remote.c: refactor setup of branch->merge list

When we call branch_get() to lookup or create a "struct
branch", we make sure the "merge" field is filled in so that
callers can access it. But the conditions under which we do
so are a little confusing, and can lead to two funny
situations:

1. If there's no branch.*.remote config, we cannot provide
branch->merge (because it is really just an application
of branch.*.merge to our remote's refspecs). But
branch->merge_nr may be non-zero, leading callers to be
believe they can access branch->merge (e.g., in
branch_merge_matches and elsewhere).

It doesn't look like this can cause a segfault in
practice, as most code paths dealing with merge config
will bail early if there is no remote defined. But it's
a bit of a dangerous construct.

We can fix this by setting merge_nr to "0" explicitly
when we realize that we have no merge config. Note that
merge_nr also counts the "merge_name" fields (which we
_do_ have; that's how merge_nr got incremented), so we
will "lose" access to them, in the sense that we forget
how many we had. But no callers actually care; we use
merge_name only while iteratively reading the config,
and then convert it to the final "merge" form the first
time somebody calls branch_get().

2. We set up the "merge" field every time branch_get is
called, even if it has already been done. This leaks
memory.

It's not a big deal in practice, since most code paths
will access only one branch, or perhaps each branch
only one time. But if you want to be pathological, you
can leak arbitrary memory with:

yes @{upstream} | head -1000 | git rev-list --stdin

We can fix this by skipping setup when branch->merge is
already non-NULL.

In addition to those two fixes, this patch pushes the "do we
need to setup merge?" logic down into set_merge, where it is
a bit easier to follow.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/remote.c b/remote.c
index bec8b311b0f725da93cb034523ab2f3ddac9bed9..ac17e66c09595a5b85ac3f40bec7168d17d8bfa6 100644 (file)
--- a/remote.c
+++ b/remote.c
@@ -1636,6 +1636,19 @@ static void set_merge(struct branch *ret)
        unsigned char sha1[20];
        int i;
 
+       if (!ret)
+               return; /* no branch */
+       if (ret->merge)
+               return; /* already run */
+       if (!ret->remote_name || !ret->merge_nr) {
+               /*
+                * no merge config; let's make sure we don't confuse callers
+                * with a non-zero merge_nr but a NULL merge
+                */
+               ret->merge_nr = 0;
+               return;
+       }
+
        ret->merge = xcalloc(ret->merge_nr, sizeof(*ret->merge));
        for (i = 0; i < ret->merge_nr; i++) {
                ret->merge[i] = xcalloc(1, sizeof(**ret->merge));
@@ -1660,11 +1673,9 @@ struct branch *branch_get(const char *name)
                ret = current_branch;
        else
                ret = make_branch(name, 0);
-       if (ret && ret->remote_name) {
+       if (ret && ret->remote_name)
                ret->remote = remote_get(ret->remote_name);
-               if (ret->merge_nr)
-                       set_merge(ret);
-       }
+       set_merge(ret);
        return ret;
 }