Prevent recording output path replacement

recording/recorder.go

@@ -57,18 +57,29 @@ func New(directory string, format string, now time.Time, frameSize int, interval
 	if err := os.MkdirAll(directory, 0755); err != nil {
 		return nil, err
 	}
-	path, err := reservePath(directory, now, format)
+	output, path, err := reserveOutput(directory, now, format)
 	if err != nil {
 		return nil, err
 	}
-	args := ffmpegArgs(format, path)
+	args := ffmpegArgs(format)
 	cmd := exec.Command("ffmpeg", args...)
+	// Pass the reserved file descriptor directly to ffmpeg. The file is never
+	// reopened by pathname, preventing replacement between reservation and use.
+	cmd.ExtraFiles = []*os.File{output}
 	stdin, err := cmd.StdinPipe()
 	if err != nil {
+		_ = output.Close()
 		_ = os.Remove(path)
 		return nil, err
 	}
 	if err := cmd.Start(); err != nil {
+		_ = output.Close()
+		_ = os.Remove(path)
+		return nil, err
+	}
+	if err := output.Close(); err != nil {
+		_ = cmd.Process.Kill()
+		_ = cmd.Wait()
 		_ = os.Remove(path)
 		return nil, err
 	}
@@ -96,7 +107,7 @@ func NormalizeFormat(format string) string {
 	return format
 }
 
-func reservePath(directory string, now time.Time, format string) (string, error) {
+func reserveOutput(directory string, now time.Time, format string) (*os.File, string, error) {
 	for {
 		path := UniquePath(directory, now, format)
 		file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0600)
@@ -104,16 +115,25 @@ func reservePath(directory string, now time.Time, format string) (string, error)
 			continue
 		}
 		if err != nil {
-			return "", err
+			return nil, "", err
 		}
-		if err := file.Close(); err != nil {
-			_ = os.Remove(path)
-			return "", err
-		}
-		return path, nil
+		return file, path, nil
 	}
 }
 
+// reservePath remains available for callers that only need to reserve a name.
+func reservePath(directory string, now time.Time, format string) (string, error) {
+	file, path, err := reserveOutput(directory, now, format)
+	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)
@@ -270,7 +290,7 @@ func writePCM(w io.Writer, samples []int16) error {
 	return err
 }
 
-func ffmpegArgs(format string, path string) []string {
+func ffmpegArgs(format string) []string {
 	args := []string{
 		"-loglevel", "error",
 		"-f", "s16le",
@@ -281,5 +301,5 @@ func ffmpegArgs(format string, path string) []string {
 	if format == FormatOpus {
 		args = append(args, "-c:a", "libopus")
 	}
-	return append(args, "-y", path)
+	return append(args, "-f", format, "pipe:3")
 }