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Back-patch fix for race condition in heap_update (make sure we hold

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the buffer lock while checking page free space).
This commit is contained in:
Tom Lane
2001-05-17 00:48:45 +00:00
parent 6d85003098
commit a246bf935f
1 changed files with 57 additions and 11 deletions
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66
src/backend/access/heap/heapam.c
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@@ -8,7 +8,7 @@
* *
* *
* IDENTIFICATION * IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/access/heap/heapam.c,v 1.113 2001/03/25 23:23:58 tgl Exp $ * $Header: /cvsroot/pgsql/src/backend/access/heap/heapam.c,v 1.113.2.1 2001/05/17 00:48:45 tgl Exp $
* *
* *
* INTERFACE ROUTINES * INTERFACE ROUTINES
@@ -1578,6 +1578,8 @@ heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
newbuf; newbuf;
bool need_toast, bool need_toast,
already_marked; already_marked;
Size newtupsize,
pagefree;
int result; int result;
/* increment access statistics */ /* increment access statistics */
@@ -1685,8 +1687,10 @@ l2:
HeapTupleHasExtended(newtup) || HeapTupleHasExtended(newtup) ||
(MAXALIGN(newtup->t_len) > TOAST_TUPLE_THRESHOLD)); (MAXALIGN(newtup->t_len) > TOAST_TUPLE_THRESHOLD));
if (need_toast || newtupsize = MAXALIGN(newtup->t_len);
(unsigned) MAXALIGN(newtup->t_len) > PageGetFreeSpace((Page) dp)) pagefree = PageGetFreeSpace((Page) dp);
if (need_toast || newtupsize > pagefree)
{ {
_locked_tuple_.node = relation->rd_node; _locked_tuple_.node = relation->rd_node;
_locked_tuple_.tid = oldtup.t_self; _locked_tuple_.tid = oldtup.t_self;
@@ -1704,19 +1708,61 @@ l2:
/* Let the toaster do its thing */ /* Let the toaster do its thing */
if (need_toast) if (need_toast)
{
heap_tuple_toast_attrs(relation, newtup, &oldtup); heap_tuple_toast_attrs(relation, newtup, &oldtup);
newtupsize = MAXALIGN(newtup->t_len);
}
/* Now, do we need a new page for the tuple, or not? */ /*
if ((unsigned) MAXALIGN(newtup->t_len) <= PageGetFreeSpace((Page) dp)) * Now, do we need a new page for the tuple, or not? This is a bit
newbuf = buffer; * tricky since someone else could have added tuples to the page
else * while we weren't looking. We have to recheck the available space
* after reacquiring the buffer lock. But don't bother to do that
* if the former amount of free space is still not enough; it's
* unlikely there's more free now than before.
*
* What's more, if we need to get a new page, we will need to acquire
* buffer locks on both old and new pages. To avoid deadlock against
* some other backend trying to get the same two locks in the other
* order, we must be consistent about the order we get the locks in.
* We use the rule "lock the higher-numbered page of the relation
* first". To implement this, we must do RelationGetBufferForTuple
* while not holding the lock on the old page. In 7.1, that routine
* always returns the last page of the relation, so the rule holds.
* (7.2 will use a different approach, but the same ordering rule.)
*/
if (newtupsize > pagefree)
{
/* Assume there's no chance to put newtup on same page. */
newbuf = RelationGetBufferForTuple(relation, newtup->t_len); newbuf = RelationGetBufferForTuple(relation, newtup->t_len);
/* Now reacquire lock on old tuple's page. */
/* Re-acquire the lock on the old tuple's page. */
/* this seems to be deadlock free... */
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE); LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
} }
else else
{
/* Re-acquire the lock on the old tuple's page. */
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
/* Re-check using the up-to-date free space */
pagefree = PageGetFreeSpace((Page) dp);
if (newtupsize > pagefree)
{
/*
* Rats, it doesn't fit anymore. We must now unlock and
* relock to avoid deadlock. Fortunately, this path should
* seldom be taken.
*/
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
newbuf = RelationGetBufferForTuple(relation, newtup->t_len);
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
}
else
{
/* OK, it fits here, so we're done. */
newbuf = buffer;
}
}
}
else
{ {
/* No TOAST work needed, and it'll fit on same page */ /* No TOAST work needed, and it'll fit on same page */
already_marked = false; already_marked = false;