pgstat_heap() creates a BufferAccessStrategy and attaches it to a HeapScanDesc. It continues to use that strategy after calling heap_endscan(), which frees the strategy. This is only a risk when the table contains empty pages at the end. I get a crash in an assert-enabled build with this test procedure, after disabling autovacuum:
-- session 1 create table t (c) as select * from generate_series(1,20000); delete from t where c > 10000; -- session 2 begin; lock table t in access share mode; -- session 1 vacuum t; -- restart PostgreSQL to clear shared buffers -- session 3 select * from pgstattuple('t'); The simplest fix is to move the heap_endscan() call past the last use of the strategy. However, I don't think this function ought to be creating a strategy explicitly. It should use the one that initscan() creates, if any. -- Noah Misch EnterpriseDB http://www.enterprisedb.com
diff --git a/contrib/pgstattuple/pgstattuple.c b/contrib/pgstattuple/pgstattuple.c index edc603f..1007748 100644 --- a/contrib/pgstattuple/pgstattuple.c +++ b/contrib/pgstattuple/pgstattuple.c @@ -274,7 +274,6 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo) BlockNumber tupblock; Buffer buffer; pgstattuple_type stat = {0}; - BufferAccessStrategy bstrategy; SnapshotData SnapshotDirty; /* Disable syncscan because we assume we scan from block zero upwards */ @@ -283,10 +282,6 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo) nblocks = scan->rs_nblocks; /* # blocks to be scanned */ - /* prepare access strategy for this table */ - bstrategy = GetAccessStrategy(BAS_BULKREAD); - scan->rs_strategy = bstrategy; - /* scan the relation */ while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL) { @@ -320,26 +315,28 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo) { CHECK_FOR_INTERRUPTS(); - buffer = ReadBufferExtended(rel, MAIN_FORKNUM, block, RBM_NORMAL, bstrategy); + buffer = ReadBufferExtended(rel, MAIN_FORKNUM, block, + RBM_NORMAL, scan->rs_strategy); LockBuffer(buffer, BUFFER_LOCK_SHARE); stat.free_space += PageGetHeapFreeSpace((Page) BufferGetPage(buffer)); UnlockReleaseBuffer(buffer); block++; } } - heap_endscan(scan); while (block < nblocks) { CHECK_FOR_INTERRUPTS(); - buffer = ReadBufferExtended(rel, MAIN_FORKNUM, block, RBM_NORMAL, bstrategy); + buffer = ReadBufferExtended(rel, MAIN_FORKNUM, block, + RBM_NORMAL, scan->rs_strategy); LockBuffer(buffer, BUFFER_LOCK_SHARE); stat.free_space += PageGetHeapFreeSpace((Page) BufferGetPage(buffer)); UnlockReleaseBuffer(buffer); block++; } + heap_endscan(scan); relation_close(rel, AccessShareLock); stat.table_len = (uint64) nblocks *BLCKSZ;
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