[RFC v2] refs: strip out not allowed flags from ref_transaction_update

Callers are only allowed to pass certain flags into ref_transaction_update, other flags are internal to it. To prevent mistakes from the callers, strip the internal only flags out before continuing.

This was noticed because of a compiler warning gcc 7.1.1 issued about passing a NULL parameter as second parameter to memcpy (through hashcpy):

In file included from refs.c:5:0:
refs.c: In function ‘ref_transaction_verify’:
cache.h:948:2: error: argument 2 null where non-null expected [-Werror=nonnull]
  memcpy(sha_dst, sha_src, GIT_SHA1_RAWSZ);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from git-compat-util.h:165:0,
                 from cache.h:4,
                 from refs.c:5:
/usr/include/string.h:43:14: note: in a call to function ‘memcpy’ declared here
 extern void *memcpy (void *__restrict __dest, const void *__restrict __src,
              ^~~~~~

The call to hascpy in ref_transaction_add_update is protected by the passed in flags, but as we only add flags there, gcc notices REF_HAVE_NEW or REF_HAVE_OLD flags could be passed in from the outside, which would potentially result in passing in NULL as second parameter to memcpy.

Fix both the compiler warning, and make the interface safer for its users by stripping the internal flags out.

Suggested-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
---

Here's my attempt at doing this.

The odd flag out as the flag that's advertised as internal but can't stripped out is REF_ISPRUNING. REF_ISPRUNING is passed in as argument to 'ref_transaction_delete()' in 'prune_ref()'.

Maybe this flag should be public, or maybe I'm missing something here? Having only this internal flags as part of the allowed flags feels a bit ugly, but I'm also unfamiliar with the refs code, hence the RFC. If someone has more suggestions they would be very welcome :)

 refs.c | 2 ++
 refs.h | 8 ++++++++
 2 files changed, 10 insertions(+)