dump: Simplify writeNode and use fewer goroutines

This changes Dumper.writeNode to spawn loader goroutines as needed
instead of as a pool. The code is shorter, fewer goroutines are spawned
for small files, and crash dumps (also for unrelated errors) should be
smaller.

internal/dump/common.go

@@ -6,7 +6,6 @@ import (
 	"path"
 
 	"github.com/restic/restic/internal/bloblru"
-	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/restic"
 	"github.com/restic/restic/internal/walker"
 	"golang.org/x/sync/errgroup"
@@ -104,75 +103,50 @@ func (d *Dumper) WriteNode(ctx context.Context, node *restic.Node) error {
 }
 
 func (d *Dumper) writeNode(ctx context.Context, w io.Writer, node *restic.Node) error {
-	type loadTask struct {
-		id  restic.ID
-		out chan<- []byte
-	}
-	type writeTask struct {
-		data <-chan []byte
-	}
-
-	loaderCh := make(chan loadTask)
-	// per worker: allows for one blob that gets download + one blob thats queue for writing
-	writerCh := make(chan writeTask, d.repo.Connections()*2)
-
 	wg, ctx := errgroup.WithContext(ctx)
+	limit := d.repo.Connections() - 1 // See below for the -1.
+	blobs := make(chan (<-chan []byte), limit)
 
 	wg.Go(func() error {
-		defer close(loaderCh)
-		defer close(writerCh)
-		for _, id := range node.Content {
-			// non-blocking blob handover to allow the loader to load the next blob
-			// while the old one is still written
-			ch := make(chan []byte, 1)
+		for ch := range blobs {
 			select {
-			case loaderCh <- loadTask{id: id, out: ch}:
 			case <-ctx.Done():
 				return ctx.Err()
-			}
-
-			select {
-			case writerCh <- writeTask{data: ch}:
-			case <-ctx.Done():
-				return ctx.Err()
-			}
-		}
-		return nil
-	})
-
-	for i := uint(0); i < d.repo.Connections(); i++ {
-		wg.Go(func() error {
-			for task := range loaderCh {
-				blob, err := d.cache.GetOrCompute(task.id, func() ([]byte, error) {
-					return d.repo.LoadBlob(ctx, restic.DataBlob, task.id, nil)
-				})
-				if err != nil {
+			case blob := <-ch:
+				if _, err := w.Write(blob); err != nil {
 					return err
 				}
-
-				select {
-				case task.out <- blob:
-				case <-ctx.Done():
-					return ctx.Err()
-				}
-			}
-			return nil
-		})
-	}
-
-	wg.Go(func() error {
-		for result := range writerCh {
-			select {
-			case data := <-result.data:
-				if _, err := w.Write(data); err != nil {
-					return errors.Wrap(err, "Write")
-				}
-			case <-ctx.Done():
-				return ctx.Err()
 			}
 		}
 		return nil
 	})
 
+	// Start short-lived goroutines to load blobs.
+	// There will be at most 1+cap(blobs) calling LoadBlob at any moment.
+loop:
+	for _, id := range node.Content {
+		// This needs to be buffered, so that loaders can quit
+		// without waiting for the writer.
+		ch := make(chan []byte, 1)
+
+		wg.Go(func() error {
+			blob, err := d.cache.GetOrCompute(id, func() ([]byte, error) {
+				return d.repo.LoadBlob(ctx, restic.DataBlob, id, nil)
+			})
+
+			if err == nil {
+				ch <- blob
+			}
+			return err
+		})
+
+		select {
+		case blobs <- ch:
+		case <-ctx.Done():
+			break loop
+		}
+	}
+
+	close(blobs)
 	return wg.Wait()
 }