Reserve recording paths atomically

recording/recorder.go

@@ -57,14 +57,19 @@ func New(directory string, format string, now time.Time, frameSize int, interval
 	if err := os.MkdirAll(directory, 0755); err != nil {
 		return nil, err
 	}
-	path := UniquePath(directory, now, format)
+	path, err := reservePath(directory, now, format)
+	if err != nil {
+		return nil, err
+	}
 	args := ffmpegArgs(format, path)
 	cmd := exec.Command("ffmpeg", args...)
 	stdin, err := cmd.StdinPipe()
 	if err != nil {
+		_ = os.Remove(path)
 		return nil, err
 	}
 	if err := cmd.Start(); err != nil {
+		_ = os.Remove(path)
 		return nil, err
 	}
 	recorder := &Recorder{
@@ -91,6 +96,24 @@ func NormalizeFormat(format string) string {
 	return format
 }
 
+func reservePath(directory string, now time.Time, format string) (string, error) {
+	for {
+		path := UniquePath(directory, now, format)
+		file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0600)
+		if os.IsExist(err) {
+			continue
+		}
+		if err != nil {
+			return "", err
+		}
+		if err := file.Close(); err != nil {
+			_ = os.Remove(path)
+			return "", err
+		}
+		return path, nil
+	}
+}
+
 func UniquePath(directory string, now time.Time, format string) string {
 	base := fmt.Sprintf("barnard-recording-%s", now.Format("20060102-150405"))
 	path := filepath.Join(directory, base+"."+format)

recording/recorder_test.go

@@ -4,6 +4,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"sync"
 	"testing"
 	"time"
 )
@@ -58,6 +59,39 @@ func TestUniquePathAvoidsCollision(t *testing.T) {
 	}
 }
 
+func TestReservePathPreventsConcurrentRecordingCollisions(t *testing.T) {
+	dir := t.TempDir()
+	now := time.Date(2026, 5, 14, 12, 30, 0, 0, time.Local)
+	paths := make(chan string, 2)
+	errs := make(chan error, 2)
+	var wg sync.WaitGroup
+	for range 2 {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			path, err := reservePath(dir, now, "flac")
+			if err != nil {
+				errs <- err
+				return
+			}
+			paths <- path
+		}()
+	}
+	wg.Wait()
+	close(paths)
+	close(errs)
+	for err := range errs {
+		t.Fatal(err)
+	}
+	var reserved []string
+	for path := range paths {
+		reserved = append(reserved, path)
+	}
+	if len(reserved) != 2 || reserved[0] == reserved[1] {
+		t.Fatalf("reserved paths = %#v", reserved)
+	}
+}
+
 func TestNormalizeStereoFrame(t *testing.T) {
 	mono := NormalizeStereoFrame([]int16{1, -2}, 3)
 	wantMono := []int16{1, 1, -2, -2, 0, 0}