Date: Tue, 29 Mar 2022 17:06:21 +0900 (JST)
Message-Id: <20220329.170621.2101149643828659444.horikyota.ntt@gmail.com>
To: er@xs4all.nl
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: TRAP: FailedAssertion("HaveRegisteredOrActiveSnapshot()",
 File: "toast_internals.c", Line: 670, PID: 19403)
From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
In-Reply-To: <20220328.183646.1041002117568499041.horikyota.ntt@gmail.com>
References: <dc9dd229-ed30-6c62-4c41-d733ffff776b@xs4all.nl>
	<20220328.183646.1041002117568499041.horikyota.ntt@gmail.com>
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At Mon, 28 Mar 2022 18:36:46 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in 
> Inserting EnsurePortalSnapshotExists() to RunFromStore fixes this but
> I'm not sure where is the right place to do this..

Then, I found that portal->holdSnapshot is that.  I came up with the
attached.  It does the follows:

1. Teach PlannedStmtRequiresSnapshot() to return true for FetchStmt.

2. Use holdSnapshot in RunFromStore if any.


The rerpducer is reduced to as small as the following.

CREATE TABLE t (a text);
INSERT INTO t VALUES('some random text');
BEGIN;
DECLARE c CURSOR FOR SELECT * FROM t;
FETCH ALL IN c;

But I haven't come up with a reasonable way to generate the 'some
random text' yet.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

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 filename="use_holdsnapshot_to_read_cursor.patch"

diff --git a/src/backend/tcop/pquery.c b/src/backend/tcop/pquery.c
index 5aa5a350f3..8354029f2a 100644
--- a/src/backend/tcop/pquery.c
+++ b/src/backend/tcop/pquery.c
@@ -1068,6 +1068,18 @@ RunFromStore(Portal portal, ScanDirection direction, uint64 count,
 
 	dest->rStartup(dest, CMD_SELECT, portal->tupDesc);
 
+	/*
+	 * If holdSnapshot is set, that means we should use the snapshot to read
+	 * this store.
+	 */
+	if (portal->holdSnapshot)
+	{
+		Assert(portal->portalSnapshot == 0);
+		PushActiveSnapshotWithLevel(portal->holdSnapshot,
+									portal->createLevel);
+		portal->portalSnapshot = GetActiveSnapshot();
+	}
+	
 	if (ScanDirectionIsNoMovement(direction))
 	{
 		/* do nothing except start/stop the destination */
@@ -1114,6 +1126,13 @@ RunFromStore(Portal portal, ScanDirection direction, uint64 count,
 
 	dest->rShutdown(dest);
 
+	if (portal->holdSnapshot)
+	{
+		Assert(portal->portalSnapshot == GetActiveSnapshot());
+		PopActiveSnapshot();
+		portal->portalSnapshot = NULL;
+	}
+
 	ExecDropSingleTupleTableSlot(slot);
 
 	return current_tuple_count;
@@ -1756,7 +1775,6 @@ PlannedStmtRequiresSnapshot(PlannedStmt *pstmt)
 		IsA(utilityStmt, VariableShowStmt) ||
 		IsA(utilityStmt, ConstraintsSetStmt) ||
 	/* efficiency hacks from here down */
-		IsA(utilityStmt, FetchStmt) ||
 		IsA(utilityStmt, ListenStmt) ||
 		IsA(utilityStmt, NotifyStmt) ||
 		IsA(utilityStmt, UnlistenStmt) ||

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