From bdf7db81921deb99fd9d489cbcc635906c89e215 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 12 Apr 2016 11:13:52 -0400
Subject: [PATCH] In generic WAL application and replay, ensure page "hole" is
always zero.
The previous coding could allow the contents of the "hole" between pd_lower
and pd_upper to diverge during replay from what it had been when the update
was originally applied. This would pose a problem if checksums were in
use, and in any case would complicate forensic comparisons between master
and slave servers. So force the "hole" to contain zeroes, both at initial
application of a generically-logged action, and at replay.
Alexander Korotkov, adjusted slightly by me
---
src/backend/access/transam/generic_xlog.c | 40 +++++++++++++++++++++--
1 file changed, 37 insertions(+), 3 deletions(-)
diff --git a/src/backend/access/transam/generic_xlog.c b/src/backend/access/transam/generic_xlog.c
index 072838a5e7a..e07bd836d2e 100644
--- a/src/backend/access/transam/generic_xlog.c
+++ b/src/backend/access/transam/generic_xlog.c
@@ -338,17 +338,31 @@ GenericXLogFinish(GenericXLogState *state)
{
PageData *pageData = &state->pages[i];
Page page;
+ PageHeader pageHeader;
if (BufferIsInvalid(pageData->buffer))
continue;
page = BufferGetPage(pageData->buffer, NULL, NULL,
BGP_NO_SNAPSHOT_TEST);
+ pageHeader = (PageHeader) pageData->image;
if (pageData->fullImage)
{
- /* A full page image does not require anything special */
- memcpy(page, pageData->image, BLCKSZ);
+ /*
+ * A full-page image does not require us to supply any xlog
+ * data. Just apply the image, being careful to zero the
+ * "hole" between pd_lower and pd_upper in order to avoid
+ * divergence between actual page state and what replay would
+ * produce.
+ */
+ memcpy(page, pageData->image, pageHeader->pd_lower);
+ memset(page + pageHeader->pd_lower, 0,
+ pageHeader->pd_upper - pageHeader->pd_lower);
+ memcpy(page + pageHeader->pd_upper,
+ pageData->image + pageHeader->pd_upper,
+ BLCKSZ - pageHeader->pd_upper);
+
XLogRegisterBuffer(i, pageData->buffer,
REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
}
@@ -359,7 +373,15 @@ GenericXLogFinish(GenericXLogState *state)
* associated with this page.
*/
computeDelta(pageData, page, (Page) pageData->image);
- memcpy(page, pageData->image, BLCKSZ);
+
+ /* Apply the image, with zeroed "hole" as above */
+ memcpy(page, pageData->image, pageHeader->pd_lower);
+ memset(page + pageHeader->pd_lower, 0,
+ pageHeader->pd_upper - pageHeader->pd_lower);
+ memcpy(page + pageHeader->pd_upper,
+ pageData->image + pageHeader->pd_upper,
+ BLCKSZ - pageHeader->pd_upper);
+
XLogRegisterBuffer(i, pageData->buffer, REGBUF_STANDARD);
XLogRegisterBufData(i, pageData->delta, pageData->deltaLen);
}
@@ -395,6 +417,7 @@ GenericXLogFinish(GenericXLogState *state)
BGP_NO_SNAPSHOT_TEST),
pageData->image,
BLCKSZ);
+ /* We don't worry about zeroing the "hole" in this case */
MarkBufferDirty(pageData->buffer);
}
END_CRIT_SECTION();
@@ -473,6 +496,7 @@ generic_redo(XLogReaderState *record)
if (action == BLK_NEEDS_REDO)
{
Page page;
+ PageHeader pageHeader;
char *blockDelta;
Size blockDeltaSize;
@@ -481,6 +505,16 @@ generic_redo(XLogReaderState *record)
blockDelta = XLogRecGetBlockData(record, block_id, &blockDeltaSize);
applyPageRedo(page, blockDelta, blockDeltaSize);
+ /*
+ * Since the delta contains no information about what's in the
+ * "hole" between pd_lower and pd_upper, set that to zero to
+ * ensure we produce the same page state that application of the
+ * logged action by GenericXLogFinish did.
+ */
+ pageHeader = (PageHeader) page;
+ memset(page + pageHeader->pd_lower, 0,
+ pageHeader->pd_upper - pageHeader->pd_lower);
+
PageSetLSN(page, lsn);
MarkBufferDirty(buffers[block_id]);
}
--
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