[PATCH 03/10] merge: do not point "branch" to a resolve_ref()'s static buffer

resolve_ref() may return a pointer to a static buffer. Callers that
use this value longer than a couple of statements should copy the
value to avoid some hidden resolve_ref() call that may change the
static buffer's value.

The bug found by Tony Wang [off-list ref] in builtin/merge.c
demonstrates this. The first call is in cmd_merge()

branch = resolve_ref("HEAD", head_sha1, 0, &flag);

Then deep in lookup_commit_or_die() a few lines after, resolve_ref()
may be called again and destroy "branch".

lookup_commit_or_die
 lookup_commit_reference
  lookup_commit_reference_gently
   parse_object
    lookup_replace_object
     do_lookup_replace_object
      prepare_replace_object
       for_each_replace_ref
        do_for_each_ref
         get_loose_refs
          get_ref_dir
           get_ref_dir
            resolve_ref

Ask resolve_ref() to allocate a new buffer instead of returning static
buffer.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
---
 builtin/merge.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/builtin/merge.c b/builtin/merge.c
index 1be6f3b..1a3095f 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c

@@ -1087,6 +1087,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	struct commit_list *common = NULL;
 	const char *best_strategy = NULL, *wt_strategy = NULL;
 	struct commit_list **remotes = &remoteheads;
+	char *branch_ref;
 
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage_with_options(builtin_merge_usage, builtin_merge_options);

@@ -1095,7 +1096,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	 * Check if we are _not_ on a detached HEAD, i.e. if there is a
 	 * current branch.
 	 */
-	branch = resolve_ref("HEAD", head_sha1, 0, &flag, 0);
+	branch = branch_ref = resolve_ref("HEAD", head_sha1, 0, &flag, 1);
 	if (branch && !prefixcmp(branch, "refs/heads/"))
 		branch += 11;
 	if (!branch || is_null_sha1(head_sha1))

@@ -1497,5 +1498,6 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		ret = suggest_conflicts(option_renormalize);
 
 done:
+	free(branch_ref);
 	return ret;
 }

-- 
1.7.4.74.g639db