res_ari_channels: Fix a deadlock when originating multiple channels close to eachother.

If a Stasis application is specified an implicit subscription is done on the originated
channel. This was previously done with the channel lock held which is dangerous as the
underlying code locks the container and iterates items. This change releases the lock
on the originated channel before subscribing occurs.

(closes issue ASTERISK-22768)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/2979/
........

Merged revisions 402346 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@402347 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/stasis_app.h

@@ -140,6 +140,8 @@ enum stasis_app_subscribe_res {
  *             after adding the subscription.
  *
  * \return \ref stasis_app_subscribe_res return code.
+ *
+ * \note Do not hold any channel locks if subscribing to a channel.
  */
 enum stasis_app_subscribe_res stasis_app_subscribe(const char *app_name,
 	const char **event_source_uris, int event_sources_count,

res/ari/resource_channels.c

@@ -642,6 +642,9 @@ void ast_ari_originate(struct ast_variable *headers,
 		return;
 	}
 
+	snapshot = ast_channel_snapshot_create(chan);
+	ast_channel_unlock(chan);
+
 	if (!ast_strlen_zero(args->app)) {
 		/* channel: + channel ID + null terminator */
 		char uri[9 + strlen(ast_channel_uniqueid(chan))];
@@ -651,10 +654,7 @@ void ast_ari_originate(struct ast_variable *headers,
 		stasis_app_subscribe(args->app, uris, 1, NULL);
 	}
 
-	snapshot = ast_channel_snapshot_create(chan);
 	ast_ari_response_ok(response, ast_channel_snapshot_to_json(snapshot));
-
-	ast_channel_unlock(chan);
 	ast_channel_unref(chan);
 }