Print progress at once off a terminal, keep stderr to one writer (closes #13)

On a pipe, a file or /dev/null each scan phase now prints its zero-state
line the moment it starts instead of after its first finished item, so
a slow first read under cron no longer leaves the log silent. Terminal
detection uses golang.org/x/term, now a direct dependency listed in the
README; the old character-device test took /dev/null for a terminal.
The walk spinner no longer redraws from the library's own goroutine; it
advances only as items complete, so a redraw cannot garble a warning.
Building the bar moved into newBar so the terminal display can be
tested without a terminal.

Lint suppressed: gosec on two test file opens, paralleltest on three
tests that replace os.Stderr.

Model: opus-5-5

README.md

@@ -113,7 +113,8 @@ Goals, in order:
     `sfdupes`.
 -   Dependencies: standard library, `github.com/spf13/cobra` for the
     CLI, **one progress-bar library**
-    (`github.com/schollz/progressbar/v3`), and **one SQLite driver**
+    (`github.com/schollz/progressbar/v3`), `golang.org/x/term` to tell
+    whether stderr is a terminal, and **one SQLite driver**
     (`modernc.org/sqlite`, pure Go, so builds keep cgo disabled).
     `github.com/spf13/viper` is permitted if configuration-file support
     is ever needed, but is not currently used. No other third-party
@@ -533,10 +534,13 @@ hash: [12345/98765] 12% |████      | 92 files/s elapsed 2:32 eta 17:54
 
 Additional requirements:
 
--   When stderr is not a TTY, do not emit ANSI redraws: print a plain
+-   When stderr is not a terminal (a pipe, a file, `/dev/null`), do not
-    one-line progress update no more often than every 5 seconds instead.
+    emit ANSI redraws: print a plain one-line progress update the moment
+    each phase starts, then no more often than every 5 seconds.
 -   Progress updates are driven from the main goroutine and must be
-    non-blocking with respect to the worker pool.
+    non-blocking with respect to the worker pool. Nothing redraws in the
+    background (the spinner advances only as items complete), so a
+    warning printed during a phase always lands on its own line.
 -   `report` and `trees` modes need no progress display, only their
     stderr summaries.
 

TODO.md

@@ -29,6 +29,9 @@
 
 # Completed Steps
 
+- progress prints at once on a non-terminal, uses a real terminal test,
+  and has a single writer to stderr (2026-10-03,
+  https://git.eeqj.de/sneak/sfdupes/issues/13)
 - stamp the git tag or short commit in a plain `docker build .`
   instead of `dev` (2026-10-02, branch `next`, closes
   https://git.eeqj.de/sneak/sfdupes/issues/67): `.dockerignore` now

go.mod

@@ -5,6 +5,7 @@ go 1.25.7
 require (
 	github.com/schollz/progressbar/v3 v3.19.1
 	github.com/spf13/cobra v1.10.2
+	golang.org/x/term v0.44.0
 	modernc.org/sqlite v1.54.0
 )
 
@@ -19,7 +20,6 @@ require (
 	github.com/rivo/uniseg v0.4.7 // indirect
 	github.com/spf13/pflag v1.0.9 // indirect
 	golang.org/x/sys v0.46.0 // indirect
-	golang.org/x/term v0.44.0 // indirect
 	modernc.org/libc v1.74.1 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect

progress.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/schollz/progressbar/v3"
+	"golang.org/x/term"
 )
 
 // plainInterval is the minimum time between progress lines when stderr
@@ -24,24 +25,21 @@ const percentScale = 100
 
 // stderrIsTTY reports whether stderr is attached to a terminal.
 func stderrIsTTY() bool {
-	fi, err := os.Stderr.Stat()
+	return term.IsTerminal(int(os.Stderr.Fd()))
-	if err != nil {
-		return false
-	}
-
-	return fi.Mode()&os.ModeCharDevice != 0
 }
 
 // progress renders one scan pass's progress on stderr. On a TTY it
 // delegates to the progressbar library (spinner style when the total is
 // unknown, full bar with count/percent/rate/elapsed/ETA otherwise). When
 // stderr is not a TTY it emits no ANSI redraws: it prints a plain
-// one-line update no more often than every plainInterval.
+// one-line update as the pass starts, then no more often than every
+// plainInterval.
 //
-// All methods must be called from the main goroutine only. A nil
+// All methods must be called from the main goroutine only; nothing
-// *progress is a valid no-display receiver: every method is a no-op,
+// else writes to stderr while a pass runs. A nil *progress is a valid
-// so batched database flushes during the streaming pass can reuse the
+// no-display receiver: every method is a no-op, so batched database
-// update-pass helpers without rendering anything.
+// flushes during the streaming pass can reuse the update-pass helpers
+// without rendering anything.
 type progress struct {
 	label string
 	total int64 // -1 when unknown (walk pass)
@@ -53,10 +51,22 @@ type progress struct {
 
 func newProgress(label string, total int64) *progress {
 	p := &progress{label: label, total: total, start: time.Now()}
-	if !stderrIsTTY() {
+	if stderrIsTTY() {
+		p.bar = newBar(label, total)
+
 		return p
 	}
 
