archiver: Asynchronously complete FutureFile

After reading and chunking all data in a file, the FutureFile still has
to wait until the FutureBlobs are completed. This was done synchronously
which results in blocking the file saver and prevents the next file from
being read.

By replacing the FutureBlob with a callback, it becomes possible to
complete the FutureFile asynchronously.

internal/archiver/blob_saver_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"runtime"
+	"sync"
 	"sync/atomic"
 	"testing"
 
@@ -45,16 +46,22 @@ func TestBlobSaver(t *testing.T) {
 
 	b := NewBlobSaver(ctx, wg, saver, uint(runtime.NumCPU()))
 
-	var results []FutureBlob
+	var wait sync.WaitGroup
+	var results []SaveBlobResponse
 
+	wait.Add(20)
 	for i := 0; i < 20; i++ {
 		buf := &Buffer{Data: []byte(fmt.Sprintf("foo%d", i))}
-		fb := b.Save(ctx, restic.DataBlob, buf)
-		results = append(results, fb)
+		idx := i
+		results = append(results, SaveBlobResponse{})
+		b.Save(ctx, restic.DataBlob, buf, func(res SaveBlobResponse) {
+			results[idx] = res
+			wait.Done()
+		})
 	}
 
-	for i, blob := range results {
-		sbr := blob.Take(ctx)
+	wait.Wait()
+	for i, sbr := range results {
 		if sbr.known {
 			t.Errorf("blob %v is known, that should not be the case", i)
 		}
@@ -94,7 +101,7 @@ func TestBlobSaverError(t *testing.T) {
 
 			for i := 0; i < test.blobs; i++ {
 				buf := &Buffer{Data: []byte(fmt.Sprintf("foo%d", i))}
-				b.Save(ctx, restic.DataBlob, buf)
+				b.Save(ctx, restic.DataBlob, buf, func(res SaveBlobResponse) {})
 			}
 
 			b.TriggerShutdown()