Don't call PageGetSpecialPointer() on page until it's been initialized.

After calling XLogInitBufferForRedo(), the page might be all-zeros if it was
not in page cache already. btree_xlog_unlink_page initialized the page
correctly, but it called PageGetSpecialPointer before initializing it, which
would lead to a corrupt page at WAL replay, if the unlinked page is not in
page cache.

Backpatch to 9.4, the bug came with the rewrite of B-tree page deletion.

diff --git a/src/backend/access/nbtree/nbtxlog.c b/src/backend/access/nbtree/nbtxlog.c
index 2f8586725a2d7609d002f243f7b432084171d2a5..da28e21d5cbd46655a287dcdf0b850b6676dc47d 100644 (file)
--- a/src/backend/access/nbtree/nbtxlog.c
+++ b/src/backend/access/nbtree/nbtxlog.c
@@ -864,9 +864,10 @@ btree_xlog_unlink_page(uint8 info, XLogReaderState *record)
 
                buffer = XLogInitBufferForRedo(record, 3);
                page = (Page) BufferGetPage(buffer);
-               pageop = (BTPageOpaque) PageGetSpecialPointer(page);
 
                _bt_pageinit(page, BufferGetPageSize(buffer));
+               pageop = (BTPageOpaque) PageGetSpecialPointer(page);
+
                pageop->btpo_flags = BTP_HALF_DEAD | BTP_LEAF;
                pageop->btpo_prev = xlrec->leafleftsib;
                pageop->btpo_next = xlrec->leafrightsib;