Stop demanding that top xact must be seen before subxact in decoding.

Manifested as

ERROR:  subtransaction logged without previous top-level txn record

this check forbids legit behaviours like
 - First xl_xact_assignment record is beyond reading, i.e. earlier
   restart_lsn.
 - After restart_lsn there is some change of a subxact.
 - After that, there is second xl_xact_assignment (for another subxact)
   revealing the relationship between top and first subxact.

Such a transaction won't be streamed anyway because we hadn't seen it in
full.  Saying for sure whether xact of some record encountered after
the snapshot was deserialized can be streamed or not requires to know
whether it wrote something before deserialization point --if yes, it
hasn't been seen in full and can't be decoded. Snapshot doesn't have such
info, so there is no easy way to relax the check.

Reported-by: Hsu, John
Diagnosed-by: Arseny Sher
Author: Arseny Sher, Amit Kapila
Reviewed-by: Amit Kapila, Dilip Kumar
Backpatch-through: 9.5
Discussion: https://postgr.es/m/AB5978B2-1772-4FEE-A245-74C91704ECB0@amazon.com

contrib/test_decoding/Makefile

@@ -7,7 +7,7 @@ REGRESS = ddl xact rewrite toast permissions decoding_in_xact \
 	decoding_into_rel binary prepared replorigin time messages \
 	spill slot truncate
 ISOLATION = mxact delayed_startup ondisk_startup concurrent_ddl_dml \
-	oldest_xmin snapshot_transfer
+	oldest_xmin snapshot_transfer subxact_without_top
 
 REGRESS_OPTS = --temp-config $(top_srcdir)/contrib/test_decoding/logical.conf
 ISOLATION_OPTS = --temp-config $(top_srcdir)/contrib/test_decoding/logical.conf

contrib/test_decoding/expected/subxact_without_top.out

+Parsed test spec with 3 sessions
+
+starting permutation: s0_begin s0_first_subxact s2_checkpoint s1_begin s1_dml s0_many_subxacts s0_commit s2_checkpoint s2_get_changes_suppress_output s2_get_changes_suppress_output s1_commit s2_get_changes
+step s0_begin: BEGIN;
+step s0_first_subxact: 
+    DO LANGUAGE plpgsql $$
+      BEGIN
+        BEGIN
+          INSERT INTO harvest VALUES (41);
+	EXCEPTION WHEN OTHERS THEN RAISE;
+	END;
+      END $$;
+
+step s2_checkpoint: CHECKPOINT;
+step s1_begin: BEGIN;
+step s1_dml: INSERT INTO harvest VALUES (43);
+step s0_many_subxacts: select subxacts();
+subxacts       
+
+               
+step s0_commit: COMMIT;
+step s2_checkpoint: CHECKPOINT;
+step s2_get_changes_suppress_output: SELECT null n FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1') GROUP BY n;
+n              
+
+               
+step s2_get_changes_suppress_output: SELECT null n FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1') GROUP BY n;
+n              
+
+step s1_commit: COMMIT;
+step s2_get_changes: SELECT data FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+data           
+
+BEGIN          
+table public.harvest: INSERT: apples[integer]:43
+COMMIT         
+?column?       
+
+stop           

contrib/test_decoding/specs/subxact_without_top.spec

+# Test decoding of subtransactions whose top-transaction is before restart
+# point.  Such transactions won't be streamed as we stream only complete
+# transactions, but it is good to test that they don't cause any problem.
+
+setup
+{
+    SELECT 'init' FROM pg_create_logical_replication_slot('isolation_slot', 'test_decoding'); -- must be first write in xact
+    CREATE TABLE harvest(apples integer);
+    CREATE OR REPLACE FUNCTION subxacts() returns void as $$
+    BEGIN
+      FOR i in 1 .. 128 LOOP
+        BEGIN
+          INSERT INTO harvest VALUES (42);
+        EXCEPTION
+        WHEN OTHERS THEN
+	  RAISE;
+        END;
+    END LOOP;
+    END; $$LANGUAGE 'plpgsql';
+}
+
+teardown
+{
+    DROP TABLE IF EXISTS harvest;
+    SELECT 'stop' FROM pg_drop_replication_slot('isolation_slot');
+}
+
+session "s0"
+setup { SET synchronous_commit=on; }
+step "s0_begin" { BEGIN; }
+step "s0_first_subxact" {
+    DO LANGUAGE plpgsql $$
+      BEGIN
+        BEGIN
+          INSERT INTO harvest VALUES (41);
+	EXCEPTION WHEN OTHERS THEN RAISE;
+	END;
+      END $$;
+}
+step "s0_many_subxacts" { select subxacts(); }
+step "s0_commit" { COMMIT; }
+
+session "s1"
+setup { SET synchronous_commit=on; }
+step "s1_begin" { BEGIN; }
+step "s1_dml" { INSERT INTO harvest VALUES (43); }
+step "s1_commit" { COMMIT; }
+
+session "s2"
+setup { SET synchronous_commit=on; }
+step "s2_checkpoint" { CHECKPOINT; }
+step "s2_get_changes" { SELECT data FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1'); }
+step "s2_get_changes_suppress_output" { SELECT null n FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1') GROUP BY n; }
+
+# The first checkpoint establishes the potential restart point (aka
+# restart_lsn) for the slot after the initial subxact.  The second checkpoint
+# followed by get_changes will ensure that the potential restart point will
+# become the actual restart point.  We do get_changes twice because if one
+# more xl_running_xacts record had slipped before our s0_commit, then the
+# potential restart point won't become actual restart point.  The s1's open
+# transaction till get_changes holds the potential restart point to our first
+# checkpoint location.
+permutation "s0_begin" "s0_first_subxact" "s2_checkpoint" "s1_begin" "s1_dml" "s0_many_subxacts" "s0_commit" "s2_checkpoint" "s2_get_changes_suppress_output" "s2_get_changes_suppress_output" "s1_commit" "s2_get_changes"

src/backend/replication/logical/reorderbuffer.c

@@ -838,9 +838,6 @@ ReorderBufferAssignChild(ReorderBuffer *rb, TransactionId xid,
 	txn = ReorderBufferTXNByXid(rb, xid, true, &new_top, lsn, true);
 	subtxn = ReorderBufferTXNByXid(rb, subxid, true, &new_sub, lsn, false);
 
-	if (new_top && !new_sub)
-		elog(ERROR, "subtransaction logged without previous top-level txn record");
-
 	if (!new_sub)
 	{
 		if (rbtxn_is_known_subxact(subtxn))