Fix race condition in the SSE code

Author: mathieucarbou

src/AsyncEventSource.cpp

@@ -199,16 +199,16 @@ AsyncEventSourceClient::~AsyncEventSourceClient() {
 }
 
 bool AsyncEventSourceClient::_queueMessage(const char *message, size_t len) {
-  if (_messageQueue.size() >= SSE_MAX_QUEUED_MESSAGES) {
-    async_ws_log_w("Event message queue overflow: discard message");
-    return false;
-  }
-
 #ifdef ESP32
   // length() is not thread-safe, thus acquiring the lock before this call..
   std::lock_guard<std::recursive_mutex> lock(_lockmq);
 #endif
 
+  if (_messageQueue.size() >= SSE_MAX_QUEUED_MESSAGES) {
+    async_ws_log_w("Event message queue overflow: discard message");
+    return false;
+  }
+
   if (_client) {
     _messageQueue.emplace_back(message, len);
   } else {
@@ -230,16 +230,16 @@ bool AsyncEventSourceClient::_queueMessage(const char *message, size_t len) {
 }
 
 bool AsyncEventSourceClient::_queueMessage(AsyncEvent_SharedData_t &&msg) {
-  if (_messageQueue.size() >= SSE_MAX_QUEUED_MESSAGES) {
-    async_ws_log_w("Event message queue overflow: discard message");
-    return false;
-  }
-
 #ifdef ESP32
   // length() is not thread-safe, thus acquiring the lock before this call..
   std::lock_guard<std::recursive_mutex> lock(_lockmq);
 #endif
 
+  if (_messageQueue.size() >= SSE_MAX_QUEUED_MESSAGES) {
+    async_ws_log_w("Event message queue overflow: discard message");
+    return false;
+  }
+
   if (_client) {
     _messageQueue.emplace_back(std::move(msg));
   } else {