backup: implement --skip-if-unchanged

internal/archiver/archiver.go

@@ -767,6 +767,8 @@ type SnapshotOptions struct {
 	Time           time.Time
 	ParentSnapshot *restic.Snapshot
 	ProgramVersion string
+	// SkipIfUnchanged omits the snapshot creation if it is identical to the parent snapshot.
+	SkipIfUnchanged bool
 }
 
 // loadParentTree loads a tree referenced by snapshot id. If id is null, nil is returned.
@@ -880,6 +882,13 @@ func (arch *Archiver) Snapshot(ctx context.Context, targets []string, opts Snaps
 		return nil, restic.ID{}, nil, err
 	}
 
+	if opts.ParentSnapshot != nil && opts.SkipIfUnchanged {
+		ps := opts.ParentSnapshot
+		if ps.Tree != nil && rootTreeID.Equal(*ps.Tree) {
+			return nil, restic.ID{}, arch.summary, nil
+		}
+	}
+
 	sn, err := restic.NewSnapshot(targets, opts.Tags, opts.Hostname, opts.Time)
 	if err != nil {
 		return nil, restic.ID{}, nil, err

internal/ui/backup/json.go

@@ -164,6 +164,11 @@ func (b *JSONProgress) ReportTotal(start time.Time, s archiver.ScanStats) {
 
 // Finish prints the finishing messages.
 func (b *JSONProgress) Finish(snapshotID restic.ID, start time.Time, summary *archiver.Summary, dryRun bool) {
+	id := ""
+	// empty if snapshot creation was skipped
+	if !snapshotID.IsNull() {
+		id = snapshotID.String()
+	}
 	b.print(summaryOutput{
 		MessageType:         "summary",
 		FilesNew:            summary.Files.New,
@@ -179,7 +184,7 @@ func (b *JSONProgress) Finish(snapshotID restic.ID, start time.Time, summary *ar
 		TotalFilesProcessed: summary.Files.New + summary.Files.Changed + summary.Files.Unchanged,
 		TotalBytesProcessed: summary.ProcessedBytes,
 		TotalDuration:       time.Since(start).Seconds(),
-		SnapshotID:          snapshotID.String(),
+		SnapshotID:          id,
 		DryRun:              dryRun,
 	})
 }
@@ -235,6 +240,6 @@ type summaryOutput struct {
 	TotalFilesProcessed uint    `json:"total_files_processed"`
 	TotalBytesProcessed uint64  `json:"total_bytes_processed"`
 	TotalDuration       float64 `json:"total_duration"` // in seconds
-	SnapshotID          string  `json:"snapshot_id"`
+	SnapshotID          string  `json:"snapshot_id,omitempty"`
 	DryRun              bool    `json:"dry_run,omitempty"`
 }

internal/ui/backup/text.go

@@ -126,7 +126,7 @@ func (b *TextProgress) Reset() {
 }
 
 // Finish prints the finishing messages.
-func (b *TextProgress) Finish(_ restic.ID, start time.Time, summary *archiver.Summary, dryRun bool) {
+func (b *TextProgress) Finish(id restic.ID, start time.Time, summary *archiver.Summary, dryRun bool) {
 	b.P("\n")
 	b.P("Files:       %5d new, %5d changed, %5d unmodified\n", summary.Files.New, summary.Files.Changed, summary.Files.Unchanged)
 	b.P("Dirs:        %5d new, %5d changed, %5d unmodified\n", summary.Dirs.New, summary.Dirs.Changed, summary.Dirs.Unchanged)
@@ -145,4 +145,12 @@ func (b *TextProgress) Finish(_ restic.ID, start time.Time, summary *archiver.Su
 		ui.FormatBytes(summary.ProcessedBytes),
 		ui.FormatDuration(time.Since(start)),
 	)
+
+	if !dryRun {
+		if id.IsNull() {
+			b.P("skipped creating snapshot\n")
+		} else {
+			b.P("snapshot %s saved\n", id.Str())
+		}
+	}
 }