transport-helper, connect: use clean_on_exit to reap children on abnormal exit

When a long-running service (e.g., a source indexer) runs as PID 1
inside a container and repeatedly spawns git, git may in turn spawn
child processes such as git-remote-https or ssh. If git exits abnormally
(e.g., via exit(128) on a transport error), the normal cleanup paths
(disconnect_helper, finish_connect) are bypassed, and these children are
never waited on. The children are reparented to PID 1, which does not
reap them, so they accumulate as zombies over time.

Set clean_on_exit and wait_after_clean on child_process structs in both
transport-helper.c and connect.c so that the existing run-command
cleanup infrastructure handles reaping on any exit path. This avoids
rolling custom atexit handlers that call finish_command(), which could
deadlock if the child is blocked waiting for the parent to close a pipe.

The clean_on_exit mechanism sends SIGTERM first, then waits, ensuring
the child terminates promptly. It also handles signal-based exits, not
just atexit.

Signed-off-by: Andrew Au <cshung@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: cshung

connect.c

@@ -1054,6 +1054,8 @@ static struct child_process *git_proxy_connect(int fd[2], char *host)
 	strvec_push(&proxy->args, port);
 	proxy->in = -1;
 	proxy->out = -1;
+	proxy->clean_on_exit = 1;
+	proxy->wait_after_clean = 1;
 	if (start_command(proxy))
 		die(_("cannot start proxy %s"), git_proxy_command);
 	fd[0] = proxy->out; /* read from proxy stdout */
@@ -1515,6 +1517,8 @@ struct child_process *git_connect(int fd[2], const char *url,
 		}
 		strvec_push(&conn->args, cmd.buf);
 
+		conn->clean_on_exit = 1;
+		conn->wait_after_clean = 1;
 		if (start_command(conn))
 			die(_("unable to fork"));
 

transport-helper.c

@@ -154,6 +154,8 @@ static struct child_process *get_helper(struct transport *transport)
 
 	helper->trace2_child_class = helper->args.v[0]; /* "remote-<name>" */
 
+	helper->clean_on_exit = 1;
+	helper->wait_after_clean = 1;
 	code = start_command(helper);
 	if (code < 0 && errno == ENOENT)
 		die(_("unable to find remote helper for '%s'"), data->name);