Fix race condition in PageManagerFlushThread during database shutdown

Author: tae898

engine/src/main/java/com/arcadedb/engine/PageManagerFlushThread.java

@@ -153,6 +153,9 @@ else if (!pagesToFlush.pages.isEmpty()) {
           // SET THE PAGES TO FLUSH TO BE RETRIEVED BY A CONCURRENT DB CLOSE = FORCE FLUSH OF PAGES
           nextPagesToFlush.set(pagesToFlush);
           try {
+            if (!pagesToFlush.database.isOpen())
+              return;
+
             synchronized (pagesToFlush.pages) {
               for (final MutablePage page : pagesToFlush.pages) {
                 try {

engine/src/main/java/com/arcadedb/engine/PaginatedComponentFile.java

@@ -147,6 +147,9 @@ public int write(final MutablePage page) throws IOException {
     // NO NEED TO SYNCHRONIZE THE BUFFER BECAUSE MUTABLE PAGES ARE NOT SHARED
     buffer.rewind();
     try {
+      if (channel == null)
+        throw new ClosedChannelException();
+
       channel.write(buffer, (page.getPhysicalSize() * (long) pageNumber));
     } catch (final ClosedChannelException e) {
       LogManager.instance().log(this, Level.SEVERE, "File '%s' was closed on write. Reopen it and retry...", null, fileName);
@@ -189,6 +192,9 @@ public void read(final CachedPage page) throws IOException {
     final ByteBuffer buffer = page.getByteBuffer();
 
     try {
+      if (channel == null)
+        throw new ClosedChannelException();
+
       channel.read(buffer, page.getPhysicalSize() * (long) page.getPageId().getPageNumber());
     } catch (final ClosedChannelException e) {
       LogManager.instance().log(this, Level.SEVERE, "File '%s' was closed on read. Reopen it and retry...", null, fileName);