thread safety enhancements

docs/ares_init_options.3

@@ -227,8 +227,9 @@ The value of
 .I sock_state_cb_data
 will be passed as the
 .I data
-argument.  Since this may be called cross-thread, a channel lock is NOT held
-when this callback is called.
+argument.  The channel lock is held during this callback, if calling
+cross-thread care must be taken to ensure no other c-ares functions are
+called from this callback.
 
 Cannot be used with \fBARES_OPT_EVENT_THREAD\fP.
 .TP 18

src/lib/ares_event_thread.c

@@ -274,16 +274,19 @@ static void *ares_event_thread(void *arg)
     const struct timeval *tvout;
     unsigned long         timeout_ms = 0; /* 0 = unlimited */
 
+    ares_event_process_updates(e);
+
+    /* Don't hold a mutex while waiting on events or calling into anything
+     * that might require a c-ares channel lock since a callback could be
+     * triggered cross-thread */
+    ares__thread_mutex_unlock(e->mutex);
+
     tvout = ares_timeout(e->channel, NULL, &tv);
     if (tvout != NULL) {
       timeout_ms =
         (unsigned long)((tvout->tv_sec * 1000) + (tvout->tv_usec / 1000) + 1);
     }
 
-    ares_event_process_updates(e);
-
-    /* Don't hold a mutex while waiting on events */
-    ares__thread_mutex_unlock(e->mutex);
     e->ev_sys->wait(e, timeout_ms);
 
     /* Each iteration should do timeout processing */

src/lib/ares_private.h

@@ -652,9 +652,7 @@ void          ares_queue_notify_empty(ares_channel_t *channel);
 #define SOCK_STATE_CALLBACK(c, s, r, w)                           \
   do {                                                            \
     if ((c)->sock_state_cb) {                                     \
-      ares__channel_unlock((c));                                  \
       (c)->sock_state_cb((c)->sock_state_cb_data, (s), (r), (w)); \
-      ares__channel_lock((c));                                    \
     }                                                             \
   } while (0)