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.

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")
+}