[PATCH v2 1/2] sequencer.c: check for lock failure and bail early in fast_forward_to

[Ronnie Sahlberg <sahlberg@google.com>]

Change fast_forward_to() to check if locking the ref failed, print a nice
error message and bail out early.
The old code did not check if ref_lock was NULL and relied on the fact
that the write_ref_sha1() would safely detect this condition and set the
return variable ret to indicate an error.
While that is safe, it makes the code harder to read for two reasons:
* Inconsistency.  Almost all other places we do check the lock for NULL
  explicitely, so the naive reader is confused "why don't we check here".
* And relying on write_ref_sha1() to detect and return an error for when
  a previous lock_any_ref_for_update() feels obfuscated.

This change should not change any functionality or logic
aside from adding an extra error message when this condition is triggered.
(write_ref_sha1() returns an error silently for this condition)

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
---
 sequencer.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/sequencer.c b/sequencer.c
index bde5f04..0a80c58 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -281,8 +281,12 @@ static int fast_forward_to(const unsigned char *to, const unsigned char *from,
 		exit(1); /* the callee should have complained already */
 	ref_lock = lock_any_ref_for_update("HEAD", unborn ? null_sha1 : from,
 					   0, NULL);
+	if (!ref_lock)
+		return error(_("Failed to lock HEAD during fast_forward_to"));
+
 	strbuf_addf(&sb, "%s: fast-forward", action_name(opts));
 	ret = write_ref_sha1(ref_lock, to, sb.buf);
+
 	strbuf_release(&sb);
 	return ret;
 }
-- 
1.9.1.504.g5a62d94