backup: store but warn if extended metadata for item is incomplete

Files were not included in the backup if the extended metadata for the
file could not be read. This is rather drastic. Instead settle on
returning a warning but still including the file in the backup.

changelog/unreleased/pull-4977

@@ -0,0 +1,15 @@
+Change: let `backup` store files with incomplete metadata
+
+If restic failed to read the extended metadata for a file or folder while
+creating a backup, then the file or folder was not included in the resulting
+snapshot. Instead, only a warning message was printed along with exiting
+with exit code 3.
+
+Now, restic also includes items for which the extended metadata could not
+be read in a snapshot. The warning message has been changed to read
+```
+incomplete metadata for /path/to/file: details on error
+```
+
+https://github.com/restic/restic/issues/4953
+https://github.com/restic/restic/pull/4977

internal/archiver/archiver.go

@@ -263,7 +263,8 @@ func (arch *Archiver) nodeFromFileInfo(snPath, filename string, fi os.FileInfo,
 	// overwrite name to match that within the snapshot
 	node.Name = path.Base(snPath)
 	if err != nil {
-		return node, fmt.Errorf("nodeFromFileInfo %v: %w", filename, err)
+		err = fmt.Errorf("incomplete metadata for %v: %w", filename, err)
+		return node, arch.error(filename, err)
 	}
 	return node, err
 }

internal/archiver/archiver_test.go

@@ -3,6 +3,7 @@ package archiver
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"io"
 	"os"
 	"path/filepath"
@@ -2338,3 +2339,28 @@ func TestRacyFileSwap(t *testing.T) {
 		t.Errorf("Save() excluded the node, that's unexpected")
 	}
 }
+
+func TestMetadataBackupErrorFiltering(t *testing.T) {
+	tempdir := t.TempDir()
+	repo := repository.TestRepository(t)
+
+	filename := filepath.Join(tempdir, "file")
+	rtest.OK(t, os.WriteFile(filename, []byte("example"), 0o600))
+	fi, err := os.Stat(filename)
+	rtest.OK(t, err)
+
+	arch := New(repo, fs.Local{}, Options{})
+
+	var filteredErr error
+	replacementErr := fmt.Errorf("replacement")
+	arch.Error = func(item string, err error) error {
+		filteredErr = err
+		return replacementErr
+	}
+
+	// check that errors from reading extended metadata are properly filtered
+	node, err := arch.nodeFromFileInfo("file", filename+"invalid", fi, false)
+	rtest.Assert(t, node != nil, "node is missing")
+	rtest.Assert(t, err == replacementErr, "expected %v got %v", replacementErr, err)
+	rtest.Assert(t, filteredErr != nil, "missing inner error")
+}