Do not prune after a backup that skipped unreadable files (closes #382)

snapshot create --skip-errors --prune pruned even when files were
skipped, so by default it deleted the previous snapshot of that name,
the only one still holding a file that had become unreadable, along
with blobs only that snapshot referenced.

The prune is now skipped exactly when the run exits non-zero for
entries --skip-errors could not read or list: older snapshots are
kept, a warning says why, and the run still exits non-zero. A file
removed during the run does not count, so such a run still prunes.
The README's --prune text and the flag's help say so.

Model: opus-5-5

internal/cli/snapshot.go

@@ -132,7 +132,8 @@ on Linux), then /etc/vaultik/config.yml.`,
 		"Run in cron mode (silent unless warning or error)")
 	cmd.Flags().BoolVar(&opts.Prune, "prune", false,
 		"After backup, drop older snapshots of the same name and remove "+
-			"orphaned blobs")
+			"orphaned blobs (not after a backup that skipped unreadable "+
+			"files)")
 	cmd.Flags().StringVar(&opts.KeepNewerThan, "keep-newer-than", "",
 		"With --prune: keep snapshots newer than this duration "+
 			"(e.g. 30d, 4w, 6mo, 1y; m is minutes, mo is months) "+

internal/vaultik/snapshot.go

@@ -153,7 +153,15 @@ func (v *Vaultik) CreateSnapshot(opts *SnapshotCreateOptions) error {
 			len(snapshotNames), v.UI.Duration(time.Since(overallStartTime)))
 	}
 
-	if opts.Prune {
+	switch {
+	case opts.Prune && filesUnreadable > 0:
+		// An older snapshot may be the only one that still holds a file
+		// this run could not read.
+		log.Warn("Skipping post-backup prune",
+			"files_unreadable", filesUnreadable)
+		v.UI.Warningf("Skipped the post-backup prune because %d file(s) "+
+			"could not be read; older snapshots are kept.", filesUnreadable)
+	case opts.Prune:
 		err = v.runPostBackupPrune(snapshotNames, opts.KeepNewerThan)
 		if err != nil {
 			return fmt.Errorf("%w: %w", ErrPostBackupPruneFailed, err)

internal/vaultik/snapshot_skip_errors_test.go

@@ -215,3 +215,69 @@ func TestBackupIgnoresExcludedFileRemovedBeforeWalkReachesIt(t *testing.T) {
 	assert.Contains(t, env.out.String(), filesLine(1, 1, 0))
 	assert.Contains(t, env.out.String(), "Finished successfully.")
 }
+
+// A backup under --skip-errors --prune that skipped a file it could not
+// read does not prune: the previous snapshot, the only one that holds
+// the file, is kept, and the output says the prune was skipped. See
+// https://git.eeqj.de/sneak/vaultik/issues/382.
+func TestSkipErrorsBackupDoesNotPruneWhenItSkips(t *testing.T) {
+	log.Initialize(log.Config{})
+	t.Parallel()
+
+	env := newDeletedCountEnv(t, map[string][]byte{
+		"other.txt":   []byte("other"),
+		"skipped.txt": []byte("skipped"),
+	})
+	env.setSnapshot("photos", "")
+	env.backUp(t, "photos")
+
+	// An unchanged file is not read again, so the file is changed before
+	// it becomes unreadable.
+	skipped := filepath.Join(env.src, "skipped.txt")
+	require.NoError(t, os.WriteFile(skipped, []byte("changed"), 0o600))
+	env.v.Fs = &openFailFs{path: skipped, err: os.ErrPermission}
+
+	env.out.Reset()
+	err := env.v.CreateSnapshot(&vaultik.SnapshotCreateOptions{
+		Prune:      true,
+		SkipErrors: true,
+		Snapshots:  []string{"photos"},
+	})
+	require.ErrorIs(t, err, vaultik.ErrFilesUnreadable)
+	assert.Contains(t, env.out.String(),
+		"Skipped the post-backup prune because 1 file(s) could not be read")
+	assert.NotContains(t, env.out.String(), "Running post-backup prune.")
+
+	snapshots, err := env.v.Repositories.Snapshots.ListRecent(
+		context.Background(), listRecentTestLimit)
+	require.NoError(t, err)
+	assert.Len(t, snapshots, 2, "the previous snapshot is kept")
+}
+
+// A file removed during a backup under --skip-errors --prune is not a
+// skipped file, so the backup prunes as usual. See
+// https://git.eeqj.de/sneak/vaultik/issues/382.
+func TestSkipErrorsBackupPrunesWhenAFileIsRemovedDuringIt(t *testing.T) {
+	log.Initialize(log.Config{})
+	t.Parallel()
+
+	env := newDeletedCountEnv(t, map[string][]byte{
+		"other.txt":   []byte("other"),
+		"removed.txt": []byte("removed"),
+	})
+	env.setSnapshot("photos", "")
+	env.backUp(t, "photos")
+
+	env.v.Fs = &removeOnLookupFs{path: filepath.Join(env.src, "removed.txt")}
+
+	require.NoError(t, env.v.CreateSnapshot(&vaultik.SnapshotCreateOptions{
+		Prune:      true,
+		SkipErrors: true,
+		Snapshots:  []string{"photos"},
+	}))
+
+	snapshots, err := env.v.Repositories.Snapshots.ListRecent(
+		context.Background(), listRecentTestLimit)
+	require.NoError(t, err)
+	assert.Len(t, snapshots, 1, "the previous snapshot is pruned")
+}