shell: fix potential deadlock in fs.meta.save BFS traversal

Refactored doTraverseBfsAndSaving to use context cancellation.
If the saving process fails, the traversal is stopped immediately
to prevent workers from blocking on the output channel.

weed/shell/command_fs_meta_save.go

@@ -102,7 +102,7 @@ func (c *commandFsMetaSave) Do(args []string, commandEnv *CommandEnv, writer io.
 		cipherKey = util.GenCipherKey()
 	}
 
-	err = doTraverseBfsAndSaving(commandEnv, writer, path, *verbose, func(entry *filer_pb.FullEntry, outputChan chan interface{}) (err error) {
+	err = doTraverseBfsAndSaving(commandEnv, writer, path, *verbose, func(ctx context.Context, entry *filer_pb.FullEntry, outputChan chan interface{}) (err error) {
 		if !entry.Entry.IsDirectory {
 			ext := filepath.Ext(entry.Entry.Name)
 			if encrypted, encErr := util.Encrypt([]byte(entry.Entry.Name), cipherKey); encErr == nil {
@@ -116,7 +116,11 @@ func (c *commandFsMetaSave) Do(args []string, commandEnv *CommandEnv, writer io.
 			return
 		}
 
-		outputChan <- bytes
+		select {
+		case outputChan <- bytes:
+		case <-ctx.Done():
+			return ctx.Err()
+		}
 		return nil
 	}, func(outputChan chan interface{}) error {
 		sizeBuf := make([]byte, 4)
@@ -143,14 +147,21 @@ func (c *commandFsMetaSave) Do(args []string, commandEnv *CommandEnv, writer io.
 
 }
 
-func doTraverseBfsAndSaving(filerClient filer_pb.FilerClient, writer io.Writer, path string, verbose bool, genFn func(entry *filer_pb.FullEntry, outputChan chan interface{}) error, saveFn func(outputChan chan interface{}) error) error {
+func doTraverseBfsAndSaving(filerClient filer_pb.FilerClient, writer io.Writer, path string, verbose bool, genFn func(ctx context.Context, entry *filer_pb.FullEntry, outputChan chan interface{}) error, saveFn func(outputChan chan interface{}) error) error {
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
 
 	var wg sync.WaitGroup
 	wg.Add(1)
 	outputChan := make(chan interface{}, 1024)
 	saveErrChan := make(chan error, 1)
 	go func() {
-		saveErrChan <- saveFn(outputChan)
+		saveErr := saveFn(outputChan)
+		saveErrChan <- saveErr
+		if saveErr != nil {
+			cancel()
+		}
 		wg.Done()
 	}()
 
@@ -160,7 +171,7 @@ func doTraverseBfsAndSaving(filerClient filer_pb.FilerClient, writer io.Writer,
 	var hasErr atomic.Bool
 
 	// also save the directory itself (path) if it exists in the filer
-	if e, getErr := filer_pb.GetEntry(context.Background(), filerClient, util.FullPath(path)); getErr != nil {
+	if e, getErr := filer_pb.GetEntry(ctx, filerClient, util.FullPath(path)); getErr != nil {
 		// Entry not found is expected and can be ignored; log other errors.
 		if !errors.Is(getErr, filer_pb.ErrNotFound) {
 			fmt.Fprintf(writer, "failed to get entry %s: %v\n", path, getErr)
@@ -172,7 +183,7 @@ func doTraverseBfsAndSaving(filerClient filer_pb.FilerClient, writer io.Writer,
 			Entry: e,
 		}
 
-		if genErr := genFn(protoMessage, outputChan); genErr != nil {
+		if genErr := genFn(ctx, protoMessage, outputChan); genErr != nil {
 			once.Do(func() {
 				firstErr = genErr
 				hasErr.Store(true)
@@ -187,7 +198,6 @@ func doTraverseBfsAndSaving(filerClient filer_pb.FilerClient, writer io.Writer,
 		}
 	}
 
-	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	err := filer_pb.TraverseBfs(ctx, filerClient, util.FullPath(path), func(parentPath util.FullPath, entry *filer_pb.Entry) error {
 
@@ -205,7 +215,7 @@ func doTraverseBfsAndSaving(filerClient filer_pb.FilerClient, writer io.Writer,
 			// fail-fast: stop traversal once an error is observed.
 			return firstErr
 		}
-		if genErr := genFn(protoMessage, outputChan); genErr != nil {
+		if genErr := genFn(ctx, protoMessage, outputChan); genErr != nil {
 			once.Do(func() {
 				firstErr = genErr
 				hasErr.Store(true)