[PATCH 5/5] run-command: add note about forking and threading

[Brandon Williams <bmwill@google.com>]

Allocation was pushed before forking 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.

Signed-off-by: Brandon Williams <bmwill@google.com>
---
 run-command.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/run-command.c b/run-command.c
index 84c63b209..2b3249de4 100644
--- a/run-command.c
+++ b/run-command.c
@@ -458,6 +458,14 @@ int start_command(struct child_process *cmd)
 		argv_array_pushv(&argv, cmd->argv);
 	}
 
+	/*
+	 * 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.
+	 */
 	cmd->pid = fork();
 	failed_errno = errno;
 	if (!cmd->pid) {
-- 
2.12.2.715.g7642488e1d-goog