Michael Haggerty [off-list ref] writes:

quoted

Could you explain s/sha1/NULL/ here in the proposed log message?

Good question.

Passing sha1 to delete_ref() doesn't add any safety, because the same
sha1 was just read a moment before, and it is not used for anything
else.

"... and it is guaranteed that no other process in the meantime
wanted to update the ref we are trying to delete because it wants to
see the ref with its updated value." is something I expected to see
at the end.

So the check only protects us from a concurrent update to
newrefname between the call to read_ref_full() and the call to
delete_ref(). But such a race is indistinguishable from the case that a
modification to newrefname happens just before the call to
read_ref_full(), which would have the same outcome as the new code.

In other words, when a transaction that contains a deletion of a ref
races with another one that updates the ref, the latter transaction
may come after the former one, but the ref may not survive in the
end result and can be left deleted?

Puzzled...