[PATCH v2 07/24] expire_reflog(): use a lock_file for rewriting the reflog file

[Michael Haggerty <mhagger@alum.mit.edu>]

We don't actually need the locking functionality, because we already
hold the lock on the reference itself, which is how the reflog file is
locked. But the lock_file code can do some of the bookkeeping for us,
and it is more careful than the old code here was. For example:

* It correctly handles the case that the reflog lock file already
  exists for some reason or cannot be opened.

* It correctly cleans up the lockfile if the program dies.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
 builtin/reflog.c | 60 ++++++++++++++++++++++++++++++++++++++------------------
 1 file changed, 41 insertions(+), 19 deletions(-)

diff --git a/builtin/reflog.c b/builtin/reflog.c
index 37b33c9..ba5b3d3 100644
--- a/builtin/reflog.c
+++ b/builtin/reflog.c
@@ -352,9 +352,10 @@ static int push_tip_to_list(const char *refname, const unsigned char *sha1, int
 static int expire_reflog(const char *refname, const unsigned char *sha1,
 			 struct cmd_reflog_expire_cb *cmd)
 {
+	static struct lock_file reflog_lock;
 	struct expire_reflog_cb cb;
 	struct ref_lock *lock;
-	char *log_file, *newlog_path = NULL;
+	char *log_file;
 	struct commit *tip_commit;
 	struct commit_list *tips;
 	int status = 0;
@@ -362,8 +363,9 @@ static int expire_reflog(const char *refname, const unsigned char *sha1,
 	memset(&cb, 0, sizeof(cb));
 
 	/*
-	 * we take the lock for the ref itself to prevent it from
-	 * getting updated.
+	 * The reflog file is locked by holding the lock on the
+	 * reference itself, plus we might need to update the
+	 * reference if --updateref was specified:
 	 */
 	lock = lock_any_ref_for_update(refname, sha1, 0, NULL);
 	if (!lock)
@@ -372,10 +374,29 @@ static int expire_reflog(const char *refname, const unsigned char *sha1,
 		unlock_ref(lock);
 		return 0;
 	}
+
 	log_file = git_pathdup("logs/%s", refname);
 	if (!cmd->dry_run) {
-		newlog_path = git_pathdup("logs/%s.lock", refname);
-		cb.newlog = fopen(newlog_path, "w");
+		/*
+		 * Even though holding $GIT_DIR/logs/$reflog.lock has
+		 * no locking implications, we use the lock_file
+		 * machinery here anyway because it does a lot of the
+		 * work we need, including cleaning up if the program
+		 * exits unexpectedly.
+		 */
+		if (hold_lock_file_for_update(&reflog_lock, log_file, 0) < 0) {
+			struct strbuf err = STRBUF_INIT;
+			unable_to_lock_message(log_file, errno, &err);
+			error("%s", err.buf);
+			strbuf_release(&err);
+			goto failure;
+		}
+		cb.newlog = fdopen_lock_file(&reflog_lock, "w");
+		if (!cb.newlog) {
+			error("cannot fdopen %s (%s)",
+			      reflog_lock.filename.buf, strerror(errno));
+			goto failure;
+		}
 	}
 
 	cb.cmd = cmd;
@@ -423,32 +444,33 @@ static int expire_reflog(const char *refname, const unsigned char *sha1,
 	}
 
 	if (cb.newlog) {
-		if (fclose(cb.newlog)) {
-			status |= error("%s: %s", strerror(errno),
-					newlog_path);
-			unlink(newlog_path);
+		if (close_lock_file(&reflog_lock)) {
+			status |= error("couldn't write %s: %s", log_file,
+					strerror(errno));
 		} else if (cmd->updateref &&
 			(write_in_full(lock->lock_fd,
 				sha1_to_hex(cb.last_kept_sha1), 40) != 40 ||
 			 write_str_in_full(lock->lock_fd, "\n") != 1 ||
 			 close_ref(lock) < 0)) {
-			status |= error("Couldn't write %s",
+			status |= error("couldn't write %s",
 					lock->lk->filename.buf);
-			unlink(newlog_path);
-		} else if (rename(newlog_path, log_file)) {
-			status |= error("cannot rename %s to %s",
-					newlog_path, log_file);
-			unlink(newlog_path);
+			rollback_lock_file(&reflog_lock);
+		} else if (commit_lock_file(&reflog_lock)) {
+			status |= error("unable to commit reflog '%s' (%s)",
+					log_file, strerror(errno));
 		} else if (cmd->updateref && commit_ref(lock)) {
-			status |= error("Couldn't set %s", lock->ref_name);
-		} else {
-			adjust_shared_perm(log_file);
+			status |= error("couldn't set %s", lock->ref_name);
 		}
 	}
-	free(newlog_path);
 	free(log_file);
 	unlock_ref(lock);
 	return status;
+
+ failure:
+	rollback_lock_file(&reflog_lock);
+	free(log_file);
+	unlock_ref(lock);
+	return -1;
 }
 
 static int collect_reflog(const char *ref, const unsigned char *sha1, int unused, void *cb_data)
-- 
2.1.3