FS-10634 [mod_spandsp]: txfax failing with 488 Not Acceptable Here race

src/include/switch_channel.h

@@ -101,6 +101,9 @@ SWITCH_DECLARE(void) switch_channel_wait_for_state_timeout(switch_channel_t *oth
 SWITCH_DECLARE(switch_status_t) switch_channel_wait_for_flag(switch_channel_t *channel,
 															 switch_channel_flag_t want_flag,
 															 switch_bool_t pres, uint32_t to, switch_channel_t *super_channel);
+SWITCH_DECLARE(switch_status_t) switch_channel_wait_for_app_flag(switch_channel_t *channel,
+																 uint32_t app_flag,
+																 const char *key, switch_bool_t pres, uint32_t to);
 
 SWITCH_DECLARE(switch_channel_state_t) switch_channel_perform_set_state(switch_channel_t *channel,
 																		const char *file, const char *func, int line, switch_channel_state_t state);

src/switch_channel.c

@@ -1748,6 +1748,39 @@ SWITCH_DECLARE(switch_status_t) switch_channel_wait_for_flag(switch_channel_t *c
 	return SWITCH_STATUS_SUCCESS;
 }
 
+SWITCH_DECLARE(switch_status_t) switch_channel_wait_for_app_flag(switch_channel_t *channel,
+																 uint32_t app_flag,
+																 const char *key, switch_bool_t pres, uint32_t to)
+{
+	if (to) {
+		to++;
+	}
+
+	for (;;) {
+		if (pres) {
+			if (switch_channel_test_app_flag_key(key, channel, app_flag)) {
+				break;
+			}
+		} else {
+			if (!switch_channel_test_app_flag_key(key, channel, app_flag)) {
+				break;
+			}
+		}
+
+		switch_cond_next();
+
+		if (switch_channel_down(channel)) {
+			return SWITCH_STATUS_FALSE;
+		}
+
+		if (to && !--to) {
+			return SWITCH_STATUS_FALSE;
+		}
+	}
+
+	return SWITCH_STATUS_SUCCESS;
+}
+
 
 SWITCH_DECLARE(void) switch_channel_set_cap_value(switch_channel_t *channel, switch_channel_cap_t cap, uint32_t value)
 {

src/switch_core_media.c

@@ -5223,9 +5223,8 @@ SWITCH_DECLARE(uint8_t) switch_core_media_negotiate_sdp(switch_core_session_t *s
 					}
 				}
 
-
 				/* do nothing here, mod_fax will trigger a response (if it's listening =/) */
-				if (switch_channel_test_app_flag_key("T38", channel, CF_APP_T38_POSSIBLE)) {
+				if (switch_channel_wait_for_app_flag(channel, CF_APP_T38_POSSIBLE, "T38", SWITCH_TRUE, 2000)) {
 					fmatch = 1;
 				} else {