lockfile: LOCK_SILENT_ON_ERRORjc/lockfile-silent-on-error

Recent "libify merge machinery" stopped from passing die_on_error
bit to hold_locked_index(), and lost an error message when there are
competing update in progress with "git merge --ff-only $commit", for
example.  The command still exits with a non-zero status, but that
is not of much help for an interactive user.  The last thing the
command says is "Updating $from..$to".  We used to follow it with a
big error message that makes it clear that "merge --ff-only" did not
succeed.

Introduce a new bit "LOCK_SILENT_ON_ERROR" that can be passed by
callers that do want to silence the message (because they either
make it a non-error by doing something else, or they show their own
error message to explain the situation), and show the error message
we used to give for everybody else, including the caller that was
touched by the libification in question.

I would not be surprised if some existing calls to hold_lock*()
functions that pass die_on_error=0 need to be updated to pass
LOCK_SILENT_ON_ERROR, and when this fix is taken alone, it may look
like a regression, but we are better off starting louder and squelch
the ones that we find safe to make silent than the other way around.

Reported-by: Robbie Iannucci <iannucci@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 16 insertions, 3 deletions

diff --git a/lockfile.c b/lockfile.c
index 9268cdf325..f7e8104449 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -174,8 +174,15 @@ int hold_lock_file_for_update_timeout(struct lock_file *lk, const char *path,
 				      int flags, long timeout_ms)
 {
 	int fd = lock_file_timeout(lk, path, flags, timeout_ms);
-	if (fd < 0 && (flags & LOCK_DIE_ON_ERROR))
-		unable_to_lock_die(path, errno);
+	if (fd < 0) {
+		if (flags & LOCK_DIE_ON_ERROR)
+			unable_to_lock_die(path, errno);
+		else if (!(flags & LOCK_SILENT_ON_ERROR)) {
+			struct strbuf buf = STRBUF_INIT;
+			unable_to_lock_message(path, errno, &buf);
+			error("%s", buf.buf);
+		}
+	}
 	return fd;
 }
 
diff --git a/lockfile.h b/lockfile.h
index d26ad27b2b..98b4862254 100644
--- a/lockfile.h
+++ b/lockfile.h
@@ -129,9 +129,15 @@ struct lock_file {
 /*
  * If a lock is already taken for the file, `die()` with an error
  * message. If this flag is not specified, trying to lock a file that
- * is already locked returns -1 to the caller.
+ * is already locked gives the same error message and returns -1 to
+ * the caller.
  */
 #define LOCK_DIE_ON_ERROR 1
+/*
+ * ... or the function can be told to be totally silent and return
+ * -1 to the caller upon error with this flag
+ */
+#define LOCK_SILENT_ON_ERROR 2
 
 /*
  * Usually symbolic links in the destination path are resolved. This