fix(audio): stop reading ffprobe output past a cap and call the file unreadable (#49)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/adapters/audio/ffprobe.go

@@ -4,6 +4,7 @@
 package audio
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
@@ -19,6 +20,45 @@ import (
 type FFprobe struct {
 	// Bin is the ffprobe executable ("ffprobe" resolves via PATH).
 	Bin string
+	// MaxOutput bounds how much of ffprobe's stdout is read; zero means DefaultMaxOutput.
+	MaxOutput int
+}
+
+// DefaultMaxOutput is far above the narrowed probe output of a legitimate file (a few
+// hundred bytes) and far below what a hostile one can make ffprobe print.
+const DefaultMaxOutput = 1 << 20
+
+// maxStderr is how much of ffprobe's stderr is kept for the error message.
+const maxStderr = 4096
+
+// errOutputCap is the write error that stops ffprobe once it prints past the cap.
+var errOutputCap = errors.New("audio: ffprobe output exceeds the cap")
+
+// capWriter collects up to max bytes and then refuses further writes, which closes the
+// pipe and ends the process instead of buffering whatever it prints.
+type capWriter struct {
+	buf bytes.Buffer
+	max int
+}
+
+func (w *capWriter) Write(p []byte) (int, error) {
+	if w.buf.Len()+len(p) > w.max {
+		return 0, errOutputCap
+	}
+	return w.buf.Write(p)
+}
+
+// headWriter keeps the first max bytes and silently drops the rest.
+type headWriter struct {
+	buf bytes.Buffer
+	max int
+}
+
+func (w *headWriter) Write(p []byte) (int, error) {
+	if room := w.max - w.buf.Len(); room > 0 {
+		w.buf.Write(p[:min(room, len(p))])
+	}
+	return len(p), nil
 }
 
 // AllowedDemuxers are the input formats an upload may be. The input is
@@ -64,18 +104,26 @@ func (f *FFprobe) Probe(ctx context.Context, path string) (uploads.Probe, error)
 	args = append(args, "-print_format", "json", "-show_entries", probeEntries, path)
 	cmd := exec.CommandContext(ctx, bin, args...)
 	cmd.WaitDelay = killGrace
-	out, err := cmd.Output()
+	maxOut := f.MaxOutput
-	if err != nil {
+	if maxOut <= 0 {
+		maxOut = DefaultMaxOutput
+	}
+	stdout, stderr := &capWriter{max: maxOut}, &headWriter{max: maxStderr}
+	cmd.Stdout, cmd.Stderr = stdout, stderr
+	if err := cmd.Run(); err != nil {
 		if ctxErr := ctx.Err(); ctxErr != nil {
 			return uploads.Probe{}, fmt.Errorf("audio: ffprobe: %w", ctxErr)
 		}
+		if errors.Is(err, errOutputCap) {
+			return uploads.Probe{}, fmt.Errorf("%w: ffprobe printed more than %d bytes", uploads.ErrUnreadable, maxOut)
+		}
 		var exit *exec.ExitError
 		if errors.As(err, &exit) {
-			return uploads.Probe{}, fmt.Errorf("%w: ffprobe: %s", uploads.ErrUnreadable, exit.Stderr)
+			return uploads.Probe{}, fmt.Errorf("%w: ffprobe: %s", uploads.ErrUnreadable, stderr.buf.String())
 		}
 		return uploads.Probe{}, fmt.Errorf("audio: run ffprobe: %w", err)
 	}
-	p, err := ParseProbe(out)
+	p, err := ParseProbe(stdout.buf.Bytes())
 	if err != nil {
 		return uploads.Probe{}, fmt.Errorf("audio: ffprobe output: %w", err)
 	}