archiver: ignore files removed in the meantime

changelog/unreleased/issue-2165

@@ -0,0 +1,16 @@
+Bugfix: Ignore disappeared backup source files
+
+If during a backup files were removed between restic listing the directory
+content and backing up the file in question, the following error could occur:
+
+```
+error: lstat /some/file/name: no such file or directory
+```
+
+The backup command now ignores this particular error and silently skips the
+removed file.
+
+https://github.com/restic/restic/issues/2165
+https://github.com/restic/restic/issues/3098
+https://github.com/restic/restic/pull/5143
+https://github.com/restic/restic/pull/5145

internal/archiver/archiver.go

@@ -464,6 +464,12 @@ func (arch *Archiver) save(ctx context.Context, snPath, target string, previous
 		}
 		return futureNode{}, true, nil
 	}
+	filterNotExist := func(err error) error {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil
+		}
+		return err
+	}
 	// exclude files by path before running Lstat to reduce number of lstat calls
 	if !arch.SelectByName(abstarget) {
 		debug.Log("%v is excluded by path", target)
@@ -473,7 +479,8 @@ func (arch *Archiver) save(ctx context.Context, snPath, target string, previous
 	meta, err := arch.FS.OpenFile(target, fs.O_NOFOLLOW, true)
 	if err != nil {
 		debug.Log("open metadata for %v returned error: %v", target, err)
-		return filterError(err)
+		// ignore if file disappeared since it was returned by readdir
+		return filterError(filterNotExist(err))
 	}
 	closeFile := true
 	defer func() {
@@ -489,7 +496,8 @@ func (arch *Archiver) save(ctx context.Context, snPath, target string, previous
 	fi, err := meta.Stat()
 	if err != nil {
 		debug.Log("lstat() for %v returned error: %v", target, err)
-		return filterError(err)
+		// ignore if file disappeared since it was returned by readdir
+		return filterError(filterNotExist(err))
 	}
 	if !arch.Select(abstarget, fi, arch.FS) {
 		debug.Log("%v is excluded", target)

internal/archiver/archiver_test.go

@@ -2479,3 +2479,48 @@ func TestIrregularFile(t *testing.T) {
 		t.Errorf("Save() excluded the node, that's unexpected")
 	}
 }
+
+type missingFS struct {
+	fs.FS
+	errorOnOpen bool
+}
+
+func (fs *missingFS) OpenFile(name string, flag int, metadataOnly bool) (fs.File, error) {
+	if fs.errorOnOpen {
+		return nil, os.ErrNotExist
+	}
+
+	return &missingFile{}, nil
+}
+
+type missingFile struct {
+	fs.File
+}
+
+func (f *missingFile) Stat() (os.FileInfo, error) {
+	return nil, os.ErrNotExist
+}
+
+func (f *missingFile) Close() error {
+	// prevent segfault in test
+	return nil
+}
+
+func TestDisappearedFile(t *testing.T) {
+	tempdir, repo := prepareTempdirRepoSrc(t, TestDir{})
+
+	back := rtest.Chdir(t, tempdir)
+	defer back()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	// depending on the underlying FS implementation a missing file may be detected by OpenFile or
+	// the subsequent file.Stat() call. Thus test both cases.
+	for _, errorOnOpen := range []bool{false, true} {
+		arch := New(repo, fs.Track{FS: &missingFS{FS: &fs.Local{}, errorOnOpen: errorOnOpen}}, Options{})
+		_, excluded, err := arch.save(ctx, "/", filepath.Join(tempdir, "testdir"), nil)
+		rtest.OK(t, err)
+		rtest.Assert(t, excluded, "testfile should have been excluded")
+	}
+}