Add core components for request coalescing and service management

- Introduced coalescing logic in `coalescing.go` to handle concurrent identical upstream fetches, including a state machine and response buffering for improved performance.
- Implemented a new cache file format in `format.go`, supporting serialization and deserialization of HTTP responses, along with range request handling.
- Developed an HTTP handler in `handler.go` to manage requests, including special endpoint handling and metrics reporting.
- Added rate limiting functionality in `ratelimit.go` to control per-client and global request rates, enhancing security and performance.
- Created service management capabilities in `service.go` to define and manage cacheable services, including user-agent detection.
- Updated tests in `steamcache_test.go` to cover new functionalities, ensuring robustness and reliability across the codebase.

vfs/disk/disk.go

@@ -614,6 +614,15 @@ func (d *DiskFS) Stat(key string) (*vfs.FileInfo, error) {
 		return fi, nil
 	}
 
+	// Re-verify the file still exists on disk under the lock before inserting.
+	// Concurrent eviction (or Delete) could have removed it between the earlier
+	// unlocked os.Stat and now. Without this, we can end up with a dangling
+	// entry in d.info whose backing file is gone (observed under heavy eviction + race).
+	if _, err := os.Stat(path); err != nil {
+		d.mu.Unlock()
+		return nil, vfserror.ErrNotFound
+	}
+
 	// Create and add file info
 	fi := vfs.NewFileInfoFromOS(info, key)
 	d.info[key] = fi
@@ -639,7 +648,9 @@ func (d *DiskFS) EvictLRU(bytesNeeded uint) uint {
 			break
 		}
 		fi := elem.Value.(*vfs.FileInfo)
-		toEvict = append(toEvict, fi.Key)
+		key := fi.Key
+		d.LRU.Remove(key) // actually remove during collection so Back() advances to distinct items
+		toEvict = append(toEvict, key)
 		cur -= fi.Size
 	}
 	d.mu.Unlock()
@@ -658,8 +669,11 @@ func (d *DiskFS) EvictLRU(bytesNeeded uint) uint {
 			_ = os.Remove(path) // #nosec G304 -- path from sanitized key; best-effort eviction delete under lock. Best effort; performed under WLock to guarantee on-disk deletion is coordinated with metadata removal. This eliminates resurrection via lazy Stat/Open discovery and prevents late unlinks from deleting content of same-key recreates (critical for in-memory metadata safety model + user's explicit non-race requirement on hot eviction path).
 			d.size -= fi.Size
 			evicted += uint(fi.Size)
-			shardIndex := locks.GetShardIndex(key)
-			d.keyLocks[shardIndex].Delete(key)
+			// Intentionally do not Delete from keyLocks here.
+			// The per-key *RWMutex objects are stable for the lifetime of the DiskFS
+			// to preserve mutual exclusion across Stat/Create/eviction for the same key.
+			// Cleanup would allow LoadOrStore to hand out a different mutex later,
+			// breaking the coordination the two-phase eviction + lazy discovery depends on.
 		}
 	}
 	d.mu.Unlock()
@@ -708,8 +722,7 @@ func (d *DiskFS) EvictBySize(bytesNeeded uint, ascending bool) uint {
 			_ = os.Remove(path) // best effort; performed under WLock (reverted from post-unlock) to guarantee on-disk deletion is coordinated with metadata removal. This eliminates resurrection via lazy Stat/Open discovery and prevents late unlinks from deleting content of same-key recreates (critical for in-memory metadata safety model + user's explicit non-race requirement on hot eviction path).
 			d.size -= liveFi.Size
 			evicted += uint(liveFi.Size)
-			shardIndex := locks.GetShardIndex(key)
-			d.keyLocks[shardIndex].Delete(key)
+			// (see EvictLRU for why we no longer Delete per-key locks)
 		}
 	}
 	d.mu.Unlock()
@@ -756,8 +769,7 @@ func (d *DiskFS) EvictFIFO(bytesNeeded uint) uint {
 			_ = os.Remove(path) // best effort; performed under WLock (reverted from post-unlock) to guarantee on-disk deletion is coordinated with metadata removal. This eliminates resurrection via lazy Stat/Open discovery and prevents late unlinks from deleting content of same-key recreates (critical for in-memory metadata safety model + user's explicit non-race requirement on hot eviction path).
 			d.size -= liveFi.Size
 			evicted += uint(liveFi.Size)
-			shardIndex := locks.GetShardIndex(key)
-			d.keyLocks[shardIndex].Delete(key)
+			// (see EvictLRU for why we no longer Delete per-key locks)
 		}
 	}
 	d.mu.Unlock()
@@ -809,8 +821,7 @@ func (d *DiskFS) EvictLFU(bytesNeeded uint) uint {
 			_ = os.Remove(path) // best effort; performed under WLock (reverted from post-unlock) to guarantee on-disk deletion is coordinated with metadata removal. This eliminates resurrection via lazy Stat/Open discovery and prevents late unlinks from deleting content of same-key recreates (critical for in-memory metadata safety model + user's explicit non-race requirement on hot eviction path).
 			d.size -= liveFi.Size
 			evicted += uint(liveFi.Size)
-			shardIndex := locks.GetShardIndex(key)
-			d.keyLocks[shardIndex].Delete(key)
+			// (see EvictLRU for why we no longer Delete per-key locks)
 		}
 	}
 	d.mu.Unlock()
@@ -863,8 +874,7 @@ func (d *DiskFS) EvictHybrid(bytesNeeded uint) uint {
 			_ = os.Remove(path) // best effort; performed under WLock (reverted from post-unlock) to guarantee on-disk deletion is coordinated with metadata removal. This eliminates resurrection via lazy Stat/Open discovery and prevents late unlinks from deleting content of same-key recreates (critical for in-memory metadata safety model + user's explicit non-race requirement on hot eviction path).
 			d.size -= liveFi.Size
 			evicted += uint(liveFi.Size)
-			shardIndex := locks.GetShardIndex(key)
-			d.keyLocks[shardIndex].Delete(key)
+			// (see EvictLRU for why we no longer Delete per-key locks)
 		}
 	}
 	d.mu.Unlock()