Fix dynahash.c to suppress hash bucket splits while a hash_seq_search() scan

is in progress on the same hashtable.  This seems the least invasive way to
fix the recently-recognized problem that a split could cause the scan to
visit entries twice or (with much lower probability) miss them entirely.
The only field-reported problem caused by this is the "failed to re-find
shared lock object" PANIC in COMMIT PREPARED reported by Michel Dorochevsky,
which was caused by multiply visited entries.  However, it seems certain
that mdsync() is vulnerable to missing required fsync's due to missed
entries, and I am fearful that RelationCacheInitializePhase2() might be at
risk as well.  Because of that and the generalized hazard presented by this
bug, back-patch all the supported branches.

Along the way, fix pg_prepared_statement() and pg_cursor() to not assume
that the hashtables they are examining will stay static between calls.
This is risky regardless of the newly noted dynahash problem, because
hash_seq_search() has never promised to cope with deletion of table entries
other than the just-returned one.  There may be no bug here because the only
supported way to call these functions is via ExecMakeTableFunctionResult()
which will cycle them to completion before doing anything very interesting,
but it seems best to get rid of the assumption.  This affects 8.2 and HEAD
only, since those functions weren't there earlier.

1 files changed, 71 insertions, 70 deletions

diff --git a/src/backend/utils/mmgr/portalmem.c b/src/backend/utils/mmgr/portalmem.c
index 4f8cba9ba5a..3e3505caf05 100644
--- a/src/backend/utils/mmgr/portalmem.c
+++ b/src/backend/utils/mmgr/portalmem.c
@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/mmgr/portalmem.c,v 1.97 2006/11/23 01:14:59 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/mmgr/portalmem.c,v 1.97.2.1 2007/04/26 23:24:57 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -22,7 +22,6 @@
 #include "access/xact.h"
 #include "catalog/pg_type.h"
 #include "commands/portalcmds.h"
-#include "funcapi.h"
 #include "miscadmin.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
@@ -577,7 +576,9 @@ AtCommit_Portals(void)
 		/* Zap all non-holdable portals */
 		PortalDrop(portal, true);
 
-		/* Restart the iteration */
+		/* Restart the iteration in case that led to other drops */
+		/* XXX is this really necessary? */
+		hash_seq_term(&status);
 		hash_seq_init(&status, PortalHashTable);
 	}
 }
@@ -806,79 +807,68 @@ AtSubCleanup_Portals(SubTransactionId mySubid)
 Datum
 pg_cursor(PG_FUNCTION_ARGS)
 {
-	FuncCallContext *funcctx;
-	HASH_SEQ_STATUS *hash_seq;
+	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+	TupleDesc	tupdesc;
+	Tuplestorestate *tupstore;
+	MemoryContext per_query_ctx;
+	MemoryContext oldcontext;
+	HASH_SEQ_STATUS hash_seq;
 	PortalHashEnt *hentry;
 
-	/* stuff done only on the first call of the function */
-	if (SRF_IS_FIRSTCALL())
-	{
-		MemoryContext oldcontext;
-		TupleDesc	tupdesc;
-
-		/* create a function context for cross-call persistence */
-		funcctx = SRF_FIRSTCALL_INIT();
-
-		/*
-		 * switch to memory context appropriate for multiple function calls
-		 */
-		oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
-
-		if (PortalHashTable)
-		{
-			hash_seq = (HASH_SEQ_STATUS *) palloc(sizeof(HASH_SEQ_STATUS));
-			hash_seq_init(hash_seq, PortalHashTable);
-			funcctx->user_fctx = (void *) hash_seq;
-		}
-		else
-			funcctx->user_fctx = NULL;
-
-		/*
-		 * build tupdesc for result tuples. This must match the definition of
-		 * the pg_cursors view in system_views.sql
-		 */
-		tupdesc = CreateTemplateTupleDesc(6, false);
-		TupleDescInitEntry(tupdesc, (AttrNumber) 1, "name",
-						   TEXTOID, -1, 0);
-		TupleDescInitEntry(tupdesc, (AttrNumber) 2, "statement",
-						   TEXTOID, -1, 0);
-		TupleDescInitEntry(tupdesc, (AttrNumber) 3, "is_holdable",
-						   BOOLOID, -1, 0);
-		TupleDescInitEntry(tupdesc, (AttrNumber) 4, "is_binary",
-						   BOOLOID, -1, 0);
-		TupleDescInitEntry(tupdesc, (AttrNumber) 5, "is_scrollable",
-						   BOOLOID, -1, 0);
-		TupleDescInitEntry(tupdesc, (AttrNumber) 6, "creation_time",
-						   TIMESTAMPTZOID, -1, 0);
-
-		funcctx->tuple_desc = BlessTupleDesc(tupdesc);
-		MemoryContextSwitchTo(oldcontext);
-	}
-
-	/* stuff done on every call of the function */
-	funcctx = SRF_PERCALL_SETUP();
-	hash_seq = (HASH_SEQ_STATUS *) funcctx->user_fctx;
+	/* check to see if caller supports us returning a tuplestore */
+	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("set-valued function called in context that cannot accept a set")));
+	if (!(rsinfo->allowedModes & SFRM_Materialize))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("materialize mode required, but it is not " \
+						"allowed in this context")));
+
+	/* need to build tuplestore in query context */
+	per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
+	oldcontext = MemoryContextSwitchTo(per_query_ctx);
 
