Add "human" error from exec source

internal/exec/exec.go

@@ -5,6 +5,7 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
+	"io"
 	"net/url"
 	"os"
 	"os/exec"
@@ -102,14 +103,22 @@ func handlePipe(_ string, cmd *exec.Cmd, query url.Values) (core.Producer, error
 }
 
 func handleRTSP(url string, cmd *exec.Cmd, path string) (core.Producer, error) {
+	stderr := limitBuffer{buf: make([]byte, 512)}
+
+	if cmd.Stderr != nil {
+		cmd.Stderr = io.MultiWriter(cmd.Stderr, &stderr)
+	} else {
+		cmd.Stderr = &stderr
+	}
+
 	if log.Trace().Enabled() {
 		cmd.Stdout = os.Stdout
 	}
 
-	ch := make(chan core.Producer)
+	waiter := make(chan core.Producer)
 
 	waitersMu.Lock()
-	waiters[path] = ch
+	waiters[path] = waiter
 	waitersMu.Unlock()
 
 	defer func() {
@@ -127,16 +136,9 @@ func handleRTSP(url string, cmd *exec.Cmd, path string) (core.Producer, error) {
 		return nil, err
 	}
 
-	chErr := make(chan error)
-
+	done := make(chan error, 1)
 	go func() {
-		err := cmd.Wait()
-		// unblocking write to channel
-		select {
-		case chErr <- err:
-		default:
-			log.Trace().Str("url", url).Msg("[exec] close")
-		}
+		done <- cmd.Wait()
 	}()
 
 	select {
@@ -144,9 +146,10 @@ func handleRTSP(url string, cmd *exec.Cmd, path string) (core.Producer, error) {
 		_ = cmd.Process.Kill()
 		log.Error().Str("url", url).Msg("[exec] timeout")
 		return nil, errors.New("timeout")
-	case err := <-chErr:
-		return nil, fmt.Errorf("exec: %s", err)
-	case prod := <-ch:
+	case <-done:
+		// limit message size
+		return nil, errors.New("exec: " + stderr.String())
+	case prod := <-waiter:
 		log.Debug().Stringer("launch", time.Since(ts)).Msg("[exec] run")
 		return prod, nil
 	}
@@ -159,3 +162,22 @@ var (
 	waiters   = map[string]chan core.Producer{}
 	waitersMu sync.Mutex
 )
+
+type limitBuffer struct {
+	buf []byte
+	n   int
+}
+
+func (l *limitBuffer) String() string {
+	if l.n == len(l.buf) {
+		return string(l.buf) + "..."
+	}
+	return string(l.buf[:l.n])
+}
+
+func (l *limitBuffer) Write(p []byte) (int, error) {
+	if l.n < cap(l.buf) {
+		l.n += copy(l.buf[l.n:], p)
+	}
+	return len(p), nil
+}