Fix an uninitialized access in hash_xlog_squeeze_page().

Commit 861f86beea changed hash_xlog_squeeze_page() to start reading
the write buffer conditionally but forgot to initialize it leading to an
uninitialized access.

Reported-by: Alexander Lakhin
Author: Hayato Kuroda
Reviewed-by: Alexander Lakhin, Amit Kapila
Discussion: http://postgr.es/m/62ed1a9f-746a-8e86-904b-51b9b806a1d9@gmail.com

src/backend/access/hash/hash_xlog.c

@@ -632,7 +632,7 @@ hash_xlog_squeeze_page(XLogReaderState *record)
 	XLogRecPtr	lsn = record->EndRecPtr;
 	xl_hash_squeeze_page *xldata = (xl_hash_squeeze_page *) XLogRecGetData(record);
 	Buffer		bucketbuf = InvalidBuffer;
-	Buffer		writebuf;
+	Buffer		writebuf = InvalidBuffer;
 	Buffer		ovflbuf;
 	Buffer		prevbuf = InvalidBuffer;
 	Buffer		mapbuf;

src/test/regress/expected/hash_index.out

@@ -298,6 +298,20 @@ ROLLBACK;
 INSERT INTO hash_cleanup_heap SELECT 1 FROM generate_series(1, 500) as i;
 CHECKPOINT;
 VACUUM hash_cleanup_heap;
+TRUNCATE hash_cleanup_heap;
+-- Insert tuples to both the primary bucket page and overflow pages.
+INSERT INTO hash_cleanup_heap SELECT 1 FROM generate_series(1, 500) as i;
+-- Fill overflow pages by "dead" tuples.
+BEGIN;
+INSERT INTO hash_cleanup_heap SELECT 1 FROM generate_series(1, 1500) as i;
+ROLLBACK;
+-- And insert some tuples again. During squeeze operation, these will be moved
+-- to other overflow pages and also allow overflow pages filled by dead tuples
+-- to be freed. Note the main purpose of this test is to test the case where
+-- we don't need to move any tuple from the overflow page being freed.
+INSERT INTO hash_cleanup_heap SELECT 1 FROM generate_series(1, 50) as i;
+CHECKPOINT;
+VACUUM hash_cleanup_heap;
 -- Clean up.
 DROP TABLE hash_cleanup_heap;
 -- Index on temp table.

src/test/regress/sql/hash_index.sql

@@ -284,6 +284,23 @@ INSERT INTO hash_cleanup_heap SELECT 1 FROM generate_series(1, 500) as i;
 CHECKPOINT;
 VACUUM hash_cleanup_heap;
 
+TRUNCATE hash_cleanup_heap;
+
+-- Insert tuples to both the primary bucket page and overflow pages.
+INSERT INTO hash_cleanup_heap SELECT 1 FROM generate_series(1, 500) as i;
+-- Fill overflow pages by "dead" tuples.
+BEGIN;
+INSERT INTO hash_cleanup_heap SELECT 1 FROM generate_series(1, 1500) as i;
+ROLLBACK;
+-- And insert some tuples again. During squeeze operation, these will be moved
+-- to other overflow pages and also allow overflow pages filled by dead tuples
+-- to be freed. Note the main purpose of this test is to test the case where
+-- we don't need to move any tuple from the overflow page being freed.
+INSERT INTO hash_cleanup_heap SELECT 1 FROM generate_series(1, 50) as i;
+
+CHECKPOINT;
+VACUUM hash_cleanup_heap;
+
 -- Clean up.
 DROP TABLE hash_cleanup_heap;