+	// Print the zero state at once: the first item may take minutes,
+	// and a pass must never look hung.
+	p.last = p.start
+	fmt.Fprintln(os.Stderr, p.plainLine())
+
+	return p
+}
+
+// newBar builds the TTY display for newProgress.
+func newBar(label string, total int64) *progressbar.ProgressBar {
 	opts := []progressbar.Option{
 		progressbar.OptionSetWriter(os.Stderr),
 		progressbar.OptionSetDescription(label),
@@ -78,12 +88,14 @@ func newProgress(label string, total int64) *progress {
 		opts = append(opts,
 			progressbar.OptionSetPredictTime(false),
 			progressbar.OptionSpinnerType(walkSpinnerType),
+			// Advance the spinner only on Add. The library's default
+			// redraws it from its own goroutine every 100ms, which
+			// garbles a warning printed at the same moment.
+			progressbar.OptionSetSpinnerChangeInterval(0),
 		)
 	}
 
-	p.bar = progressbar.NewOptions64(total, opts...)
+	return progressbar.NewOptions64(total, opts...)
-
-	return p
 }
 
 // increment records one completed item and refreshes the display.

progress_test.go

@@ -0,0 +1,150 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+	"time"
+)
+
+// spinnerIdle outlasts the 100ms interval at which the progressbar
+// library redraws a spinner from its own goroutine unless told not to.
+const spinnerIdle = 300 * time.Millisecond
+
+// captureStderr points os.Stderr at a file for the rest of the test and
+// returns a function reading back everything written to it.
+func captureStderr(t *testing.T) func() string {
+	t.Helper()
+
+	path := filepath.Join(t.TempDir(), "stderr")
+
+	f, err := os.Create(path) //nolint:gosec // test-controlled path
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	saved := os.Stderr
+	os.Stderr = f
+
+	t.Cleanup(func() {
+		os.Stderr = saved
+
+		_ = f.Close()
+	})
+
+	return func() string {
+		b, err := os.ReadFile(path) //nolint:gosec // test-controlled path
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		return string(b)
+	}
+}
+
+//nolint:paralleltest // replaces the process-wide os.Stderr
+func TestStderrIsTTYFalseForNonTerminals(t *testing.T) {
+	r, pipe, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	regular, err := os.Create(filepath.Join(t.TempDir(), "stderr"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	devNull, err := os.OpenFile(os.DevNull, os.O_WRONLY, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	saved := os.Stderr
+
+	t.Cleanup(func() {
+		os.Stderr = saved
+
+		for _, f := range []*os.File{r, pipe, regular, devNull} {
+			_ = f.Close()
+		}
+	})
+
+	cases := map[string]*os.File{
+		"a pipe":         pipe,
+		"a regular file": regular,
+		os.DevNull:       devNull,
+	}
+
+	for name, f := range cases {
+		os.Stderr = f
+
+		if stderrIsTTY() {
+			t.Errorf("stderrIsTTY() = true with stderr on %s", name)
+		}
+	}
+}
+
+// TestNewProgressPrintsBeforeFirstItem checks that each pass shows its
+// zero state the moment it starts when stderr is not a terminal, and
+// that the next line still waits for plainInterval.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestNewProgressPrintsBeforeFirstItem(t *testing.T) {
+	stderr := captureStderr(t)
+
+	newProgress("walk", -1).increment()
+	newProgress("hash", 10).increment()
+
+	want := "walk: 0 files, elapsed 0s\n" +
+		"hash: [0/10] 0% 0 files/s elapsed 0s eta ?\n"
+	if got := stderr(); got != want {
+		t.Errorf("stderr = %q, want %q", got, want)
+	}
+}
+
+// TestProgressWarningsOnOwnLines drives the terminal display of the walk
+// pass: every warning must land on a line of its own between redraws of
+// the spinner, and nothing may redraw while no method is being called,
+// or a warning could be garbled by a redraw from another goroutine.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestProgressWarningsOnOwnLines(t *testing.T) {
+	stderr := captureStderr(t)
+
+	p := &progress{
+		label: "walk", total: -1, start: time.Now(),
+		bar: newBar("walk", -1),
+	}
+
+	for _, w := range []string{"first", "second", "third"} {
+		p.increment()
+		p.warnf("warning %s", w)
+	}
+
+	p.increment()
+
+	before := stderr()
+
+	time.Sleep(spinnerIdle)
+
+	if after := stderr(); after != before {
+		t.Errorf("stderr written while idle: %q", after[len(before):])
+	}
+
+	// A terminal shows each line as the text after its last carriage
+	// return.
+	var warnings []string
+
+	for line := range strings.SplitSeq(before, "\n") {
+		if strings.Contains(line, "warning") {
+			warnings = append(warnings, line[strings.LastIndex(line, "\r")+1:])
+		}
+	}
+
+	want := []string{"warning first", "warning second", "warning third"}
+	if !slices.Equal(warnings, want) {
+		t.Errorf("warning lines = %q, want %q", warnings, want)
+	}
+}