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

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 385616bdb5486d593129606bded963fc5ba559d7..b13dd19f5550437fcd64b815e779232ee1aaaaf9 100644 (file)
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -14331,6 +14331,12 @@ 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.
@@ -14338,6 +14344,8 @@ PreCommit_on_commit_actions(void)
                performMultipleDeletions(targetObjects, DROP_CASCADE,
                                                                 PERFORM_DELETION_INTERNAL | PERFORM_DELETION_QUIETLY);
 
+               PopActiveSnapshot();
+
 #ifdef USE_ASSERT_CHECKING
 
                /*

diff --git a/src/test/regress/expected/temp.out b/src/test/regress/expected/temp.out
index 5fa20a05bfe4768b448e918f495b7a1fae86e91a..981f76497796031dbd6aa95ae4d1cd5a7a8aca6b 100644 (file)
--- a/src/test/regress/expected/temp.out
+++ b/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

diff --git a/src/test/regress/sql/temp.sql b/src/test/regress/sql/temp.sql
index fdef2b0f978fdced17fb0eca669442e2003a842e..9414fb633614d22dfe4e65bf9e28a843e558b2db 100644 (file)
--- a/src/test/regress/sql/temp.sql
+++ b/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;