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 5f9fc49e78ca1388ab482e24c8b5a873238ae0b6..2debb870bd00af67241aaf98e5d9529687b7e8d5 100644 (file)
--- a/src/backend/access/nbtree/nbtxlog.c
+++ b/src/backend/access/nbtree/nbtxlog.c
@@ -997,9 +997,10 @@ btree_xlog_unlink_page(uint8 info, XLogRecPtr lsn, XLogRecord *record)
        buffer = XLogReadBuffer(xlrec->node, xlrec->leafblk, true);
        Assert(BufferIsValid(buffer));
        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;