chan_pjsip: Relock correct channel during "fax" redirect.

When fax detection occurs on an outbound PJSIP channel the
redirect operation will result in a masquerade occurring and
the underlying channel on the session changing. The code
incorrectly relocked the new channel instead of the old
channel when returning. This resulted in the new channel
being locked indefinitely. The code now always acts on the
expected channel.

ASTERISK-28538

Change-Id: I2b2e60d07e74383ae7e90d752c036c4b02d6b3a3

channels/chan_pjsip.c

@@ -749,7 +749,8 @@ static int chan_pjsip_answer(struct ast_channel *ast)
 }
 
 /*! \brief Internal helper function called when CNG tone is detected */
-static struct ast_frame *chan_pjsip_cng_tone_detected(struct ast_sip_session *session, struct ast_frame *f)
+static struct ast_frame *chan_pjsip_cng_tone_detected(struct ast_channel *ast, struct ast_sip_session *session,
+	struct ast_frame *f)
 {
 	const char *target_context;
 	int exists;
@@ -765,11 +766,11 @@ static struct ast_frame *chan_pjsip_cng_tone_detected(struct ast_sip_session *se
 	}
 
 	/* If already executing in the fax extension don't do anything */
-	if (!strcmp(ast_channel_exten(session->channel), "fax")) {
+	if (!strcmp(ast_channel_exten(ast), "fax")) {
 		return f;
 	}
 
-	target_context = S_OR(ast_channel_macrocontext(session->channel), ast_channel_context(session->channel));
+	target_context = S_OR(ast_channel_macrocontext(ast), ast_channel_context(ast));
 
 	/*
 	 * We need to unlock the channel here because ast_exists_extension has the
@@ -778,25 +779,30 @@ static struct ast_frame *chan_pjsip_cng_tone_detected(struct ast_sip_session *se
 	 *
 	 * ast_async_goto() has its own restriction on not holding the channel lock.
 	 */
-	ast_channel_unlock(session->channel);
+	ast_channel_unlock(ast);
 	ast_frfree(f);
 	f = &ast_null_frame;
-	exists = ast_exists_extension(session->channel, target_context, "fax", 1,
-		S_COR(ast_channel_caller(session->channel)->id.number.valid,
-			ast_channel_caller(session->channel)->id.number.str, NULL));
+	exists = ast_exists_extension(ast, target_context, "fax", 1,
+		S_COR(ast_channel_caller(ast)->id.number.valid,
+			ast_channel_caller(ast)->id.number.str, NULL));
 	if (exists) {
 		ast_verb(2, "Redirecting '%s' to fax extension due to CNG detection\n",
-			ast_channel_name(session->channel));
-		pbx_builtin_setvar_helper(session->channel, "FAXEXTEN", ast_channel_exten(session->channel));
-		if (ast_async_goto(session->channel, target_context, "fax", 1)) {
+			ast_channel_name(ast));
+		pbx_builtin_setvar_helper(ast, "FAXEXTEN", ast_channel_exten(ast));
+		if (ast_async_goto(ast, target_context, "fax", 1)) {
 			ast_log(LOG_ERROR, "Failed to async goto '%s' into fax extension in '%s'\n",
-				ast_channel_name(session->channel), target_context);
+				ast_channel_name(ast), target_context);
 		}
 	} else {
 		ast_log(LOG_NOTICE, "FAX CNG detected on '%s' but no fax extension in '%s'\n",
-			ast_channel_name(session->channel), target_context);
+			ast_channel_name(ast), target_context);
 	}
-	ast_channel_lock(session->channel);
+
+	/* It's possible for a masquerade to have occurred when doing the ast_async_goto resulting in
+	 * the channel on the session having changed. Since we need to return with the original channel
+	 * locked we lock the channel that was passed in and not session->channel.
+	 */
+	ast_channel_lock(ast);
 
 	return f;
 }
@@ -895,7 +901,11 @@ static struct ast_frame *chan_pjsip_read_stream(struct ast_channel *ast)
 			if (f->subclass.integer == 'f') {
 				ast_debug(3, "Channel driver fax CNG detected on %s\n",
 					ast_channel_name(ast));
-				f = chan_pjsip_cng_tone_detected(session, f);
+				f = chan_pjsip_cng_tone_detected(ast, session, f);
+				/* When chan_pjsip_cng_tone_detected returns it is possible for the
+				 * channel pointed to by ast and by session->channel to differ due to a
+				 * masquerade. It's best not to touch things after this.
+				 */
 			} else {
 				ast_debug(3, "* Detected inband DTMF '%c' on '%s'\n", f->subclass.integer,
 					ast_channel_name(ast));