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Commitc2ba012

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Work around gcc 4.6.0 bug that breaks WAL replay.
ReadRecord's habit of using both direct references to tmpRecPtr andreferences to *RecPtr (which is pointing at tmpRecPtr) triggers anoptimization bug in gcc 4.6.0, which apparently has forgotten aboutaliasing rules. Avoid the compiler bug, and make the code more readableto boot, by getting rid of the direct references. Improve the commentswhile at it.Back-patch to all supported versions, in case they get built with 4.6.0.Tom Lane, with some cosmetic suggestions from Alex Hunsaker
1 parentcb2d158 commitc2ba012

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2 files changed

+30
-19
lines changed

2 files changed

+30
-19
lines changed

‎src/backend/access/transam/xlog.c

Lines changed: 24 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -3728,23 +3728,32 @@ ReadRecord(XLogRecPtr *RecPtr, int emode, bool fetching_ckpt)
37283728
RecPtr=&tmpRecPtr;
37293729

37303730
/*
3731-
* Align recptr to next page if no more records can fit on the current
3732-
* page.
3731+
* RecPtr is pointing to end+1 of the previous WAL record. We must
3732+
* advance it if necessary to where the next record starts. First,
3733+
* align to next page if no more records can fit on the current page.
37333734
*/
37343735
if (XLOG_BLCKSZ- (RecPtr->xrecoff %XLOG_BLCKSZ)<SizeOfXLogRecord)
3735-
{
3736-
NextLogPage(tmpRecPtr);
3737-
/* We will account for page header size below */
3738-
}
3736+
NextLogPage(*RecPtr);
37393737

3740-
if (tmpRecPtr.xrecoff >=XLogFileSize)
3738+
/* Check for crossing of xlog segment boundary */
3739+
if (RecPtr->xrecoff >=XLogFileSize)
37413740
{
3742-
(tmpRecPtr.xlogid)++;
3743-
tmpRecPtr.xrecoff=0;
3741+
(RecPtr->xlogid)++;
3742+
RecPtr->xrecoff=0;
37443743
}
3744+
3745+
/*
3746+
* If at page start, we must skip over the page header. But we can't
3747+
* do that until we've read in the page, since the header size is
3748+
* variable.
3749+
*/
37453750
}
37463751
else
37473752
{
3753+
/*
3754+
* In this case, the passed-in record pointer should already be
3755+
* pointing to a valid record starting position.
3756+
*/
37483757
if (!XRecOffIsValid(RecPtr->xrecoff))
37493758
ereport(PANIC,
37503759
(errmsg("invalid record offset at %X/%X",
@@ -3773,11 +3782,13 @@ ReadRecord(XLogRecPtr *RecPtr, int emode, bool fetching_ckpt)
37733782
if (targetRecOff==0)
37743783
{
37753784
/*
3776-
* Can only get here in the continuing-from-prev-page case, because
3777-
* XRecOffIsValid eliminated the zero-page-offset case otherwise. Need
3778-
* to skip over the new page's header.
3785+
* At page start, so skip over page header. The Assert checks that
3786+
* we're not scribbling on caller's record pointer; it's OK because we
3787+
* can only get here in the continuing-from-prev-record case, since
3788+
* XRecOffIsValid rejected the zero-page-offset case otherwise.
37793789
*/
3780-
tmpRecPtr.xrecoff+=pageHeaderSize;
3790+
Assert(RecPtr==&tmpRecPtr);
3791+
RecPtr->xrecoff+=pageHeaderSize;
37813792
targetRecOff=pageHeaderSize;
37823793
}
37833794
elseif (targetRecOff<pageHeaderSize)

‎src/include/access/xlog_internal.h

Lines changed: 6 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -154,13 +154,13 @@ typedef XLogLongPageHeaderData *XLogLongPageHeader;
154154
/* Align a record pointer to next page */
155155
#defineNextLogPage(recptr) \
156156
do {\
157-
if (recptr.xrecoff % XLOG_BLCKSZ != 0)\
158-
recptr.xrecoff +=\
159-
(XLOG_BLCKSZ - recptr.xrecoff % XLOG_BLCKSZ);\
160-
if (recptr.xrecoff >= XLogFileSize) \
157+
if ((recptr).xrecoff % XLOG_BLCKSZ != 0)\
158+
(recptr).xrecoff +=\
159+
(XLOG_BLCKSZ -(recptr).xrecoff % XLOG_BLCKSZ);\
160+
if ((recptr).xrecoff >= XLogFileSize) \
161161
{\
162-
(recptr.xlogid)++;\
163-
recptr.xrecoff = 0; \
162+
((recptr).xlogid)++;\
163+
(recptr).xrecoff = 0; \
164164
}\
165165
} while (0)
166166

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