Fixed bug #8901 : Bugcheck during nbackup under high load

Thanks to @AlexPeshkoff for help

Author: hvlad

src/jrd/nbak.cpp

@@ -120,6 +120,7 @@ void NBackupStateLock::blockingAstHandler(thread_db* tdbb)
 	}
 
 	{	// scope
+		backup_manager->stateBlocking = true;
 		Firebird::MutexUnlockGuard counterGuard(counterMutex, FB_FUNCTION);
 		backup_manager->stateBlocking = !backup_manager->localStateLock.tryBeginWrite(FB_FUNCTION);
 		if (backup_manager->stateBlocking)

src/jrd/nbak.h

@@ -510,7 +510,7 @@ class BackupManager
 	bool shutDown;
 	bool allocIsValid;			// true, if alloc table cache is completely read from disk
 	bool master;				// this instance performs current begin\end backup process
-	bool stateBlocking;			// blocking AST handler doesn't released stateLock
+	std::atomic_bool stateBlocking;			// blocking AST handler doesn't released stateLock
 
 	NBackupStateLock* stateLock;
 	Firebird::RWLock localStateLock;	// must be acquired before global stateLock