Softmix: Fix crash when switching from softmix to another bridge technology.

The crash is caused by a race condition when switching between native RTP
and softmix bridging technologies.  In this situation, the bridging
technology is switched from native RTP to softmix, and then back to native
RTP fast enough that the softmix private data gets destroyed before the
softmix mixing thread gets started.

Thanks to Kinsey Moore for the crash analysis.

* Fix race condition when starting the softmix mixing thread and switching
to another bridge technology.

(closes issue ASTERISK-22678)
Reported by: John Bigelow
Patches:
      jira_asterisk_22678_v12.patch (license #5621) patch uploaded by rmudgett
Tested by: John Bigelow
........

Merged revisions 400849 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400850 65c4cc65-6c06-0410-ace0-fbb531ad65f3

bridges/bridge_softmix.c

@@ -119,6 +119,14 @@ struct softmix_channel {
 
 struct softmix_bridge_data {
 	struct ast_timer *timer;
+	/*!
+	 * \brief Bridge pointer passed to the softmix mixing thread.
+	 *
+	 * \note Does not need a reference because the bridge will
+	 * always exist while the mixing thread exists even if the
+	 * bridge is no longer actively using the softmix technology.
+	 */
+	struct ast_bridge *bridge;
 	/*! Lock for signaling the mixing thread. */
 	ast_mutex_t lock;
 	/*! Condition, used if we need to wake up the mixing thread. */
@@ -980,8 +988,8 @@ softmix_cleanup:
  */
 static void *softmix_mixing_thread(void *data)
 {
-	struct ast_bridge *bridge = data;
-	struct softmix_bridge_data *softmix_data;
+	struct softmix_bridge_data *softmix_data = data;
+	struct ast_bridge *bridge = softmix_data->bridge;
 
 	ast_bridge_lock(bridge);
 	if (bridge->callid) {
@@ -990,7 +998,6 @@ static void *softmix_mixing_thread(void *data)
 
 	ast_debug(1, "Bridge %s: starting mixing thread\n", bridge->uniqueid);
 
-	softmix_data = bridge->tech_pvt;
 	while (!softmix_data->stop) {
 		if (!bridge->num_active) {
 			/* Wait for something to happen to the bridge. */
@@ -1029,6 +1036,7 @@ static void softmix_bridge_data_destroy(struct softmix_bridge_data *softmix_data
 		softmix_data->timer = NULL;
 	}
 	ast_mutex_destroy(&softmix_data->lock);
+	ast_cond_destroy(&softmix_data->cond);
 	ast_free(softmix_data);
 }
 
@@ -1041,7 +1049,9 @@ static int softmix_bridge_create(struct ast_bridge *bridge)
 	if (!softmix_data) {
 		return -1;
 	}
+	softmix_data->bridge = bridge;
 	ast_mutex_init(&softmix_data->lock);
+	ast_cond_init(&softmix_data->cond, NULL);
 	softmix_data->timer = ast_timer_open();
 	if (!softmix_data->timer) {
 		ast_log(AST_LOG_WARNING, "Failed to open timer for softmix bridge\n");
@@ -1055,7 +1065,8 @@ static int softmix_bridge_create(struct ast_bridge *bridge)
 	bridge->tech_pvt = softmix_data;
 
 	/* Start the mixing thread. */
-	if (ast_pthread_create(&softmix_data->thread, NULL, softmix_mixing_thread, bridge)) {
+	if (ast_pthread_create(&softmix_data->thread, NULL, softmix_mixing_thread,
+		softmix_data)) {
 		softmix_data->thread = AST_PTHREADT_NULL;
 		softmix_bridge_data_destroy(softmix_data);
 		bridge->tech_pvt = NULL;