Patrick Steinhardt [off-list ref] writes:

On Thu, May 15, 2025 at 04:07:26PM +0200, Karthik Nayak wrote:

quoted

diff --git a/builtin/fetch.c b/builtin/fetch.c
index 5279997c96..15eac2b1c2 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c

@@ -1688,6 +1644,37 @@ static int set_head(const struct ref *remote_refs, struct remote *remote)
 	return result;
 }

+struct ref_rejection_data {
+	int *retcode;
+	int conflict_msg_shown;
+	const char *remote_name;
+};
+
+static void ref_transaction_rejection_handler(const char *refname,
+					      const struct object_id *old_oid UNUSED,
+					      const struct object_id *new_oid UNUSED,
+					      const char *old_target UNUSED,
+					      const char *new_target UNUSED,
+					      enum ref_transaction_error err,
+					      void *cb_data)
+{
+	struct ref_rejection_data *data = (struct ref_rejection_data *)cb_data;

Nit: unnecessary cast.

quoted

+	if (err == REF_TRANSACTION_ERROR_NAME_CONFLICT && !data->conflict_msg_shown) {
+		error(_("some local refs could not be updated; try running\n"
+			" 'git remote prune %s' to remove any old, conflicting "
+			"branches"), data->remote_name);
+		data->conflict_msg_shown = 1;
+	} else {
+		char *reason = ref_transaction_error_msg(err);
+
+		error(_("fetching ref %s failed: %s"), refname, reason);
+		free(reason);
+	}
+
+	*data->retcode = 1;
+}

Okay, we stopped ignoring generic errors now and will print them. What
I'm still unclear about: which exact errors do we accept now that
`REF_TRANSACTION_ALLOW_FAILURE` is specified? Most of the error codes we
probably want to accept, but what about `REF_TRANSACTION_ERROR_GENERIC`?

The current mechanism in `ref_transaction_maybe_set_rejected()` doesn't
handle `REF_TRANSACTION_ERROR_GENERIC` errors. This was a design choice
(more of a requirement of what this error represents), where
`REF_TRANSACTION_ERROR_GENERIC` errors cannot be resolved on an
individual reference level. It includes:

  - System errors such as I/O errors
  - Duplicates present

Both of these represent issues which are bigger than a single ref
update, so we have to propagate these errors up.

This makes me wonder a bit about the current layout of how we handle
these errors. If the rejection handler was invoked while preparing the
transaction for each reference as we go instead of afterwards we could
decide on-the-fly whether a specific error should be ignored or not.
That might lead to a design that is both more flexible and more obvious
at the same time because error handling is now handled explicitly by the
callsite that wants to ignore some errors.

I did ponder on this while I was building the batched transaction
mechanism. I decided to take it iteratively. We can, for instance,
modify `ref_transaction_maybe_set_rejected()` to work with a callback
function which would allow the users to accept/reject errors.

However, even if we go down that route, `REF_TRANSACTION_ERROR_GENERIC`
errors still cannot be overlooked, these errors will abort the entire
transaction.

That said, I'm not trying to avoid going down that route. I do agree
with the flexibility it does provide. Once we hit such a usecase, we
should make that change.

For 'git-fetch(1)' and 'git-recieve-pack(1)', do you see a usecase?

Last but not least, I think that it would also allow us to decide ahead
of time whether we want to commit. Right now we basically say "just
commit it, whatever happens". But if I'm not mistaken, all the errors
that we care about and that callers may want to ignore are already
detected at prepare time. So if we already bubbled up relevant info
while calling `ref_transaction_prepare()` the caller may then decide to
not commit at all based on some criteria.

Indeed, that is correct. I can confirm that even now all the calls to
`ref_transaction_maybe_set_rejected()` are made in the prepare phase, so
we could already do this, since `transaction->rejections` is already
populated at this stage.

Sorry, I should've probably proposed this when you introducued this
mechanism. But sometimes you only see things like that as we gain more
users.

You don't have to apologize. Such discussions are very important and you
shouldn't hesitate to bring up such points.

quoted

@@ -1808,6 +1795,24 @@ static int do_fetch(struct transport *transport,
 			retcode = 1;
 	}

+	/*
+	 * If not atomic, we can still use batched updates, which would be much
+	 * more performant. We don't initiate the transaction before pruning,
+	 * since pruning must be an independent step, to avoid F/D conflicts.
+	 *
+	 * TODO: if reference transactions gain logical conflict resolution, we
+	 * can delete and create refs (with F/D conflicts) in the same transaction
+	 * and this can be moved about the 'prune_refs()' block.

s/about/above/?

Indeed!

Patrick