restore: only report errors for blobs that actually failed to load

Previously, errors would be reported for all blobs of a packfile that
failed to stream. Now, only the not yet processed blobs are reported.

internal/restorer/filerestorer.go

@@ -246,7 +246,10 @@ func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) error {
 		return err
 	}
 
+	// track already processed blobs for precise error reporting
+	processedBlobs := restic.NewBlobSet()
 	err := repository.StreamPack(ctx, r.packLoader, r.key, pack.id, blobList, func(h restic.BlobHandle, blobData []byte, err error) error {
+		processedBlobs.Insert(h)
 		blob := blobs[h.ID]
 		if err != nil {
 			for file := range blob.files {
@@ -292,7 +295,19 @@ func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) error {
 	})
 
 	if err != nil {
-		for file := range pack.files {
+		// only report error for not yet processed blobs
+		affectedFiles := make(map[*fileInfo]struct{})
+		for _, blob := range blobList {
+			if processedBlobs.Has(blob.BlobHandle) {
+				continue
+			}
+			blob := blobs[blob.ID]
+			for file := range blob.files {
+				affectedFiles[file] = struct{}{}
+			}
+		}
+
+		for file := range affectedFiles {
 			if errFile := sanitizeError(file, err); errFile != nil {
 				return errFile
 			}

internal/restorer/filerestorer_test.go

@@ -317,3 +317,47 @@ func testPartialDownloadError(t *testing.T, part int) {
 	rtest.OK(t, err)
 	verifyRestore(t, r, repo)
 }
+
+func TestFatalDownloadError(t *testing.T) {
+	tempdir := rtest.TempDir(t)
+	content := []TestFile{
+		{
+			name: "file1",
+			blobs: []TestBlob{
+				{"data1-1", "pack1"},
+				{"data1-2", "pack1"},
+			},
+		},
+		{
+			name: "file2",
+			blobs: []TestBlob{
+				{"data2-1", "pack1"},
+				{"data2-2", "pack1"},
+				{"data2-3", "pack1"},
+			},
+		}}
+
+	repo := newTestRepo(content)
+
+	loader := repo.loader
+	repo.loader = func(ctx context.Context, h backend.Handle, length int, offset int64, fn func(rd io.Reader) error) error {
+		// only return half the data to break file2
+		return loader(ctx, h, length/2, offset, fn)
+	}
+
+	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2, false, nil)
+	r.files = repo.files
+
+	var errors []string
+	r.Error = func(s string, e error) error {
+		// ignore errors as in the `restore` command
+		errors = append(errors, s)
+		return nil
+	}
+
+	err := r.restoreFiles(context.TODO())
+	rtest.OK(t, err)
+
+	rtest.Assert(t, len(errors) == 1, "unexpected number of restore errors, expected: 1, got: %v", len(errors))
+	rtest.Assert(t, errors[0] == "file2", "expected error for file2, got: %v", errors[0])
+}