Two more RI fast-path issues

Hi,

While reviewing the RI fast path after removing batching from master,
with help from Claude Opus 5.5, I found two more places where it
behaves differently from the SPI path, apart from the cast issues
discussed in [1].

1. Fast-path doesn't refuse to take row locks in a read-only
transaction, whereas the SPI path does. In the SPI path, the executor
calls PreventCommandIfReadOnly(), but the fast-path continues without
checking. With pk as a plain table and ppk as a partitioned one
(whose checks use SPI), and both foreign keys deferred:

CREATE TABLE pk (a int PRIMARY KEY); INSERT INTO pk VALUES (1);
CREATE TABLE ppk (a int PRIMARY KEY) PARTITION BY LIST (a);
CREATE TABLE ppk1 PARTITION OF ppk DEFAULT; INSERT INTO ppk VALUES (1);
CREATE TABLE fk (a int REFERENCES pk DEFERRABLE INITIALLY DEFERRED);
CREATE TABLE pfk (a int REFERENCES ppk DEFERRABLE INITIALLY DEFERRED);

-- SPI
BEGIN; INSERT INTO pfk VALUES (1); SET TRANSACTION READ ONLY;
COMMIT;
ERROR: cannot execute SELECT FOR KEY SHARE in a read-only transaction
CONTEXT: SQL statement "SELECT 1 FROM "public"."ppk" x WHERE "a"
OPERATOR(pg_catalog.=) $1 FOR KEY SHARE OF x"

-- fast -path
BEGIN; INSERT INTO fk VALUES (1); SET TRANSACTION READ ONLY;
COMMIT;
COMMIT

2. Fast-path can produce "attempted to lock invisible tuple" error
due to a bug in how it calls table_tuple_lock(). In the SPI path the
ExecLockRows() calls table_tuple_lock() with es_output_cid, whereas
the fast-path uses GetCurrentCommandId(), which can result in the
latter case using a command ID that is different from when the initial
snapshot was taken. If user code running during the scan advances it,
and if that code also updated the referenced row, the fast path can
fail with "attempt to lock invisible tuple" when it should report an
FK violation (). In this case, user code is the index's equality
operator as shown in the test case added in 0002.

Attached patches to fix both, which I'd like to get in before RC1 freeze.

--
Thanks, Amit Langote

[1] https://www.postgresql.org/message-id/CA%2BHiwqFqUhTc_oO%2B58Cs8mww8ZzJ8b8-DL%2BKGbeTMecLVTunvQ%40mail.gmail.com