Re: [PATCH 04/20] refs_verify_refname_available(): implement once for all backends

On Mon, Mar 20, 2017 at 05:33:09PM +0100, Michael Haggerty wrote:

It turns out that we can now implement
`refs_verify_refname_available()` based on the other virtual
functions, so there is no need for it to be defined at the backend
level. Instead, define it once in `refs.c` and remove the
`files_backend` definition.

Does this mean that backends can no longer provide storage for
D/F-conflicted refnames (i.e., "refs/foo" and "refs/foo/bar")? It looks
like the global verify_refname_available() has that logic baked in.

I know that was a complex subject because of the potential compatibility
issues (e.g., fetching from a server with a more capable backend), but
I'd just worry we are shutting a door on some options. OTOH, it probably
wouldn't be that hard for the global function to check a flag specific
to the ref_store, and allow such refs when it is set.

 int refs_verify_refname_available(struct ref_store *refs,
 				  const char *refname,
-				  const struct string_list *extra,
+				  const struct string_list *extras,
 				  const struct string_list *skip,
 				  struct strbuf *err)
 {
[...]
+		/*
+		 * We are still at a leading dir of the refname (e.g.,
+		 * "refs/foo"; if there is a reference with that name,
+		 * it is a conflict, *unless* it is in skip.
+		 */
+		if (skip && string_list_has_string(skip, dirname.buf))
+			continue;
+
+		if (!refs_read_raw_ref(refs, dirname.buf, oid.hash, &referent, &type)) {
+			strbuf_addf(err, "'%s' exists; cannot create '%s'",
+				    dirname.buf, refname);
+			goto cleanup;
+		}

We don't really care about reading the ref value here; we just care if
it exists. Does that matter for efficiency (e.g., for the files backend
it's a stat() versus an open/read/close)? I guess the difference only
matters when it _does_ exist, which is the uncommon error case.

(Also, I suspect the loose ref cache always just reads everything in the
current code, though with the iterator approach in theory we could stop
doing that).

-Peff