Check rsync destination capacity before copying

internal/rsync/rsync.go

@@ -108,6 +108,68 @@ func (r *Runner) Run(ctx context.Context, runID string, job model.Job, callback
 	return summary, nil
 }
 
+// Estimate performs rsync's complete comparison without writing destination data.
+func (r *Runner) Estimate(ctx context.Context, runID string, job model.Job) (Summary, error) {
+	job.Rsync.DryRun = true
+	args := Arguments(job)
+	for index, arg := range args {
+		if strings.HasPrefix(arg, "--out-format=") {
+			args[index] = "--out-format="
+		}
+	}
+	cmd := exec.CommandContext(ctx, r.Binary, args...)
+	cmd.Env = append(os.Environ(), "LC_ALL=C", "LANG=C")
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+	cmd.Cancel = func() error {
+		if cmd.Process == nil {
+			return os.ErrProcessDone
+		}
+		if err := syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL); err != nil && !errors.Is(err, syscall.ESRCH) {
+			return err
+		}
+		return nil
+	}
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return Summary{}, err
+	}
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		return Summary{}, err
+	}
+	if err := cmd.Start(); err != nil {
+		if errors.Is(err, exec.ErrNotFound) {
+			return Summary{}, fmt.Errorf("environment: rsync binary not found: %w", err)
+		}
+		return Summary{}, err
+	}
+	unregister := r.register(runID, cmd.Process)
+	defer unregister()
+	var summary Summary
+	done := make(chan struct{})
+	go func() {
+		scanOutput(stdout, func(line string) {
+			if value, ok := parseCount(filesPattern, line); ok {
+				summary.Files = value
+			}
+			if value, ok := parseCount(bytesPattern, line); ok {
+				summary.Bytes = value
+			}
+		})
+		close(done)
+	}()
+	errBytes, _ := io.ReadAll(io.LimitReader(stderr, 1024*1024))
+	err = cmd.Wait()
+	<-done
+	if err != nil {
+		if errors.Is(ctx.Err(), context.Canceled) {
+			return Summary{}, fmt.Errorf("cancelled: %w", ctx.Err())
+		}
+		return Summary{}, fmt.Errorf("rsync preflight: %s", strings.TrimSpace(string(errBytes)))
+	}
+	return summary, nil
+}
+
 func Arguments(job model.Job) []string {
 	options := job.Rsync
 	args := []string{"--recursive", "--human-readable", "--info=progress2", "--stats", "--partial", "--out-format=FILE|%n"}