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Commit909eebf

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dshash: revise sequential scan support.
The previous coding of dshash_seq_next(), on the first call, accessedstatus->hash_table->size_log2 without holding a partition lock and withoutguaranteeing that ensure_valid_bucket_pointers() had ever been called.That oversight turns out to not have immediately visible effects, becausebucket 0 is always in partition 0, and ensure_valid_bucket_pointers() wascalled after acquiring the partition lock. However,PARTITION_FOR_BUCKET_INDEX() with a size_log2 of 0 ends up triggering formallyundefined behaviour.Simplify by accessing partition 0, without using PARTITION_FOR_BUCKET_INDEX().While at it, remove dshash_get_current(), there is no convincing usecase. Also polish a few comments.Author: Andres Freund <andres@anarazel.de>Reviewed-By: Thomas Munro <thomas.munro@gmail.com>Discussion:https://postgr.es/m/CA+hUKGL9hY_VY=+oUK+Gc1iSRx-Ls5qeYJ6q=dQVZnT3R63Taw@mail.gmail.com
1 parent55e566f commit909eebf

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+27
-30
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2 files changed

+27
-30
lines changed

‎src/backend/lib/dshash.c

Lines changed: 27 additions & 29 deletions
Original file line numberDiff line numberDiff line change
@@ -601,13 +601,12 @@ dshash_memhash(const void *v, size_t size, void *arg)
601601
}
602602

603603
/*
604-
* dshash_seq_init/_next/_term
605-
* Sequentially scan through dshash table and return all the
606-
* elements one by one, return NULL when no more.
604+
* Sequentially scan through dshash table and return all the elements one by
605+
* one, return NULL when all elements have been returned.
607606
*
608-
* dshash_seq_termshould always be called when a scan finished.
609-
*The caller maydelete returned elements midst of a scan by using
610-
*dshash_delete_current(). exclusivemust betrue to delete elements.
607+
* dshash_seq_termneeds to be called when a scan finished. The caller may
608+
* delete returned elements midst of a scan by using dshash_delete_current()
609+
*if exclusive=true.
611610
*/
612611
void
613612
dshash_seq_init(dshash_seq_status*status,dshash_table*hash_table,
@@ -625,34 +624,38 @@ dshash_seq_init(dshash_seq_status *status, dshash_table *hash_table,
625624
/*
626625
* Returns the next element.
627626
*
628-
* Returned elements are locked and the callermust notexplicitlyrelease
629-
*it. It isreleasedat the next calltodshash_next().
627+
* Returned elements are locked and the callermay not release the lock. It is
628+
* releasedby future callstodshash_seq_next() or dshash_seq_term().
630629
*/
631630
void*
632631
dshash_seq_next(dshash_seq_status*status)
633632
{
634633
dsa_pointernext_item_pointer;
635634

636-
if (status->curitem==NULL)
635+
/*
636+
* Not yet holding any partition locks. Need to determine the size of the
637+
* hash table, it could have been resized since we were looking
638+
* last. Since we iterate in partition order, we can start by
639+
* unconditionally lock partition 0.
640+
*
641+
* Once we hold the lock, no resizing can happen until the scan ends. So
642+
* we don't need to repeatedly call ensure_valid_bucket_pointers().
643+
*/
644+
if (status->curpartition==-1)
637645
{
638-
intpartition;
639-
640646
Assert(status->curbucket==0);
641647
Assert(!status->hash_table->find_locked);
642648

643-
/* first shot. grab the first item. */
644-
partition=
645-
PARTITION_FOR_BUCKET_INDEX(status->curbucket,
646-
status->hash_table->size_log2);
647-
LWLockAcquire(PARTITION_LOCK(status->hash_table,partition),
649+
status->curpartition=0;
650+
651+
LWLockAcquire(PARTITION_LOCK(status->hash_table,
652+
status->curpartition),
648653
status->exclusive ?LW_EXCLUSIVE :LW_SHARED);
649-
status->curpartition=partition;
650654

651-
/* resize doesn't happen from now until seq scan ends */
652-
status->nbuckets=
653-
NUM_BUCKETS(status->hash_table->control->size_log2);
654655
ensure_valid_bucket_pointers(status->hash_table);
655656

657+
status->nbuckets=
658+
NUM_BUCKETS(status->hash_table->control->size_log2);
656659
next_item_pointer=status->hash_table->buckets[status->curbucket];
657660
}
658661
else
@@ -714,7 +717,7 @@ dshash_seq_next(dshash_seq_status *status)
714717
/*
715718
* Terminates the seqscan and release all locks.
716719
*
717-
*Shouldbealwayscalledwhen finishing or exiting a seqscan.
720+
*Needs tobe calledafter finishing or when exiting a seqscan.
718721
*/
719722
void
720723
dshash_seq_term(dshash_seq_status*status)
@@ -726,7 +729,9 @@ dshash_seq_term(dshash_seq_status *status)
726729
LWLockRelease(PARTITION_LOCK(status->hash_table,status->curpartition));
727730
}
728731

729-
/* Remove the current entry while a seq scan. */
732+
/*
733+
* Remove the current entry of the seq scan.
734+
*/
730735
void
731736
dshash_delete_current(dshash_seq_status*status)
732737
{
@@ -746,13 +751,6 @@ dshash_delete_current(dshash_seq_status *status)
746751
delete_item(hash_table,item);
747752
}
748753

749-
/* Get the current entry while a seq scan. */
750-
void*
751-
dshash_get_current(dshash_seq_status*status)
752-
{
753-
returnENTRY_FROM_ITEM(status->curitem);
754-
}
755-
756754
/*
757755
* Print debugging information about the internal state of the hash table to
758756
* stderr. The caller must hold no partition locks.

‎src/include/lib/dshash.h

Lines changed: 0 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -101,7 +101,6 @@ extern void dshash_seq_init(dshash_seq_status *status, dshash_table *hash_table,
101101
externvoid*dshash_seq_next(dshash_seq_status*status);
102102
externvoiddshash_seq_term(dshash_seq_status*status);
103103
externvoiddshash_delete_current(dshash_seq_status*status);
104-
externvoid*dshash_get_current(dshash_seq_status*status);
105104

106105
/* Convenience hash and compare functions wrapping memcmp and tag_hash. */
107106
externintdshash_memcmp(constvoid*a,constvoid*b,size_tsize,void*arg);

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