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Prevent excess SimpleLruTruncate() deletion.

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Every core SLRU wraps around.  With the exception of pg_notify, the wrap
point can fall in the middle of a page.  Account for this in the
PagePrecedes callback specification and in SimpleLruTruncate()'s use of
said callback.  Update each callback implementation to fit the new
specification.  This changes SerialPagePrecedesLogically() from the
style of asyncQueuePagePrecedes() to the style of CLOGPagePrecedes().
(Whereas pg_clog and pg_serial share a key space, pg_serial is nothing
like pg_notify.)  The bug fixed here has the same symptoms and user
followup steps as 592a589a04.  Back-patch
to 9.5 (all supported versions).

Reviewed by Andrey Borodin and (in earlier versions) by Tom Lane.

Discussion: https://postgr.es/m/20190202083822.GC32531@gust.leadboat.com
This commit is contained in:
Noah Misch
2021-01-16 12:21:35 -08:00
parent f52db96944
commit 677f6cb1d9
8 changed files with 310 additions and 78 deletions
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16
src/include/access/slru.h
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@@ -121,9 +121,14 @@ typedef struct SlruCtlData
bool do_fsync;
/*
* Decide which of two page numbers is "older" for truncation purposes. We
* need to use comparison of TransactionIds here in order to do the right
* thing with wraparound XID arithmetic.
* Decide whether a page is "older" for truncation and as a hint for
* evicting pages in LRU order. Return true if every entry of the first
* argument is older than every entry of the second argument. Note that
* !PagePrecedes(a,b) && !PagePrecedes(b,a) need not imply a==b; it also
* arises when some entries are older and some are not. For SLRUs using
* SimpleLruTruncate(), this must use modular arithmetic. (For others,
* the behavior of this callback has no functional implications.) Use
* SlruPagePrecedesUnitTests() in SLRUs meeting its criteria.
*/
bool (*PagePrecedes) (int, int);
@@ -147,6 +152,11 @@ extern int SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno,
TransactionId xid);
extern void SimpleLruWritePage(SlruCtl ctl, int slotno);
extern void SimpleLruFlush(SlruCtl ctl, bool allow_redirtied);
#ifdef USE_ASSERT_CHECKING
extern void SlruPagePrecedesUnitTests(SlruCtl ctl, int per_page);
#else
#define SlruPagePrecedesUnitTests(ctl, per_page) do {} while (0)
#endif
extern void SimpleLruTruncate(SlruCtl ctl, int cutoffPage);
extern bool SimpleLruDoesPhysicalPageExist(SlruCtl ctl, int pageno);