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Commitd01a744

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Fix hash_search to avoid corruption of the hash table on out-of-memory.
An out-of-memory error during expand_table() on a palloc-based hash tablewould leave a partially-initialized entry in the table. This would not beharmful for transient hash tables, since they'd get thrown away anyway attransaction abort. But for long-lived hash tables, such as the relcachehash, this would effectively corrupt the table, leading to crash or othermisbehavior later.To fix, rearrange the order of operations so that table enlargement isattempted before we insert a new entry, rather than after adding itto the hash table.Problem discovered by Hitoshi Harada, though this is a bit differentfrom his proposed patch.
1 parent823f83d commitd01a744

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1 file changed

+25
-16
lines changed

1 file changed

+25
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‎src/backend/utils/hash/dynahash.c

Lines changed: 25 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -820,6 +820,27 @@ hash_search_with_hash_value(HTAB *hashp,
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hctl->accesses++;
821821
#endif
822822

823+
/*
824+
* If inserting, check if it is time to split a bucket.
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*
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* NOTE: failure to expand table is not a fatal error, it just means we
827+
* have to run at higher fill factor than we wanted. However, if we're
828+
* using the palloc allocator then it will throw error anyway on
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* out-of-memory, so we must do this before modifying the table.
830+
*/
831+
if (action==HASH_ENTER||action==HASH_ENTER_NULL)
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{
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/*
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* Can't split if running in partitioned mode, nor if frozen, nor if
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* table is the subject of any active hash_seq_search scans. Strange
836+
* order of these tests is to try to check cheaper conditions first.
837+
*/
838+
if (!IS_PARTITIONED(hctl)&& !hashp->frozen&&
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hctl->nentries / (long) (hctl->max_bucket+1) >=hctl->ffactor&&
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!has_seq_scans(hashp))
841+
(void)expand_table(hashp);
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}
843+
823844
/*
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* Do the initial lookup
825846
*/
@@ -940,24 +961,12 @@ hash_search_with_hash_value(HTAB *hashp,
940961
currBucket->hashvalue=hashvalue;
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hashp->keycopy(ELEMENTKEY(currBucket),keyPtr,keysize);
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943-
/* caller is expected to fill the data field on return */
944-
945964
/*
946-
*Check if itistime tosplit a bucket. Can't split if running
947-
*in partitioned mode, nor if table is the subject of any active
948-
*hash_seq_search scans. Strange order of these tests is to try
949-
*to check cheaper conditions first.
965+
*Callerisexpected tofill the data field on return. DO NOT
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*insert any code that could possibly throw error here, as doing
967+
*so would leave the table entry incomplete and hence corrupt the
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*caller's data structure.
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*/
951-
if (!IS_PARTITIONED(hctl)&&
952-
hctl->nentries / (long) (hctl->max_bucket+1) >=hctl->ffactor&&
953-
!has_seq_scans(hashp))
954-
{
955-
/*
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* NOTE: failure to expand table is not a fatal error, it just
957-
* means we have to run at higher fill factor than we wanted.
958-
*/
959-
expand_table(hashp);
960-
}
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return (void*)ELEMENTKEY(currBucket);
963972
}

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