refs API: add a version of refs_resolve_ref_unsafe() with "errno"

Add a new refs_werrres_ref_unsafe() function, which is like
refs_resolve_ref_unsafe() except that it explicitly saves away the
"errno" to a passed-in parameter, the refs_resolve_ref_unsafe() then
becomes a wrapper for it.

In subsequent commits we'll migrate code over to it, before finally
making "refs_resolve_ref_unsafe()" with an "errno" parameter the
canonical version, so this this function exists only so that we can
incrementally migrate callers, it will be going away in a subsequent
commit.

As the added comment notes has a rather tortured name to be the same
length as "refs_resolve_ref_unsafe", to avoid churn as we won't need
to re-indent the argument lists, similarly the documentation and
structure of it in refs.h is designed to minimize a diff in a
subsequent commit, where that documentation will be added to the new
refs_resolve_ref_unsafe().

At the end of this migration the "meaningful errno" TODO item left in
76d70dc0c6 (refs.c: make resolve_ref_unsafe set errno to something
meaningful on error, 2014-06-20) will be resolved.

As can be seen from the use of refs_read_raw_ref() we'll also need to
convert some functions that the new refs_werrres_ref_unsafe() itself
calls to take this "failure_errno". That will be done in subsequent
commits.

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

refs.c

@@ -1679,17 +1679,19 @@ int refs_read_raw_ref(struct ref_store *ref_store,
 					   type, &errno);
 }
 
-/* This function needs to return a meaningful errno on failure */
-const char *refs_resolve_ref_unsafe(struct ref_store *refs,
+const char *refs_werrres_ref_unsafe(struct ref_store *refs,
 				    const char *refname,
 				    int resolve_flags,
-				    struct object_id *oid, int *flags)
+				    struct object_id *oid,
+				    int *flags, int *failure_errno)
 {
 	static struct strbuf sb_refname = STRBUF_INIT;
 	struct object_id unused_oid;
 	int unused_flags;
 	int symref_count;
 
+	assert(failure_errno);
+
 	if (!oid)
 		oid = &unused_oid;
 	if (!flags)
@@ -1700,7 +1702,7 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 	if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
 		if (!(resolve_flags & RESOLVE_REF_ALLOW_BAD_NAME) ||
 		    !refname_is_safe(refname)) {
-			errno = EINVAL;
+			*failure_errno = EINVAL;
 			return NULL;
 		}
 
@@ -1718,9 +1720,12 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 	for (symref_count = 0; symref_count < SYMREF_MAXDEPTH; symref_count++) {
 		unsigned int read_flags = 0;
 
+		errno = 0;
 		if (refs_read_raw_ref(refs, refname,
 				      oid, &sb_refname, &read_flags)) {
 			*flags |= read_flags;
+			if (errno)
+				*failure_errno = errno;
 
 			/* In reading mode, refs must eventually resolve */
 			if (resolve_flags & RESOLVE_REF_READING)
@@ -1731,9 +1736,9 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 			 * may show errors besides ENOENT if there are
 			 * similarly-named refs.
 			 */
-			if (errno != ENOENT &&
-			    errno != EISDIR &&
-			    errno != ENOTDIR)
+			if (*failure_errno != ENOENT &&
+			    *failure_errno != EISDIR &&
+			    *failure_errno != ENOTDIR)
 				return NULL;
 
 			oidclr(oid);
@@ -1760,7 +1765,7 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 		if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
 			if (!(resolve_flags & RESOLVE_REF_ALLOW_BAD_NAME) ||
 			    !refname_is_safe(refname)) {
-				errno = EINVAL;
+				*failure_errno = EINVAL;
 				return NULL;
 			}
 
@@ -1768,10 +1773,23 @@ const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 		}
 	}
 
-	errno = ELOOP;
+	*failure_errno = ELOOP;
 	return NULL;
 }
 
+const char *refs_resolve_ref_unsafe(struct ref_store *refs, const char *refname,
+				    int resolve_flags, struct object_id *oid,
+				    int *flags)
+{
+	int failure_errno = 0;
+	const char *refn;
+	refn = refs_werrres_ref_unsafe(refs, refname, resolve_flags,
+				       oid, flags, &failure_errno);
+	if (!refn)
+		errno = failure_errno;
+	return refn;
+}
+
 /* backend functions */
 int refs_init_db(struct strbuf *err)
 {

refs.h

@@ -11,6 +11,18 @@ struct string_list;
 struct string_list_item;
 struct worktree;
 
+/*
+ * Callers should not inspect "errno" on failure, but rather pass in a
+ * "failure_errno" parameter, on failure the "errno" will indicate the
+ * type of failure encountered, but not necessarily one that came from
+ * a syscall. We might have faked it up.
+ */
+const char *refs_werrres_ref_unsafe(struct ref_store *refs,
+				    const char *refname,
+				    int resolve_flags,
+				    struct object_id *oid,
+				    int *flags, int *failure_errno);
+
 /*
  * Resolve a reference, recursively following symbolic refererences.
  *