restic: a failed command reports the tail of its output, not only its exit status

Issue 1585: the backup before a nats move failed with only 'sh: exit status 1'.

Claude-Session: https://claude.ai/code/session_01JG73KG7AfSdh6FP8VrBqCn

modules/restic/cmd/restic-backups/backups.go

@@ -61,25 +61,37 @@ func execRunner(ctx context.Context, name string, args ...string) (string, error
 	cmd := exec.CommandContext(ctx, program, argv...)
 	cmd.Stdout, cmd.Stderr = &stdout, &stderr
 	if err := cmd.Run(); err != nil {
-		said := strings.TrimSpace(stderr.String())
-		if said == "" {
-			said = strings.TrimSpace(stdout.String())
-		}
-		if program == "sudo" && strings.HasPrefix(said, "sudo:") {
-			return "", fmt.Errorf("%s needs root and the runtime's account may not run it without a prompt: %s", name, said)
-		}
-		lines := strings.Split(said, "\n")
-		if len(lines) > 3 {
-			lines = lines[len(lines)-3:]
-		}
-		if said == "" {
-			return "", fmt.Errorf("%s: %w", name, err)
-		}
-		return "", errors.New(strings.Join(lines, " / "))
+		return "", failure(program, name, err, stdout.String(), stderr.String())
 	}
 	return stdout.String(), nil
 }
 
+// failureTail bounds what a failed command's output adds to the failure it reports: its last 2 KB.
+const failureTail = 2048
+
+var secretLooking = regexp.MustCompile(`(?i)((?:pass(?:word|wd)?|secret|token|api[_-]?key)\s*[=:]\s*)\S+`)
+
+// failure is the error of a command that exited badly: its last lines of stderr (stdout when stderr is
+// empty), bounded to failureTail bytes, with what looks like a secret blotted out, so the report of a
+// failed dump says why and not only that it exited. With nothing printed it keeps the exit status.
+func failure(program, name string, err error, stdout, stderr string) error {
+	said := strings.TrimSpace(stderr)
+	if said == "" {
+		said = strings.TrimSpace(stdout)
+	}
+	if program == "sudo" && strings.HasPrefix(said, "sudo:") {
+		return fmt.Errorf("%s needs root and the runtime's account may not run it without a prompt: %s", name, said)
+	}
+	if said == "" {
+		return fmt.Errorf("%s: %w (it printed nothing)", name, err)
+	}
+	if len(said) > failureTail {
+		said = "..." + said[len(said)-failureTail:]
+	}
+	said = secretLooking.ReplaceAllString(said, "${1}***")
+	return fmt.Errorf("%s: %v: %s", name, err, strings.Join(strings.Split(said, "\n"), " / "))
+}
+
 // Declared is what one module declared.
 type Declared struct {
 	Module string   `json:"module"`

modules/restic/cmd/restic-backups/backups_test.go

@@ -333,3 +333,20 @@ func TestARestoreBesideAnotherAccountsDirectoryGoesToTheState(t *testing.T) {
 	}
 	_ = ran
 }
+
+func TestAFailedCommandReportsTheTailOfWhatItPrintedWithSecretsBlotted(t *testing.T) {
+	exit := errors.New("exit status 1")
+	long := strings.Repeat("x", 5000) + "\npg_dump: error: password=hunter2 refused"
+	for _, c := range []struct{ name, program, out, errOut, want, not string }{
+		{"stderr", "sh", "", "pg_dump: connection refused\nsecond", "sh: exit status 1: pg_dump: connection refused / second", ""},
+		{"stdout when stderr is empty", "sh", "dump failed\n", "", "sh: exit status 1: dump failed", ""},
+		{"nothing printed", "sh", "", "", "sh: exit status 1 (it printed nothing)", ""},
+		{"bounded and blotted", "sh", "", long, "password=*** refused", "hunter2"},
+		{"sudo prompt", "sudo", "", "sudo: a password is required", "needs root", ""},
+	} {
+		got := failure(c.program, "sh", exit, c.out, c.errOut).Error()
+		if !strings.Contains(got, c.want) || (c.not != "" && strings.Contains(got, c.not)) || len(got) > failureTail+100 {
+			t.Errorf("%s: got %q", c.name, got)
+		}
+	}
+}