Reinstate HEAP_XMAX_LOCK_ONLY|HEAP_KEYS_UPDATED as allowed

alvherre · alvherre · commit 28f4b61083b2 · 2021-02-23T17:30:21.000-03:00
Commit 866e24d47db1 added an assert that HEAP_XMAX_LOCK_ONLY and HEAP_KEYS_UPDATED cannot appear together, on the faulty assumption that the latter necessarily referred to an update and not a tuple lock; but that's wrong, because SELECT FOR UPDATE can use precisely that combination, as evidenced by the amcheck test case added here. Remove the Assert(), and also patch amcheck's verify_heapam.c to not complain if the combination is found. Also, out of overabundance of caution, update (across all branches) README.tuplock to be more explicit about this. Author: Julien Rouhaud <rjuju123@gmail.com> Reviewed-by: Mahendra Singh Thalor <mahi6run@gmail.com> Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com> Discussion: https://postgr.es/m/20210124061758.GA11756@nol
diff --git a/src/backend/access/heap/README.tuplock b/src/backend/access/heap/README.tuplock
@@ -146,9 +146,10 @@ The following infomask bits are applicable:
   FOR UPDATE; this is implemented by the HEAP_KEYS_UPDATED bit.
 
 - HEAP_KEYS_UPDATED
-  This bit lives in t_infomask2.  If set, indicates that the XMAX updated
-  this tuple and changed the key values, or it deleted the tuple.
-  It's set regardless of whether the XMAX is a TransactionId or a MultiXactId.
+  This bit lives in t_infomask2.  If set, indicates that the operation(s) done
+  by the XMAX compromise the tuple key, such as a SELECT FOR UPDATE, an UPDATE
+  that modifies the columns of the key, or a DELETE.  It's set regardless of
+  whether the XMAX is a TransactionId or a MultiXactId.
 
 We currently never set the HEAP_XMAX_COMMITTED when the HEAP_XMAX_IS_MULTI bit
 is set.
