refs: standardize output of refs_read_symbolic_ref

When the symbolic reference we want to read with refs_read_symbolic_ref
is actually not a symbolic reference, the files and the reftable
backends return different values (1 and -1 respectively). Standardize
the returned values so that 0 is success, -1 is a generic error and -2
is that the reference was actually non-symbolic.

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ferdinandyb

refs.h

@@ -83,6 +83,17 @@ int refs_read_ref_full(struct ref_store *refs, const char *refname,
 
 int refs_read_ref(struct ref_store *refs, const char *refname, struct object_id *oid);
 
+#define NOT_A_SYMREF -2
+
+/*
+ * Read the symbolic ref named "refname" and write its immediate referent into
+ * the provided buffer. Referent is left empty if "refname" is not a symbolic
+ * ref. It does not resolve the symbolic reference recursively in case the
+ * target is also a symbolic ref.
+ *
+ * Returns 0 on success, -2 if the "refname" is not a symbolic ref,
+ * -1 otherwise.
+ */
 int refs_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
 			   struct strbuf *referent);
 

refs/files-backend.c

@@ -596,10 +596,9 @@ static int files_read_symbolic_ref(struct ref_store *ref_store, const char *refn
 	unsigned int type;
 
 	ret = read_ref_internal(ref_store, refname, &oid, referent, &type, &failure_errno, 1);
-	if (ret)
-		return ret;
-
-	return !(type & REF_ISSYMREF);
+	if (!ret && !(type & REF_ISSYMREF))
+		return NOT_A_SYMREF;
+	return ret;
 }
 
 int parse_loose_ref_contents(const struct git_hash_algo *algop,

refs/refs-internal.h

@@ -673,6 +673,11 @@ struct ref_storage_be {
 
 	ref_iterator_begin_fn *iterator_begin;
 	read_raw_ref_fn *read_raw_ref;
+
+	/*
+	 * Please refer to `refs_read_symbolic_ref()` for the expected
+	 * behaviour.
+	 */
 	read_symbolic_ref_fn *read_symbolic_ref;
 
 	reflog_iterator_begin_fn *reflog_iterator_begin;

refs/reftable-backend.c

@@ -830,10 +830,12 @@ static int reftable_be_read_symbolic_ref(struct ref_store *ref_store,
 		return ret;
 
 	ret = reftable_stack_read_ref(stack, refname, &ref);
-	if (ret == 0 && ref.value_type == REFTABLE_REF_SYMREF)
+	if (ret)
+		ret = -1;
+	else if (ref.value_type == REFTABLE_REF_SYMREF)
 		strbuf_addstr(referent, ref.value.symref);
 	else
-		ret = -1;
+		ret = NOT_A_SYMREF;
 
 	reftable_ref_record_release(&ref);
 	return ret;