Add O_EXCL to MirroredBufferPool_DoOpen create file

This is needed to prevent relations possibly overwriting each
other. The O_EXCL is present in postgres's mdcreate() but for some
reason we don't have it here. This adds it back.
Signed-off-by: NXin Zhang <xzhang@pivotal.io>

src/backend/cdb/cdbmirroredbufferpool.c

@@ -252,8 +252,9 @@ static void MirroredBufferPool_DoOpen(
 	*primaryError = 0;
 	*mirrorDataLossOccurred = false;
 
+	/* Using O_EXCL to make it atomic operation to avoid overwriting an existing file. */
 	if (create)
-		fileFlags = O_CREAT | O_RDWR | PG_BINARY;
+		fileFlags = O_CREAT | O_RDWR | O_EXCL | PG_BINARY;
 
 	PersistentTablespace_GetPrimaryAndMirrorFilespaces(
 										relFileNode->spcNode,