From e5a3c9d9b5ce535151d3a7e3173e8d27d2d8cd58 Mon Sep 17 00:00:00 2001
From: Etsuro Fujita <efujita@postgresql.org>
Date: Sun, 1 Jun 2025 17:30:00 +0900
Subject: postgres_fdw: Inherit the local transaction's access/deferrable
 modes.

Previously, postgres_fdw always 1) opened a remote transaction in READ
WRITE mode even when the local transaction was READ ONLY, causing a READ
ONLY transaction using it that references a foreign table mapped to a
remote view executing a volatile function to write in the remote side,
and 2) opened the remote transaction in NOT DEFERRABLE mode even when
the local transaction was DEFERRABLE, causing a SERIALIZABLE READ ONLY
DEFERRABLE transaction using it to abort due to a serialization failure
in the remote side.

To avoid these, modify postgres_fdw to open a remote transaction in the
same access/deferrable modes as the local transaction.  This commit also
modifies it to open a remote subtransaction in the same access mode as
the local subtransaction.

Although these issues exist since the introduction of postgres_fdw,
there have been no reports from the field.  So it seems fine to just fix
them in master only.

Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAPmGK16n_hcUUWuOdmeUS%2Bw4Q6dZvTEDHb%3DOP%3D5JBzo-M3QmpQ%40mail.gmail.com
---
 contrib/postgres_fdw/sql/postgres_fdw.sql | 78 +++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)

(limited to 'contrib/postgres_fdw/sql/postgres_fdw.sql')

diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index e534b40de3c..20a535b99d8 100644
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -4200,6 +4200,84 @@ SELECT count(*) FROM remote_application_name
 DROP FOREIGN TABLE remote_application_name;
 DROP VIEW my_application_name;
 
+-- ===================================================================
+-- test read-only and/or deferrable transactions
+-- ===================================================================
+CREATE TABLE loct (f1 int, f2 text);
+CREATE FUNCTION locf() RETURNS SETOF loct LANGUAGE SQL AS
+  'UPDATE public.loct SET f2 = f2 || f2 RETURNING *';
+CREATE VIEW locv AS SELECT t.* FROM locf() t;
+CREATE FOREIGN TABLE remt (f1 int, f2 text)
+  SERVER loopback OPTIONS (table_name 'locv');
+CREATE FOREIGN TABLE remt2 (f1 int, f2 text)
+  SERVER loopback2 OPTIONS (table_name 'locv');
+INSERT INTO loct VALUES (1, 'foo'), (2, 'bar');
+
+START TRANSACTION READ ONLY;
+SAVEPOINT s;
+SELECT * FROM remt;  -- should fail
+ROLLBACK TO s;
+RELEASE SAVEPOINT s;
+SELECT * FROM remt;  -- should fail
+ROLLBACK;
+
+START TRANSACTION;
+SAVEPOINT s;
+SET transaction_read_only = on;
+SELECT * FROM remt;  -- should fail
+ROLLBACK TO s;
+RELEASE SAVEPOINT s;
+SET transaction_read_only = on;
+SELECT * FROM remt;  -- should fail
+ROLLBACK;
+
+START TRANSACTION;
+SAVEPOINT s;
+SELECT * FROM remt;  -- should work
+SET transaction_read_only = on;
+SELECT * FROM remt;  -- should fail
+ROLLBACK TO s;
+RELEASE SAVEPOINT s;
+SELECT * FROM remt;  -- should work
+SET transaction_read_only = on;
+SELECT * FROM remt;  -- should fail
+ROLLBACK;
+
+START TRANSACTION;
+SAVEPOINT s;
+SELECT * FROM remt;  -- should work
+SET transaction_read_only = on;
+SELECT * FROM remt2;  -- should fail
+ROLLBACK TO s;
+RELEASE SAVEPOINT s;
+SELECT * FROM remt;  -- should work
+SET transaction_read_only = on;
+SELECT * FROM remt2;  -- should fail
+ROLLBACK;
+
+DROP FOREIGN TABLE remt;
+CREATE FOREIGN TABLE remt (f1 int, f2 text)
+  SERVER loopback OPTIONS (table_name 'loct');
+
+START TRANSACTION ISOLATION LEVEL SERIALIZABLE READ ONLY;
+SELECT * FROM remt;
+COMMIT;
+
+START TRANSACTION ISOLATION LEVEL SERIALIZABLE DEFERRABLE;
+SELECT * FROM remt;
+COMMIT;
+
+START TRANSACTION ISOLATION LEVEL SERIALIZABLE READ ONLY DEFERRABLE;
+SELECT * FROM remt;
+COMMIT;
+
+-- Clean up
+DROP FOREIGN TABLE remt;
+DROP FOREIGN TABLE remt2;
+DROP VIEW locv;
+DROP FUNCTION locf();
+DROP TABLE loct;
+
 -- ===================================================================
 -- test parallel commit and parallel abort
 -- ===================================================================
-- 
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