feat: add concurrency limiter to scoutfs

This brings scoutfs in-line with the posix concurrency limiter.
This fixes a hang with scoutfs due to not correctly initializing
the concurrency in posix leading to a concurrency of 0 allowed.

This also adds a sane default to the posix concurrency when not
initialized.

backend/posix/posix.go

@@ -126,6 +126,9 @@ const (
 
 	doFalloc   = true
 	skipFalloc = false
+
+	// defaultConcurrency is the default limit for concurrent POSIX actions.
+	defaultConcurrency = 5000
 )
 
 // PosixOpts are the options for the Posix backend
@@ -219,10 +222,18 @@ func New(rootdir string, meta meta.MetadataStorer, opts PosixOpts) (*Posix, erro
 		newDirPerm:         opts.NewDirPerm,
 		forceNoTmpFile:     opts.ForceNoTmpFile,
 		validateBucketName: opts.ValidateBucketNames,
-		actionLimiter:      semaphore.NewWeighted(int64(opts.Concurrency)),
+		actionLimiter:      semaphore.NewWeighted(int64(concurrencyOrDefault(opts.Concurrency))),
 	}, nil
 }
 
+// concurrencyOrDefault returns n if it is positive, otherwise defaultConcurrency.
+func concurrencyOrDefault(n int) int {
+	if n > 0 {
+		return n
+	}
+	return defaultConcurrency
+}
+
 func validateSubDir(root, dir string) (string, error) {
 	absDir, err := filepath.Abs(dir)
 	if err != nil {

backend/scoutfs/scoutfs.go

@@ -42,6 +42,9 @@ type ScoutfsOpts struct {
 	// incorrect access to the filesystem. This is only needed if the
 	// frontend is not already validating bucket names.
 	ValidateBucketNames bool
+	// Concurrency sets the maximum number of concurrently running POSIX actions.
+	// Defaults to 5000 when unset or non-positive.
+	Concurrency int
 }
 
 var _ backend.Backend = &ScoutFS{}

backend/scoutfs/scoutfs_compat.go

@@ -81,6 +81,7 @@ func New(rootdir string, opts ScoutfsOpts) (*ScoutFS, error) {
 		NewDirPerm:          opts.NewDirPerm,
 		VersioningDir:       opts.VersioningDir,
 		ValidateBucketNames: opts.ValidateBucketNames,
+		Concurrency:         opts.Concurrency,
 	})
 	if err != nil {
 		return nil, err

cmd/versitygw/scoutfs.go

@@ -99,6 +99,13 @@ move interfaces as well as support for tiered filesystems.`,
 				EnvVars:     []string{"VGW_DISABLE_NOARCHIVE"},
 				Destination: &disableNoArchive,
 			},
+			&cli.IntFlag{
+				Name:        "concurrency",
+				Usage:       "maximum concurrent actions allowed",
+				EnvVars:     []string{"VGW_POSIX_CONCURRENCY"},
+				Value:       5000,
+				Destination: &actionsConcurrency,
+			},
 		},
 	}
 }
@@ -112,6 +119,10 @@ func runScoutfs(ctx *cli.Context) error {
 		return fmt.Errorf("invalid directory permissions: %d", dirPerms)
 	}
 
+	if actionsConcurrency <= 0 {
+		return fmt.Errorf("concurrency must be positive, got %d", actionsConcurrency)
+	}
+
 	var opts scoutfs.ScoutfsOpts
 	opts.GlacierMode = glacier
 	opts.ChownUID = chownuid
@@ -122,6 +133,7 @@ func runScoutfs(ctx *cli.Context) error {
 	opts.VersioningDir = versioningDir
 	opts.ValidateBucketNames = disableStrictBucketNames
 	opts.SetProjectID = setProjectID
+	opts.Concurrency = actionsConcurrency
 
 	be, err := scoutfs.New(ctx.Args().Get(0), opts)
 	if err != nil {