission waiter task must not panic")
+            .expect("multipart admission gate must stay open");
+        assert!(matches!(admission, ForegroundWriteAdmission::Admitted(_)));
+        assert_eq!(manager.put_object_admission_snapshot().queued, Some(0));
+    }
+
+    #[tokio::test(start_paused = true)]
+    #[serial]
+    async fn test_concurrency_manager_multipart_part_rejects_after_queue_wait_timeout() {
+        let manager = ConcurrencyManager::with_multipart_admission_queue_for_test(1, Duration::from_secs(30), 0);
+        let held = manager
+            .admit_multipart_part(1024)
+            .await
+            .expect("first multipart part admission should acquire");
+
+        let waiter_manager = manager.clone();
+        let waiter = tokio::spawn(async move { waiter_manager.admit_multipart_part(1024).await });
+        tokio::task::yield_now().await;
+        tokio::time::advance(Duration::from_secs(30)).await;
+
+        let admission = waiter
+            .await
+            .expect("multipart admission waiter task must not panic")
+            .expect("multipart admission gate must stay open");
+        assert!(matches!(admission, ForegroundWriteAdmission::Rejected));
+        assert_eq!(manager.put_object_admission_snapshot().queued, Some(0));
+        drop(held);
+    }
+
+    #[tokio::test(start_paused = true)]
+    #[serial]
+    async fn test_concurrency_manager_multipart_part_rejects_immediately_when_queue_is_full() {
+        let manager = ConcurrencyManager::with_multipart_admission_queue_for_test(1, Duration::from_secs(30), 1);
+        let held = manager
+            .admit_multipart_part(1024)
+            .await
+            .expect("first multipart part admission should acquire");
+
+        let waiter_manager = manager.clone();
+        let queued = tokio::spawn(async move { waiter_manager.admit_multipart_part(1024).await });
+        tokio::task::yield_now().await;
+        assert_eq!(manager.put_object_admission_snapshot().queued, Some(1));
+
+        let overflow = manager
+            .admit_multipart_part(1024)
+            .await
+            .expect("full multipart queue should reject, not close");
+        assert!(matches!(overflow, ForegroundWriteAdmission::Rejected));
+
+        drop(held);
+        let admission = queued
+            .await
+            .expect("queued multipart part task must not panic")
+            .expect("multipart admission gate must stay open");
+        assert!(matches!(admission, ForegroundWriteAdmission::Admitted(_)));
+    }
+
+    #[tokio::test(start_paused = true)]
+    #[serial]
+    async fn test_concurrency_manager_cancelled_multipart_waiter_releases_queue_slot() {
+        let manager = ConcurrencyManager::with_multipart_admission_queue_for_test(1, Duration::from_secs(30), 1);
+        let held = manager
+            .admit_multipart_part(1024)
+            .await
+            .expect("first multipart part admission should acquire");
+
+        let waiter_manager = manager.clone();
+        let queued = tokio::spawn(async move { waiter_manager.admit_multipart_part(1024).await });
+        tokio::task::yield_now().await;
+        assert_eq!(manager.put_object_admission_snapshot().queued, Some(1));
+        queued.abort();
+        let _ = queued.await;
+
+        assert_eq!(manager.put_object_admission_snapshot().queued, Some(0));
+        drop(held);
+    }
+
+    #[test]
+    fn test_concurrency_manager_derives_multipart_admission_max_pending_from_limit() {
+        assert_eq!(derive_multipart_admission_max_pending(7, 32), 7);
+        assert_eq!(derive_multipart_admission_max_pending(0, 32), 512);
+        assert_eq!(derive_multipart_admission_max_pending(0, 1), 16);
+    }
+
     #[test]
     fn test_concurrency_manager_derives_large_put_admission_limit_from_scheduler_cap() {
         assert_eq!(derive_large_put_admission_limit(7, 64), 7);