tevent: Fix a race condition

We can't rely on tctx to exist after we unlocked the mutex. It took a
while, but this does lead to data corruption. If *tctx is replaced with
something where tctx->wakeup_fd points to a real, existing file
descriptor, we're screwed. And by screwed, this means file corruption
on disk.

Again. I am not tall enough for this business.

http://bholley.net/blog/2015/must-be-this-tall-to-write-multi-threaded-code.html

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13130

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Sat Nov 11 03:20:09 CET 2017 on sn-devel-144

(cherry picked from commit 20cfcb7dbc5dd099384b76a76e3d35cf627100b6)

Autobuild-User(v4-6-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-6-test): Mon Nov 13 14:23:54 CET 2017 on sn-devel-144

diff --git a/lib/tevent/tevent_threads.c b/lib/tevent/tevent_threads.c
index 8197323af020e43444415da0f3d42191e8590452..197ad1dfe41145c94d7634086d6d8417eea85fd5 100644 (file)
--- a/lib/tevent/tevent_threads.c
+++ b/lib/tevent/tevent_threads.c
@@ -434,7 +434,7 @@ void _tevent_threaded_schedule_immediate(struct tevent_threaded_context *tctx,
 {
 #ifdef HAVE_PTHREAD
        struct tevent_context *ev;
-       int ret;
+       int ret, wakeup_fd;
 
        ret = pthread_mutex_lock(&tctx->event_ctx_mutex);
        if (ret != 0) {
@@ -442,6 +442,7 @@ void _tevent_threaded_schedule_immediate(struct tevent_threaded_context *tctx,
        }
 
        ev = tctx->event_ctx;
+       wakeup_fd = tctx->wakeup_fd;
 
        ret = pthread_mutex_unlock(&tctx->event_ctx_mutex);
        if (ret != 0) {
@@ -489,7 +490,7 @@ void _tevent_threaded_schedule_immediate(struct tevent_threaded_context *tctx,
         * than a noncontended one. So I'd opt for the lower footprint
         * initially. Maybe we have to change that later.
         */
-       tevent_common_wakeup_fd(tctx->wakeup_fd);
+       tevent_common_wakeup_fd(wakeup_fd);
 #else
        /*
         * tevent_threaded_context_create() returned NULL with ENOSYS...