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Fix race condition in GIN posting tree page deletion.

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If a page is deleted, and reused for something else, just as a search is
following a rightlink to it from its left sibling, the search would continue
scanning whatever the new contents of the page are. That could lead to
incorrect query results, or even something more curious if the page is
reused for a different kind of a page.

To fix, modify the search algorithm to lock the next page before releasing
the previous one, and refrain from deleting pages from the leftmost branch
of the tree.

Add a new Concurrency section to the README, explaining why this works.
There is a lot more one could say about concurrency in GIN, but that's for
another patch.

Backpatch to all supported versions.
This commit is contained in:
Heikki Linnakangas
2013-11-08 22:21:42 +02:00
parent 636b868f17
commit ac4ab97ec0
5 changed files with 121 additions and 59 deletions
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31
src/backend/access/gin/ginget.c
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@ -105,16 +105,11 @@ moveRightIfItNeeded(GinBtreeData *btree, GinBtreeStack *stack)
/*
* We scanned the whole page, so we should take right page
*/
stack->blkno = GinPageGetOpaque(page)->rightlink;
if (GinPageRightMost(page))
return false; /* no more pages */
LockBuffer(stack->buffer, GIN_UNLOCK);
stack->buffer = ReleaseAndReadBuffer(stack->buffer,
btree->index,
stack->blkno);
LockBuffer(stack->buffer, GIN_SHARE);
stack->buffer = ginStepRight(stack->buffer, btree->index, GIN_SHARE);
stack->blkno = BufferGetBlockNumber(stack->buffer);
stack->off = FirstOffsetNumber;
}
@ -132,7 +127,6 @@ scanPostingTree(Relation index, GinScanEntry scanEntry,
GinPostingTreeScan *gdi;
Buffer buffer;
Page page;
BlockNumber blkno;
/* Descend to the leftmost leaf page */
gdi = ginPrepareScanPostingTree(index, rootPostingTree, TRUE);
@ -162,10 +156,7 @@ scanPostingTree(Relation index, GinScanEntry scanEntry,
if (GinPageRightMost(page))
break; /* no more pages */
blkno = GinPageGetOpaque(page)->rightlink;
LockBuffer(buffer, GIN_UNLOCK);
buffer = ReleaseAndReadBuffer(buffer, index, blkno);
LockBuffer(buffer, GIN_SHARE);
buffer = ginStepRight(buffer, index, GIN_SHARE);
}
UnlockReleaseBuffer(buffer);
@ -543,7 +534,6 @@ static void
entryGetNextItem(GinState *ginstate, GinScanEntry entry)
{
Page page;
BlockNumber blkno;
for (;;)
{
@ -561,23 +551,18 @@ entryGetNextItem(GinState *ginstate, GinScanEntry entry)
* It's needed to go by right link. During that we should refind
* first ItemPointer greater that stored
*/
blkno = GinPageGetOpaque(page)->rightlink;
LockBuffer(entry->buffer, GIN_UNLOCK);
if (blkno == InvalidBlockNumber)
if (GinPageRightMost(page))
{
ReleaseBuffer(entry->buffer);
UnlockReleaseBuffer(entry->buffer);
ItemPointerSetInvalid(&entry->curItem);
entry->buffer = InvalidBuffer;
entry->isFinished = TRUE;
return;
}
entry->buffer = ReleaseAndReadBuffer(entry->buffer,
ginstate->index,
blkno);
LockBuffer(entry->buffer, GIN_SHARE);
entry->buffer = ginStepRight(entry->buffer,
ginstate->index,
GIN_SHARE);
page = BufferGetPage(entry->buffer);
entry->offset = InvalidOffsetNumber;