Ensure we have a snapshot while dropping ON COMMIT DROP temp tables.

Dropping a temp table could entail TOAST table access to clean out
toasted catalog entries, such as large pg_constraint.conbin strings
for complex CHECK constraints.  If we did that via ON COMMIT DROP,
we triggered the assertion in init_toast_snapshot(), because
there was no provision for setting up a snapshot for the drop
actions.  Fix that.

(I assume here that the adjacent truncation actions for ON COMMIT
DELETE ROWS don't have a similar problem: it doesn't seem like
nontransactional truncations would need to touch any toasted fields.
If that proves wrong, we could refactor a bit to have the same
snapshot acquisition cover that too.)

The test case added here does not fail before v15, because that
assertion was added in 277692220 which was not back-patched.
However, the race condition the assertion warns of surely
exists further back, so back-patch to all supported branches.

Per report from Richard Guo.

Discussion: https://postgr.es/m/CAMbWs4-x26=_QxxgdJyNbiCDzvtr2WV5ZDso_v-CukKEe6cBZw@mail.gmail.com

src/backend/commands/tablecmds.c

@@ -15859,6 +15859,12 @@ PreCommit_on_commit_actions(void)
 			add_exact_object_address(&object, targetObjects);
 		}
 
+		/*
+		 * Object deletion might involve toast table access (to clean up
+		 * toasted catalog entries), so ensure we have a valid snapshot.
+		 */
+		PushActiveSnapshot(GetTransactionSnapshot());
+
 		/*
 		 * Since this is an automatic drop, rather than one directly initiated
 		 * by the user, we pass the PERFORM_DELETION_INTERNAL flag.
@@ -15866,6 +15872,8 @@ PreCommit_on_commit_actions(void)
 		performMultipleDeletions(targetObjects, DROP_CASCADE,
 								 PERFORM_DELETION_INTERNAL | PERFORM_DELETION_QUIETLY);
 
+		PopActiveSnapshot();
+
 #ifdef USE_ASSERT_CHECKING
 
 		/*

src/test/regress/expected/temp.out

@@ -108,6 +108,26 @@ SELECT * FROM temptest;
    1
 (1 row)
 
+COMMIT;
+SELECT * FROM temptest;
+ERROR:  relation "temptest" does not exist
+LINE 1: SELECT * FROM temptest;
+                      ^
+-- Test it with a CHECK condition that produces a toasted pg_constraint entry
+BEGIN;
+do $$
+begin
+  execute format($cmd$
+    CREATE TEMP TABLE temptest (col text CHECK (col < %L)) ON COMMIT DROP
+  $cmd$,
+    (SELECT string_agg(g.i::text || ':' || random()::text, '|')
+     FROM generate_series(1, 100) g(i)));
+end$$;
+SELECT * FROM temptest;
+ col 
+-----
+(0 rows)
+
 COMMIT;
 SELECT * FROM temptest;
 ERROR:  relation "temptest" does not exist

src/test/regress/sql/temp.sql

@@ -101,6 +101,22 @@ COMMIT;
 
 SELECT * FROM temptest;
 
+-- Test it with a CHECK condition that produces a toasted pg_constraint entry
+BEGIN;
+do $$
+begin
+  execute format($cmd$
+    CREATE TEMP TABLE temptest (col text CHECK (col < %L)) ON COMMIT DROP
+  $cmd$,
+    (SELECT string_agg(g.i::text || ':' || random()::text, '|')
+     FROM generate_series(1, 100) g(i)));
+end$$;
+
+SELECT * FROM temptest;
+COMMIT;
+
+SELECT * FROM temptest;
+
 -- ON COMMIT is only allowed for TEMP
 
 CREATE TABLE temptest(col int) ON COMMIT DELETE ROWS;