run-command: add note about forking and threading

All non-Async-Signal-Safe functions (e.g. malloc and die) were removed
between 'fork' and 'exec' in start_command in order to avoid potential
deadlocking when forking while multiple threads are running.  This
deadlocking is possible when a thread (other than the one forking) has
acquired a lock and didn't get around to releasing it before the fork.
This leaves the lock in a locked state in the resulting process with no
hope of it ever being released.

Add a note describing this potential pitfall before the call to 'fork()'
so people working in this section of the code know to only use
Async-Signal-Safe functions in the child process.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -537,6 +537,15 @@ fail_pipe:
 	prepare_cmd(&argv, cmd);
 	childenv = prep_childenv(cmd->env);
 
+	/*
+	 * NOTE: In order to prevent deadlocking when using threads special
+	 * care should be taken with the function calls made in between the
+	 * fork() and exec() calls.  No calls should be made to functions which
+	 * require acquiring a lock (e.g. malloc) as the lock could have been
+	 * held by another thread at the time of forking, causing the lock to
+	 * never be released in the child process.  This means only
+	 * Async-Signal-Safe functions are permitted in the child.
+	 */
 	cmd->pid = fork();
 	failed_errno = errno;
 	if (!cmd->pid) {