Fix a bug in the handling of nested orElse

Exposed by the following snippet, courtesy of Bas van Dijk and Patrick
Palka on [email protected]:

import Control.Concurrent.STM
main = do
  x <- atomically $ do
         t <- newTVar 1
         writeTVar t 2
         ((readTVar t >> retry) `orElse` return ()) `orElse` return ()
         readTVar t
  print x

rts/STM.c

@@ -1460,10 +1460,28 @@ StgBool stmCommitNestedTransaction(Capability *cap, StgTRecHeader *trec) {
 
 	StgTVar *s;
 	s = e -> tvar;
-	if (entry_is_update(e)) {
+
+        // Careful! We might have a read entry here that we don't want
+        // to spam over the update entry in the enclosing TRec.  e.g. in
+        //
+        //   t <- newTVar 1
+        //   writeTVar t 2
+        //   ((readTVar t >> retry) `orElse` return ()) `orElse` return ()
+        //
+        // - the innermost txn first aborts, giving us a read-only entry
+        //   with e->expected_value == e->new_value == 1
+        // - the inner orElse commits into the outer orElse, which
+        //   lands us here.  If we unconditionally did
+        //   merge_update_into(), then we would overwrite the outer
+        //   TRec's update, so we must check whether the entry is an
+        //   update or not, and if not, just do merge_read_into.
+        //
+        if (entry_is_update(e)) {
             unlock_tvar(cap, trec, s, e -> expected_value, FALSE);
-	}
-	merge_update_into(cap, et, s, e -> expected_value, e -> new_value);
+            merge_update_into(cap, et, s, e -> expected_value, e -> new_value);
+        } else {
+            merge_read_into(cap, et, s, e -> expected_value);
+        }
 	ACQ_ASSERT(s -> current_value != (StgClosure *)trec);
       });
     } else {