Fix logical decoding error when system table w/ toast is repeatedly rewritten.

Repeatedly rewriting a mapped catalog table with VACUUM FULL or
CLUSTER could cause logical decoding to fail with:
ERROR, "could not map filenode \"%s\" to relation OID"

To trigger the problem the rewritten catalog had to have live tuples
with toasted columns.

The problem was triggered as during catalog table rewrites the
heap_insert() check that prevents logical decoding information to be
emitted for system catalogs, failed to treat the new heap's toast table
as a system catalog (because the new heap is not recognized as a
catalog table via RelationIsLogicallyLogged()). The relmapper, in
contrast to the normal catalog contents, does not contain historical
information. After a single rewrite of a mapped table the new relation
is known to the relmapper, but if the table is rewritten twice before
logical decoding occurs, the relfilenode cannot be mapped to a
relation anymore.  Which then leads us to error out.   This only
happens for toast tables, because the main table contents aren't
re-inserted with heap_insert().

The fix is simple, add a new heap_insert() flag that prevents logical
decoding information from being emitted, and accept during decoding
that there might not be tuple data for toast tables.

Unfortunately that does not fix pre-existing logical decoding
errors. Doing so would require not throwing an error when a filenode
cannot be mapped to a relation during decoding, and that seems too
likely to hide bugs.  If it's crucial to fix decoding for an existing
slot, temporarily changing the ERROR in ReorderBufferCommit() to a
WARNING appears to be the best fix.

Author: Andres Freund
Discussion: https://postgr.es/m/20180914021046.oi7dm4ra3ot2g2kt@alap3.anarazel.de
Backpatch: 9.4-, where logical decoding was introduced

1 files changed, 41 insertions, 1 deletions

diff --git a/contrib/test_decoding/sql/rewrite.sql b/contrib/test_decoding/sql/rewrite.sql
index 8a7329423de..4271b82bead 100644
--- a/contrib/test_decoding/sql/rewrite.sql
+++ b/contrib/test_decoding/sql/rewrite.sql
@@ -3,6 +3,35 @@ SET synchronous_commit = on;
 
 DROP TABLE IF EXISTS replication_example;
 
+-- Ensure there's tables with toast datums.  To do so, we dynamically
+-- create a function returning a large textblob.  We want tables of
+-- different kinds: mapped catalog table, unmapped catalog table,
+-- shared catalog table and usertable.
+CREATE FUNCTION exec(text) returns void language plpgsql volatile
+  AS $f$
+    BEGIN
+      EXECUTE $1;
+    END;
+$f$;
+CREATE ROLE justforcomments NOLOGIN;
+
+SELECT exec(
+    format($outer$CREATE FUNCTION iamalongfunction() RETURNS TEXT IMMUTABLE LANGUAGE SQL AS $f$SELECT text %L$f$$outer$,
+           (SELECT repeat(string_agg(to_char(g.i, 'FM0000'), ''), 50) FROM generate_series(1, 500) g(i))));
+SELECT exec(
+    format($outer$COMMENT ON FUNCTION iamalongfunction() IS %L$outer$,
+           iamalongfunction()));
+SELECT exec(
+    format($outer$COMMENT ON ROLE JUSTFORCOMMENTS IS %L$outer$,
+           iamalongfunction()));
+CREATE TABLE iamalargetable AS SELECT iamalongfunction() longfunctionoutput;
+
+-- verify toast usage
+SELECT pg_relation_size((SELECT reltoastrelid FROM pg_class WHERE oid = 'pg_proc'::regclass)) > 0;
+SELECT pg_relation_size((SELECT reltoastrelid FROM pg_class WHERE oid = 'pg_description'::regclass)) > 0;
+SELECT pg_relation_size((SELECT reltoastrelid FROM pg_class WHERE oid = 'pg_shdescription'::regclass)) > 0;
+
+
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
 CREATE TABLE replication_example(id SERIAL PRIMARY KEY, somedata int, text varchar(120));
 INSERT INTO replication_example(somedata) VALUES (1);
@@ -57,6 +86,17 @@ COMMIT;
 CHECKPOINT;
 
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
-SELECT pg_drop_replication_slot('regression_slot');
 
+-- trigger repeated rewrites of a system catalog with a toast table,
+-- that previously was buggy: 20180914021046.oi7dm4ra3ot2g2kt@alap3.anarazel.de
+VACUUM FULL pg_proc; VACUUM FULL pg_description; VACUUM FULL pg_shdescription; VACUUM FULL iamalargetable;
+INSERT INTO replication_example(somedata, testcolumn1, testcolumn3) VALUES (8, 6, 1);
+VACUUM FULL pg_proc; VACUUM FULL pg_description; VACUUM FULL pg_shdescription; VACUUM FULL iamalargetable;
+INSERT INTO replication_example(somedata, testcolumn1, testcolumn3) VALUES (9, 7, 1);
+SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+
+SELECT pg_drop_replication_slot('regression_slot');
 DROP TABLE IF EXISTS replication_example;
+DROP FUNCTION iamalongfunction();
+DROP FUNCTION exec(text);
+DROP ROLE justforcomments;