refs debug: add a wrapper for "read_symbolic_ref"

In cd475b3b03 (refs: add ability for backends to special-case reading
of symbolic refs, 2022-03-01) when the "read_symbolic_ref" callback
was added we'd fall back on "refs_read_raw_ref" if there wasn't any
backend implementation of "read_symbolic_ref".

As discussed in the preceding commit this would only happen if we were
running the "debug" backend, e.g. in the "setup for ref completion"
test in t9902-completion.sh with:

    GIT_TRACE_REFS=1 git fetch --no-tags other

Let's improve the trace output, but and also eliminate the
now-redundant refs_read_raw_ref() fallback case. As noted in the
preceding commit the "packed" backend will never call
refs_read_symbolic_ref() (nor is it ever going to). For any future
backend such as reftable it's OK to ask that they either implement
this (or a wrapper) themselves.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -1676,18 +1676,7 @@ int refs_read_raw_ref(struct ref_store *ref_store, const char *refname,
 int refs_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
 			   struct strbuf *referent)
 {
-	struct object_id oid;
-	int ret, failure_errno = 0;
-	unsigned int type = 0;
-
-	if (ref_store->be->read_symbolic_ref)
-		return ref_store->be->read_symbolic_ref(ref_store, refname, referent);
-
-	ret = refs_read_raw_ref(ref_store, refname, &oid, referent, &type, &failure_errno);
-	if (ret || !(type & REF_ISSYMREF))
-		return -1;
-
-	return 0;
+	return ref_store->be->read_symbolic_ref(ref_store, refname, referent);
 }
 
 const char *refs_resolve_ref_unsafe(struct ref_store *refs,

refs/debug.c

@@ -262,6 +262,24 @@ static int debug_read_raw_ref(struct ref_store *ref_store, const char *refname,
 	return res;
 }
 
+static int debug_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
+				   struct strbuf *referent)
+{
+	struct debug_ref_store *drefs = (struct debug_ref_store *)ref_store;
+	struct ref_store *refs = drefs->refs;
+	int res;
+
+	res = refs->be->read_symbolic_ref(refs, refname, referent);
+	if (!res)
+		trace_printf_key(&trace_refs, "read_symbolic_ref: %s: (%s)\n",
+				 refname, referent->buf);
+	else
+		trace_printf_key(&trace_refs,
+				 "read_symbolic_ref: %s: %d\n", refname, res);
+	return res;
+
+}
+
 static struct ref_iterator *
 debug_reflog_iterator_begin(struct ref_store *ref_store)
 {
@@ -423,6 +441,13 @@ struct ref_storage_be refs_be_debug = {
 	.name = "debug",
 	.init = NULL,
 	.init_db = debug_init_db,
+
+	/*
+	 * None of these should be NULL. If the "files" backend (in
+	 * "struct ref_storage_be refs_be_files" in files-backend.c)
+	 * has a function we should also have a wrapper for it here.
+	 * Test the output with "GIT_TRACE_REFS=1".
+	 */
 	.transaction_prepare = debug_transaction_prepare,
 	.transaction_finish = debug_transaction_finish,
 	.transaction_abort = debug_transaction_abort,
@@ -436,7 +461,7 @@ struct ref_storage_be refs_be_debug = {
 
 	.iterator_begin = debug_ref_iterator_begin,
 	.read_raw_ref = debug_read_raw_ref,
-	.read_symbolic_ref = NULL,
+	.read_symbolic_ref = debug_read_symbolic_ref,
 
 	.reflog_iterator_begin = debug_reflog_iterator_begin,
 	.for_each_reflog_ent = debug_for_each_reflog_ent,