Relax overly strict assertion

Ever since its birth, ReorderBufferBuildTupleCidHash() has contained an
assertion that a catalog tuple cannot change Cmax after acquiring one.  But
that's wrong: if a subtransaction executes DDL that affects that catalog
tuple, and later aborts and another DDL affects the same tuple, it will
change Cmax.  Relax the assertion to merely verify that the Cmax remains
valid and monotonically increasing, instead.

Add a test that tickles the relevant code.

Diagnosed by, and initial patch submitted by: Arseny Sher
Co-authored-by: Arseny Sher
Discussion: https://postgr.es/m/874l9p8hyw.fsf@ars-thinkpad

1 files changed, 9 insertions, 5 deletions

diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index 152f99b4547..7da681154cd 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -1326,15 +1326,19 @@ ReorderBufferBuildTupleCidHash(ReorderBuffer *rb, ReorderBufferTXN *txn)
 		}
 		else
 		{
+			/*
+			 * Maybe we already saw this tuple before in this transaction,
+			 * but if so it must have the same cmin.
+			 */
 			Assert(ent->cmin == change->data.tuplecid.cmin);
-			Assert(ent->cmax == InvalidCommandId ||
-				   ent->cmax == change->data.tuplecid.cmax);
 
 			/*
-			 * if the tuple got valid in this transaction and now got deleted
-			 * we already have a valid cmin stored. The cmax will be
-			 * InvalidCommandId though.
+			 * cmax may be initially invalid, but once set it can only grow,
+			 * and never become invalid again.
 			 */
+			Assert((ent->cmax == InvalidCommandId) ||
+				   ((change->data.tuplecid.cmax != InvalidCommandId) &&
+					(change->data.tuplecid.cmax > ent->cmax)));
 			ent->cmax = change->data.tuplecid.cmax;
 		}
 	}