Check contents in archiver

When backing up with a parent snapshot and the file is not changed, also
check if contents are still available in index.

changelog/unreleased/issue-2571

@@ -0,0 +1,16 @@
+Enhancement: Self-heal missing file parts during backup of unchanged files
+
+We've improved the resilience of restic to certain types of repository corruption.
+
+For files that are unchanged since the parent snapshot, the backup command now
+verifies that all parts of the files still exist in the repository. Parts that are
+missing, e.g. from a damaged repository, are backed up again. This verification
+was already run for files that were modified since the parent snapshot, but is
+now also done for unchanged files.
+
+Note that restic will not backup file parts that are referenced in the index but
+where the actual data is not present on disk, as this situation can only be
+detected by restic check. Please ensure that you run `restic check` regularly.
+
+https://github.com/restic/restic/issues/2571
+https://github.com/restic/restic/pull/2827

cmd/restic/integration_test.go

@@ -359,6 +359,53 @@ func TestBackupNonExistingFile(t *testing.T) {
 	testRunBackup(t, "", dirs, opts, env.gopts)
 }
 
+func TestBackupSelfHealing(t *testing.T) {
+	env, cleanup := withTestEnvironment(t)
+	defer cleanup()
+
+	testRunInit(t, env.gopts)
+
+	p := filepath.Join(env.testdata, "test/test")
+	rtest.OK(t, os.MkdirAll(filepath.Dir(p), 0755))
+	rtest.OK(t, appendRandomData(p, 5))
+
+	opts := BackupOptions{}
+
+	testRunBackup(t, filepath.Dir(env.testdata), []string{filepath.Base(env.testdata)}, opts, env.gopts)
+	testRunCheck(t, env.gopts)
+
+	r, err := OpenRepository(env.gopts)
+	rtest.OK(t, err)
+
+	// Get all tree packs
+	rtest.OK(t, r.LoadIndex(env.gopts.ctx))
+	treePacks := restic.NewIDSet()
+	for _, idx := range r.Index().(*repository.MasterIndex).All() {
+		for _, id := range idx.TreePacks() {
+			treePacks.Insert(id)
+		}
+	}
+
+	// remove all data packs
+	rtest.OK(t, r.List(env.gopts.ctx, restic.DataFile, func(id restic.ID, size int64) error {
+		if treePacks.Has(id) {
+			return nil
+		}
+		return r.Backend().Remove(env.gopts.ctx, restic.Handle{Type: restic.DataFile, Name: id.String()})
+	}))
+
+	testRunRebuildIndex(t, env.gopts)
+	// now the repo is also missing the data blob in the index; check should report this
+	rtest.Assert(t, runCheck(CheckOptions{}, env.gopts, nil) != nil,
+		"check should have reported an error")
+
+	// second backup should report an error but "heal" this situation
+	err = testRunBackupAssumeFailure(t, filepath.Dir(env.testdata), []string{filepath.Base(env.testdata)}, opts, env.gopts)
+	rtest.Assert(t, err != nil,
+		"backup should have reported an error")
+	testRunCheck(t, env.gopts)
+}
+
 func includes(haystack []string, needle string) bool {
 	for _, s := range haystack {
 		if s == needle {

internal/archiver/archiver.go

@@ -302,6 +302,18 @@ func (fn *FutureNode) wait(ctx context.Context) {
 	}
 }
 
+// allBlobsPresent checks if all blobs (contents) of the given node are
+// present in the index.
+func (arch *Archiver) allBlobsPresent(previous *restic.Node) bool {
+	// check if all blobs are contained in index
+	for _, id := range previous.Content {
+		if !arch.Repo.Index().Has(id, restic.DataBlob) {
+			return false
+		}
+	}
+	return true
+}
+
 // Save saves a target (file or directory) to the repo. If the item is
 // excluded, this function returns a nil node and error, with excluded set to
 // true.
@@ -388,19 +400,26 @@ func (arch *Archiver) Save(ctx context.Context, snPath, target string, previous
 
 		// use previous list of blobs if the file hasn't changed
 		if previous != nil && !fileChanged(fi, previous, arch.IgnoreInode) {
-			debug.Log("%v hasn't changed, using old list of blobs", target)
-			arch.CompleteItem(snPath, previous, previous, ItemStats{}, time.Since(start))
-			arch.CompleteBlob(snPath, previous.Size)
-			fn.node, err = arch.nodeFromFileInfo(target, fi)
-			if err != nil {
-				return FutureNode{}, false, err
+			if arch.allBlobsPresent(previous) {
+				debug.Log("%v hasn't changed, using old list of blobs", target)
+				arch.CompleteItem(snPath, previous, previous, ItemStats{}, time.Since(start))
+				arch.CompleteBlob(snPath, previous.Size)
+				fn.node, err = arch.nodeFromFileInfo(target, fi)
+				if err != nil {
+					return FutureNode{}, false, err
+				}
+
+				// copy list of blobs
+				fn.node.Content = previous.Content
+
+				_ = file.Close()
+				return fn, false, nil
+			} else {
+				debug.Log("%v hasn't changed, but contents are missing!", target)
+				// There are contents missing - inform user!
+				err := errors.Errorf("parts of %v not found in the repository index; storing the file again", target)
+				arch.error(abstarget, fi, err)
 			}
-
-			// copy list of blobs
-			fn.node.Content = previous.Content
-
-			_ = file.Close()
-			return fn, false, nil
 		}
 
 		fn.isFile = true