[Pick 2.0](index writer) fix docWriter is null, when reenter close (#217

)

* [Fix](index writer) fix core if docWriter nullptr when IndexWriter doFlush (#213)

* [Fix](index writer) fix docWriter is null, when reenter close (#214)

src/core/CLucene/index/IndexWriter.cpp

@@ -535,7 +535,10 @@ void IndexWriter::closeInternal(bool waitForMerges) {
     try {
         if (infoStream != NULL)
             message(string("now flush at close"));
-
+        // if docWriter is nullptr, maybe it's been flushed already
+        if (docWriter == nullptr) {
+            return;
+        }
         docWriter->close();
 
         // Only allow a _CLNEW merge to be triggered if we are
@@ -2048,6 +2051,11 @@ void IndexWriter::flush(bool triggerMerge, bool _flushDocStores) {
 bool IndexWriter::doFlush(bool _flushDocStores) {
     SCOPED_LOCK_MUTEX(THIS_LOCK)
 
+    // if docWriter is nullptr, maybe it's been flushed already
+    if (docWriter == nullptr) {
+        return false;
+    }
+
     // Make sure no threads are actively adding a document
 
     // Returns true if docWriter is currently aborting, in