Try to wait for or stop any outstanding write transactions when termi…

…nating a thread

src/env.cpp

@@ -120,6 +120,12 @@ void EnvWrap::cleanupStrayTxns() {
 		fprintf(stderr, "Deleting running worker\n");
 		delete worker;
 	}*/
+	pthread_mutex_lock(writingLock);
+	if (this->writeWorker) {
+		// signal that it is cancelled
+		this->writeWorker->env = nullptr;
+	}
+	pthread_mutex_unlock(writingLock);
 	while (this->readTxns.size()) {
 		TxnWrap *tw = *this->readTxns.begin();
 		mdb_txn_abort(tw->txn);

src/writer.cpp

@@ -465,6 +465,7 @@ void WriteWorker::Write() {
 		envForTxn->lastReaderCheck = now;
 	}
 	pthread_mutex_lock(envForTxn->writingLock);
+	if (!env) return;// already closed
 	#ifndef _WIN32
 	int retries = 0;
 	retry: