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Commite2d64f8

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Avoid potential deadlock in InitCatCachePhase2().
Opening a catcache's index could require reading from that cache's owncatalog, which of course would acquire AccessShareLock on the catalog.So the original coding here risks locking index before heap, which coulddeadlock against another backend trying to get exclusive locks in thenormal order. Because InitCatCachePhase2 is only called when a backendhas to start up without a relcache init file, the deadlock was seldom seenin the field. (And by the same token, there's no need to worry about anyperformance disadvantage; so not much point in trying to distinguishexactly which catalogs have the risk.)Bug report, diagnosis, and patch by Nikhil Sontakke. Additional commentaryby me. Back-patch to all supported branches.
1 parent151e463 commite2d64f8

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‎src/backend/utils/cache/catcache.c

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@@ -24,6 +24,7 @@
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#ifdefCATCACHE_STATS
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#include"storage/ipc.h"/* for on_proc_exit */
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#endif
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#include"storage/lmgr.h"
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#include"utils/builtins.h"
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#include"utils/fmgroids.h"
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#include"utils/memutils.h"
@@ -1009,8 +1010,16 @@ InitCatCachePhase2(CatCache *cache, bool touch_index)
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{
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Relationidesc;
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/*
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* We must lock the underlying catalog before opening the index to
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* avoid deadlock, since index_open could possibly result in reading
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* this same catalog, and if anyone else is exclusive-locking this
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* catalog and index they'll be doing it in that order.
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*/
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LockRelationOid(cache->cc_reloid,AccessShareLock);
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idesc=index_open(cache->cc_indexoid,AccessShareLock);
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index_close(idesc,AccessShareLock);
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UnlockRelationOid(cache->cc_reloid,AccessShareLock);
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}
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}
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‎src/backend/utils/cache/relcache.c

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@@ -1541,6 +1541,12 @@ RelationClose(Relation relation)
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*We assume that at the time we are called, we have at least AccessShareLock
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*on the target index. (Note: in the calls from RelationClearRelation,
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*this is legitimate because we know the rel has positive refcount.)
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*
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*If the target index is an index on pg_class or pg_index, we'd better have
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*previously gotten at least AccessShareLock on its underlying catalog,
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*else we are at risk of deadlock against someone trying to exclusive-lock
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*the heap and index in that order. This is ensured in current usage by
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*only applying this to indexes being opened or having positive refcount.
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*/
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staticvoid
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RelationReloadClassinfo(Relationrelation)

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