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Commitf9e66f2

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Fix past pd_upper write in ginRedoRecompress()
ginRedoRecompress() replays actions over compressed segments of posting listin-place. However, it might lead to write past pg_upper, because intermediatestate during playing the changes can take more space than both original stateand final state. This commit fixes that by refuse from in-place modification.Instead page tail is copied once modification is started, and then it's usedas the source of original segments. Backpatch to 9.4 where posting listcompression was introduced.Reported-by: Sivasubramanian RamasubramanianDiscussion:https://postgr.es/m/1536091151804.6588%40amazon.comAuthor: Alexander Korotkov based on patch from and ideas by Sivasubramanian RamasubramanianReview: Sivasubramanian RamasubramanianBackpatch-through: 9.4
1 parent82ebf39 commitf9e66f2

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+59
-21
lines changed

1 file changed

+59
-21
lines changed

‎src/backend/access/gin/ginxlog.c

Lines changed: 59 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -133,6 +133,14 @@ ginRedoInsertEntry(Buffer buffer, bool isLeaf, BlockNumber rightblkno, void *rda
133133
}
134134
}
135135

136+
/*
137+
* Redo recompression of posting list. Doing all the changes in-place is not
138+
* always possible, because it might require more space than we've on the page.
139+
* Instead, once modification is required we copy unprocessed tail of the page
140+
* into separately allocated chunk of memory for further reading original
141+
* versions of segments. Thanks to that we don't bother about moving page data
142+
* in-place.
143+
*/
136144
staticvoid
137145
ginRedoRecompress(Pagepage,ginxlogRecompressDataLeaf*data)
138146
{
@@ -142,6 +150,9 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
142150
Pointersegmentend;
143151
char*walbuf;
144152
inttotalsize;
153+
PointertailCopy=NULL;
154+
PointerwritePtr;
155+
Pointersegptr;
145156

146157
/*
147158
* If the page is in pre-9.4 format, convert to new format first.
@@ -181,6 +192,7 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
181192
}
182193

183194
oldseg=GinDataLeafPageGetPostingList(page);
195+
writePtr= (Pointer)oldseg;
184196
segmentend= (Pointer)oldseg+GinDataLeafPageGetPostingListSize(page);
185197
segno=0;
186198

@@ -198,8 +210,6 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
198210
ItemPointerData*newitems;
199211
intnnewitems;
200212
intsegsize;
201-
Pointersegptr;
202-
intszleft;
203213

204214
/* Extract all the information we need from the WAL record */
205215
if (a_action==GIN_SEGMENT_INSERT||
@@ -222,6 +232,17 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
222232
Assert(segno <=a_segno);
223233
while (segno<a_segno)
224234
{
235+
/*
236+
* Once modification is started and page tail is copied, we've
237+
* to copy unmodified segments.
238+
*/
239+
segsize=SizeOfGinPostingList(oldseg);
240+
if (tailCopy)
241+
{
242+
Assert(writePtr+segsize<PageGetSpecialPointer(page));
243+
memcpy(writePtr, (Pointer)oldseg,segsize);
244+
}
245+
writePtr+=segsize;
225246
oldseg=GinNextPostingListSegment(oldseg);
226247
segno++;
227248
}
@@ -262,36 +283,42 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
262283
Assert(a_action==GIN_SEGMENT_INSERT);
263284
segsize=0;
264285
}
265-
szleft=segmentend-segptr;
286+
287+
/*
288+
* We're about to start modification of the page. So, copy tail of the
289+
* page if it's not done already.
290+
*/
291+
if (!tailCopy&&segptr!=segmentend)
292+
{
293+
inttailSize=segmentend-segptr;
294+
295+
tailCopy= (Pointer)palloc(tailSize);
296+
memcpy(tailCopy,segptr,tailSize);
297+
segptr=tailCopy;
298+
oldseg= (GinPostingList*)segptr;
299+
segmentend=segptr+tailSize;
300+
}
266301

267302
switch (a_action)
268303
{
269304
caseGIN_SEGMENT_DELETE:
270-
memmove(segptr,segptr+segsize,szleft-segsize);
271-
segmentend-=segsize;
272-
305+
segptr+=segsize;
273306
segno++;
274307
break;
275308

276309
caseGIN_SEGMENT_INSERT:
277-
/* make room for the new segment */
278-
memmove(segptr+newsegsize,segptr,szleft);
279310
/* copy the new segment in place */
280-
memcpy(segptr,newseg,newsegsize);
281-
segmentend+=newsegsize;
282-
segptr+=newsegsize;
311+
Assert(writePtr+newsegsize <=PageGetSpecialPointer(page));
312+
memcpy(writePtr,newseg,newsegsize);
313+
writePtr+=newsegsize;
283314
break;
284315

285316
caseGIN_SEGMENT_REPLACE:
286-
/* shift the segments that follow */
287-
memmove(segptr+newsegsize,
288-
segptr+segsize,
289-
szleft-segsize);
290-
/* copy the replacement segment in place */
291-
memcpy(segptr,newseg,newsegsize);
292-
segmentend-=segsize;
293-
segmentend+=newsegsize;
294-
segptr+=newsegsize;
317+
/* copy the new version of segment in place */
318+
Assert(writePtr+newsegsize <=PageGetSpecialPointer(page));
319+
memcpy(writePtr,newseg,newsegsize);
320+
writePtr+=newsegsize;
321+
segptr+=segsize;
295322
segno++;
296323
break;
297324

@@ -301,7 +328,18 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
301328
oldseg= (GinPostingList*)segptr;
302329
}
303330

304-
totalsize=segmentend- (Pointer)GinDataLeafPageGetPostingList(page);
331+
/* Copy the rest of unmodified segments if any. */
332+
segptr= (Pointer)oldseg;
333+
if (segptr!=segmentend&&tailCopy)
334+
{
335+
intrestSize=segmentend-segptr;
336+
337+
Assert(writePtr+restSize <=PageGetSpecialPointer(page));
338+
memcpy(writePtr,segptr,restSize);
339+
writePtr+=restSize;
340+
}
341+
342+
totalsize=writePtr- (Pointer)GinDataLeafPageGetPostingList(page);
305343
GinDataPageSetDataSize(page,totalsize);
306344
}
307345

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