repository: don't hang when copying using a single connection

changelog/unreleased/issue-3897

@@ -0,0 +1,7 @@
+Bugfix: Fix stuck `copy` command when setting backend connections to 1
+
+When calling the copy command using `copy -o <backend>.connections=1` this
+caused the command to be stuck permanently. This has been fixed.
+
+https://github.com/restic/restic/issues/3897
+https://github.com/restic/restic/pull/3898

internal/repository/repack.go

@@ -114,6 +114,10 @@ func repack(ctx context.Context, repo restic.Repository, dstRepo restic.Reposito
 	// as packs are streamed the concurrency is limited by IO
 	// reduce by one to ensure that uploading is always possible
 	repackWorkerCount := int(repo.Connections() - 1)
+	if repo != dstRepo {
+		// no need to share the upload and download connections for different repositories
+		repackWorkerCount = int(repo.Connections())
+	}
 	for i := 0; i < repackWorkerCount; i++ {
 		wg.Go(worker)
 	}

internal/repository/repack_test.go

@@ -292,12 +292,24 @@ func TestRepackCopy(t *testing.T) {
 	repository.TestAllVersions(t, testRepackCopy)
 }
 
+type oneConnectionRepo struct {
+	restic.Repository
+}
+
+func (r oneConnectionRepo) Connections() uint {
+	return 1
+}
+
 func testRepackCopy(t *testing.T, version uint) {
 	repo, cleanup := repository.TestRepositoryWithVersion(t, version)
 	defer cleanup()
 	dstRepo, dstCleanup := repository.TestRepositoryWithVersion(t, version)
 	defer dstCleanup()
 
+	// test with minimal possible connection count
+	repoWrapped := &oneConnectionRepo{repo}
+	dstRepoWrapped := &oneConnectionRepo{dstRepo}
+
 	seed := time.Now().UnixNano()
 	rand.Seed(seed)
 	t.Logf("rand seed is %v", seed)
@@ -308,7 +320,7 @@ func testRepackCopy(t *testing.T, version uint) {
 	_, keepBlobs := selectBlobs(t, repo, 0.2)
 	copyPacks := findPacksForBlobs(t, repo, keepBlobs)
 
-	_, err := repository.Repack(context.TODO(), repo, dstRepo, copyPacks, keepBlobs, nil)
+	_, err := repository.Repack(context.TODO(), repoWrapped, dstRepoWrapped, copyPacks, keepBlobs, nil)
 	if err != nil {
 		t.Fatal(err)
 	}