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

[Thomas Gummerer <t.gummerer@gmail.com>]

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>
---

> This might be a nice change to have anyway, to isolate
> `ref_transaction_update()` from mistakes by its callers. For that
> matter, one might want to be even more selective about what bits are
> allowed in the `flags` argument to `ref_transaction_update()`'s
> callers:
>
> >         flags &= REF_ALLOWED_FLAGS; /* value would need to be determined */

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(+)

diff --git a/refs.c b/refs.c
index ba22f4acef..fad61be1da 100644
--- a/refs.c
+++ b/refs.c
@@ -921,6 +921,8 @@ int ref_transaction_update(struct ref_transaction *transaction,
 		return -1;
 	}
 
+	flags &= REF_TRANSACTION_UPDATE_ALLOWED_FLAGS;
+
 	flags |= (new_sha1 ? REF_HAVE_NEW : 0) | (old_sha1 ? REF_HAVE_OLD : 0);
 
 	ref_transaction_add_update(transaction, refname, flags,
diff --git a/refs.h b/refs.h
index 6daa78eb50..4d75c207e1 100644
--- a/refs.h
+++ b/refs.h
@@ -354,6 +354,14 @@ int refs_pack_refs(struct ref_store *refs, unsigned int flags);
 #define REF_NODEREF	0x01
 #define REF_FORCE_CREATE_REFLOG 0x40
 
+/*
+ * Flags that can be passed in to ref_transaction_update
+ */
+#define REF_TRANSACTION_UPDATE_ALLOWED_FLAGS \
+	REF_ISPRUNING |                      \
+	REF_FORCE_CREATE_REFLOG |            \
+	REF_NODEREF
+
 /*
  * Setup reflog before using. Fill in err and return -1 on failure.
  */
-- 
2.14.1.480.gb18f417b89