Add check of the resolved HEAD reference while printing of a commit summary.
resolve_ref_unsafe() may return NULL pointer if underlying calls of lstat() or
open() fail in files_read_raw_ref().
Such situation can be caused by race: file becomes inaccessible to this moment.

Signed-off-by: Andrey Okoshkin <a.okosh...@samsung.com>
---
Hello,
I think this way is better for user experience:
* git doesn't crash;
* warning is shown;
* commit has been successfully created then it's safe to show a summary message
with already known information and without resolved HEAD.

Best regards,
Andrey

 builtin/commit.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/builtin/commit.c b/builtin/commit.c
index 1a0da71a4..647d6ab3e 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1483,7 +1483,10 @@ static void print_summary(const char *prefix, const 
struct object_id *oid,
        diff_setup_done(&rev.diffopt);
 
        head = resolve_ref_unsafe("HEAD", 0, junk_oid.hash, NULL);
-       if (!strcmp(head, "HEAD"))
+       if (!head) {
+               warning_errno(_("unable to resolve HEAD after creating 
commit"));
+               head = _("unresolvable HEAD");
+       } else if (!strcmp(head, "HEAD"))
                head = _("detached HEAD");
        else
                skip_prefix(head, "refs/heads/", &head);
-- 
2.14.2

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