Backported fix for CORE-3859: Too many different dynamic events crash…

… server

src/jrd/event.cpp

@@ -529,6 +529,10 @@ SLONG EVENT_que(ISC_STATUS* status_vector,
 }
 
 
+// Protects EVENT_header from being changed by another thread
+// when MUTEX is already passed to ISC_mutex_lock()
+static Firebird::Mutex remapMutex;
+
 static EVH acquire(void)
 {
 /**************************************
@@ -542,6 +546,8 @@ static EVH acquire(void)
  *
  **************************************/
 
+	Firebird::MutexLockGuard g(remapMutex);
+
 	int mutex_state;
 #ifdef MULTI_THREAD
 	if (mutex_state = ISC_mutex_lock(MUTEX))
@@ -615,6 +621,8 @@ static FRB alloc_global(UCHAR type, ULONG length, bool recurse)
 	}
 
 	if (!best && !recurse) {
+		Firebird::MutexLockGuard g(remapMutex);
+
 		const SLONG old_length = EVENT_data.sh_mem_length_mapped;
 		const SLONG ev_length = old_length + EVENT_EXTEND_SIZE;