From 3afd75eaac8aaccf5aeebc52548c396b84d85516 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 10 Jan 2018 15:50:54 -0500
Subject: [PATCH] Remove dubious micro-optimization in
ckpt_buforder_comparator().
It seems incorrect to assume that the list of CkptSortItems can never
contain duplicate page numbers: concurrent activity could result in some
page getting dropped from a low-numbered buffer and later loaded into a
high-numbered buffer while BufferSync is scanning the buffer pool.
If that happened, the comparator would give self-inconsistent results,
potentially confusing qsort(). Saving one comparison step is not worth
possibly getting the sort wrong.
So far as I can tell, nothing would actually go wrong given our current
implementation of qsort(). It might get a bit slower than expected
if there were a large number of duplicates of one value, but that's
surely a probability-epsilon case. Still, the comment is wrong,
and if we ever switched to another sort implementation it might be
less forgiving.
In passing, avoid casting away const-ness of the argument pointers;
I've not seen any compiler complaints from that, but it seems likely
that some compilers would not like it.
Back-patch to 9.6 where this code came in, just in case I've underestimated
the possible consequences.
Discussion: https://postgr.es/m/18437.1515607610@sss.pgh.pa.us
---
src/backend/storage/buffer/bufmgr.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 4e44336332..01eabe5706 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -4064,8 +4064,8 @@ local_buffer_write_error_callback(void *arg)
static int
rnode_comparator(const void *p1, const void *p2)
{
- RelFileNode n1 = *(RelFileNode *) p1;
- RelFileNode n2 = *(RelFileNode *) p2;
+ RelFileNode n1 = *(const RelFileNode *) p1;
+ RelFileNode n2 = *(const RelFileNode *) p2;
if (n1.relNode < n2.relNode)
return -1;
@@ -4174,8 +4174,8 @@ buffertag_comparator(const void *a, const void *b)
static int
ckpt_buforder_comparator(const void *pa, const void *pb)
{
- const CkptSortItem *a = (CkptSortItem *) pa;
- const CkptSortItem *b = (CkptSortItem *) pb;
+ const CkptSortItem *a = (const CkptSortItem *) pa;
+ const CkptSortItem *b = (const CkptSortItem *) pb;
/* compare tablespace */
if (a->tsId < b->tsId)
@@ -4195,8 +4195,10 @@ ckpt_buforder_comparator(const void *pa, const void *pb)
/* compare block number */
else if (a->blockNum < b->blockNum)
return -1;
- else /* should not be the same block ... */
+ else if (a->blockNum > b->blockNum)
return 1;
+ /* equal page IDs are unlikely, but not impossible */
+ return 0;
}
/*