-	/* if the hash table is uninitialized, we're done */
-	if (hash_seq == NULL)
-		SRF_RETURN_DONE(funcctx);
+	/*
+	 * build tupdesc for result tuples. This must match the definition of
+	 * the pg_cursors view in system_views.sql
+	 */
+	tupdesc = CreateTemplateTupleDesc(6, false);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "name",
+					   TEXTOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "statement",
+					   TEXTOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "is_holdable",
+					   BOOLOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 4, "is_binary",
+					   BOOLOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 5, "is_scrollable",
+					   BOOLOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 6, "creation_time",
+					   TIMESTAMPTZOID, -1, 0);
 
-	/* loop until we find a visible portal or hit the end of the list */
-	while ((hentry = hash_seq_search(hash_seq)) != NULL)
-	{
-		if (hentry->portal->visible)
-			break;
-	}
+	/*
+	 * We put all the tuples into a tuplestore in one scan of the hashtable.
+	 * This avoids any issue of the hashtable possibly changing between calls.
+	 */
+	tupstore = tuplestore_begin_heap(true, false, work_mem);
 
-	if (hentry)
+	hash_seq_init(&hash_seq, PortalHashTable);
+	while ((hentry = hash_seq_search(&hash_seq)) != NULL)
 	{
-		Portal		portal;
-		Datum		result;
+		Portal		portal = hentry->portal;
 		HeapTuple	tuple;
 		Datum		values[6];
 		bool		nulls[6];
 
-		portal = hentry->portal;
+		/* report only "visible" entries */
+		if (!portal->visible)
+			continue;
+
+		/* generate junk in short-term context */
+		MemoryContextSwitchTo(oldcontext);
+
 		MemSet(nulls, 0, sizeof(nulls));
 
 		values[0] = DirectFunctionCall1(textin, CStringGetDatum(portal->name));
@@ -892,10 +882,21 @@ pg_cursor(PG_FUNCTION_ARGS)
 		values[4] = BoolGetDatum(portal->cursorOptions & CURSOR_OPT_SCROLL);
 		values[5] = TimestampTzGetDatum(portal->creation_time);
 
-		tuple = heap_form_tuple(funcctx->tuple_desc, values, nulls);
-		result = HeapTupleGetDatum(tuple);
-		SRF_RETURN_NEXT(funcctx, result);
+		tuple = heap_form_tuple(tupdesc, values, nulls);
+
+		/* switch to appropriate context while storing the tuple */
+		MemoryContextSwitchTo(per_query_ctx);
+		tuplestore_puttuple(tupstore, tuple);
 	}
 
-	SRF_RETURN_DONE(funcctx);
+	/* clean up and return the tuplestore */
+	tuplestore_donestoring(tupstore);
+
+	MemoryContextSwitchTo(oldcontext);
+
+	rsinfo->returnMode = SFRM_Materialize;
+	rsinfo->setResult = tupstore;
+	rsinfo->setDesc = tupdesc;
+
+	return (Datum) 0;
 }