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
76d70dc0c63 (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>

1 files changed, 12 insertions, 0 deletions

diff --git a/refs.h b/refs.h
index d5099d4984..c8afde6bb5 100644
--- a/refs.h
+++ b/refs.h
@@ -12,6 +12,18 @@ 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.
  *
  * Return the name of the non-symbolic reference that ultimately pointed