op-challenger: Flush logs after program exits.

The last line emitted by programs typically doesn't include a newline.

op-challenger/fault/cannon/executor.go

@@ -80,8 +80,12 @@ func (e *Executor) GenerateProof(ctx context.Context, dir string, i uint64) erro
 
 func runCmd(ctx context.Context, l log.Logger, binary string, args ...string) error {
 	cmd := exec.CommandContext(ctx, binary, args...)
-	cmd.Stdout = oplog.NewWriter(l, log.LvlInfo)
-	cmd.Stderr = oplog.NewWriter(l, log.LvlError)
+	stdOut := oplog.NewWriter(l, log.LvlInfo)
+	defer stdOut.Close()
+	stdErr := oplog.NewWriter(l, log.LvlError)
+	defer stdErr.Close()
+	cmd.Stdout = stdOut
+	cmd.Stderr = stdErr
 	return cmd.Run()
 }
 

op-service/log/writer.go

@@ -50,3 +50,13 @@ func (w *Writer) Write(b []byte) (int, error) {
 	}
 	return len(b), nil
 }
+
+func (w *Writer) Close() error {
+	w.lock.Lock()
+	defer w.lock.Unlock()
+	if len(w.pending) > 0 {
+		w.log(string(w.pending))
+		w.pending = nil
+	}
+	return nil
+}