Re: [PATCH v2 5/8] run-command: add an "ungroup" option to run_process_parallel()

+ * If the "ungroup" option isn't specified the callbacks will get a
+ * pointer to a "struct strbuf *out", and must not write to stdout or
+ * stderr as such output will mess up the output of the other parallel
+ * processes. If "ungroup" option is specified callbacks will get a
+ * NULL "struct strbuf *out" parameter, and are responsible for
+ * emitting their own output, including dealing with any race
+ * conditions due to writing in parallel to stdout and stderr.
  */
 int run_processes_parallel(struct run_process_parallel_opts *opts);
 

diff --git a/t/helper/test-run-command.c b/t/helper/test-run-command.c
index 56a806f228b..986acbce5f2 100644
--- a/t/helper/test-run-command.c
+++ b/t/helper/test-run-command.c

@@ -31,7 +31,11 @@ static int parallel_next(struct child_process *cp,
 		return 0;
 
 	strvec_pushv(&cp->args, d->args.v);
-	strbuf_addstr(err, "preloaded output of a child\n");
+	if (err)
+		strbuf_addstr(err, "preloaded output of a child\n");
+	else
+		fprintf(stderr, "preloaded output of a child\n");
+