!3914 修复一处插件并发加载插件问题

Merge pull request !3914 from ljy/master_fix_filescan_list

src/common/backend/utils/init/postinit.cpp

@@ -2828,6 +2828,9 @@ void PostgresInitializer::InitExtensionVariable()
         (void**)MemoryContextAllocZero(u_sess->self_mem_cxt, (Size)(initExtArraySize * sizeof(void*)));
 
     DynamicFileList* file_scanner = NULL;
+    AutoMutexLock libraryLock(&file_list_lock);
+    libraryLock.lock();
+
     for (file_scanner = file_list; file_scanner != NULL; file_scanner = file_scanner->next) {
         /* 
         * If the library has a init_session_vars() function, call it for
@@ -2835,8 +2838,10 @@ void PostgresInitializer::InitExtensionVariable()
         */
         init_session_vars = (void(*)(void))pg_dlsym(file_scanner->handle, "init_session_vars");
         if (init_session_vars != NULL)
-            (*init_session_vars)();
+            (*init_session_vars)();   /*It is assumed that this does not cause a deadlock for the file_list_lock*/
     }
+
+    libraryLock.unLock();
     
     /* check whether the extension has been created 
     *  at most one will be true.