From bab7823a49bb210b8920ae59e5126d27e4d63833 Mon Sep 17 00:00:00 2001
From: Fujii Masao <fujii@postgresql.org>
Date: Mon, 29 Aug 2016 14:34:58 +0900
Subject: [PATCH] Fix pg_xlogdump so that it handles cross-page
XLP_FIRST_IS_CONTRECORD record.
Previously pg_xlogdump failed to dump the contents of the WAL file
if the file starts with the continuation WAL record which spans
more than one pages. Since pg_xlogdump assumed that the continuation
record always fits on a page, it could not find the valid WAL record to
start reading from in that case.
This patch changes pg_xlogdump so that it can handle a continuation
WAL record which crosses a page boundary and find the valid record
to start reading from.
Back-patch to 9.3 where pg_xlogdump was introduced.
Author: Pavan Deolasee
Reviewed-By: Michael Paquier and Craig Ringer
Discussion: CABOikdPsPByMiG6J01DKq6om2+BNkxHTPkOyqHM2a4oYwGKsqQ@mail.gmail.com
---
src/backend/access/transam/xlogreader.c | 91 +++++++++++++++++--------
1 file changed, 64 insertions(+), 27 deletions(-)
diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index dcf747c6334..f2da5058924 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -866,46 +866,83 @@ XLogRecPtr
XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr)
{
XLogReaderState saved_state = *state;
- XLogRecPtr targetPagePtr;
XLogRecPtr tmpRecPtr;
- int targetRecOff;
XLogRecPtr found = InvalidXLogRecPtr;
- uint32 pageHeaderSize;
XLogPageHeader header;
- int readLen;
char *errormsg;
Assert(!XLogRecPtrIsInvalid(RecPtr));
- targetRecOff = RecPtr % XLOG_BLCKSZ;
+ /*
+ * skip over potential continuation data, keeping in mind that it may span
+ * multiple pages
+ */
+ tmpRecPtr = RecPtr;
+ while (true)
+ {
+ XLogRecPtr targetPagePtr;
+ int targetRecOff;
+ uint32 pageHeaderSize;
+ int readLen;
- /* scroll back to page boundary */
- targetPagePtr = RecPtr - targetRecOff;
+ /*
+ * Compute targetRecOff. It should typically be equal or greater than
+ * short page-header since a valid record can't start anywhere before
+ * that, except when caller has explicitly specified the offset that
+ * falls somewhere there or when we are skipping multi-page
+ * continuation record. It doesn't matter though because
+ * ReadPageInternal() is prepared to handle that and will read at least
+ * short page-header worth of data
+ */
+ targetRecOff = tmpRecPtr % XLOG_BLCKSZ;
- /* Read the page containing the record */
- readLen = ReadPageInternal(state, targetPagePtr, targetRecOff);
- if (readLen < 0)
- goto err;
+ /* scroll back to page boundary */
+ targetPagePtr = tmpRecPtr - targetRecOff;
- header = (XLogPageHeader) state->readBuf;
+ /* Read the page containing the record */
+ readLen = ReadPageInternal(state, targetPagePtr, targetRecOff);
+ if (readLen < 0)
+ goto err;
- pageHeaderSize = XLogPageHeaderSize(header);
+ header = (XLogPageHeader) state->readBuf;
- /* make sure we have enough data for the page header */
- readLen = ReadPageInternal(state, targetPagePtr, pageHeaderSize);
- if (readLen < 0)
- goto err;
+ pageHeaderSize = XLogPageHeaderSize(header);
- /* skip over potential continuation data */
- if (header->xlp_info & XLP_FIRST_IS_CONTRECORD)
- {
- /* record headers are MAXALIGN'ed */
- tmpRecPtr = targetPagePtr + pageHeaderSize
- + MAXALIGN(header->xlp_rem_len);
- }
- else
- {
- tmpRecPtr = targetPagePtr + pageHeaderSize;
+ /* make sure we have enough data for the page header */
+ readLen = ReadPageInternal(state, targetPagePtr, pageHeaderSize);
+ if (readLen < 0)
+ goto err;
+
+ /* skip over potential continuation data */
+ if (header->xlp_info & XLP_FIRST_IS_CONTRECORD)
+ {
+ /*
+ * If the length of the remaining continuation data is more than
+ * what can fit in this page, the continuation record crosses over
+ * this page. Read the next page and try again. xlp_rem_len in the
+ * next page header will contain the remaining length of the
+ * continuation data
+ *
+ * Note that record headers are MAXALIGN'ed
+ */
+ if (MAXALIGN(header->xlp_rem_len) > (XLOG_BLCKSZ - pageHeaderSize))
+ tmpRecPtr = targetPagePtr + XLOG_BLCKSZ;
+ else
+ {
+ /*
+ * The previous continuation record ends in this page. Set
+ * tmpRecPtr to point to the first valid record
+ */
+ tmpRecPtr = targetPagePtr + pageHeaderSize
+ + MAXALIGN(header->xlp_rem_len);
+ break;
+ }
+ }
+ else
+ {
+ tmpRecPtr = targetPagePtr + pageHeaderSize;
+ break;
+ }
}
/*
--
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