git branch -D: give a better error message when lockfile creation fails

Previously the old error message just told the user that it was not
possible to delete the ref from the packed-refs file. Give instructions
on how to resolve the problem.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

cache.h

@@ -489,6 +489,7 @@ struct lock_file {
 };
 #define LOCK_DIE_ON_ERROR 1
 #define LOCK_NODEREF 2
+extern int unable_to_lock_error(const char *path, int err);
 extern NORETURN void unable_to_lock_index_die(const char *path, int err);
 extern int hold_lock_file_for_update(struct lock_file *, const char *path, int);
 extern int hold_lock_file_for_append(struct lock_file *, const char *path, int);

lockfile.c

@@ -155,18 +155,32 @@ static int lock_file(struct lock_file *lk, const char *path, int flags)
 	return lk->fd;
 }
 
-
-NORETURN void unable_to_lock_index_die(const char *path, int err)
+static char *unable_to_lock_message(const char *path, int err)
 {
+	struct strbuf buf = STRBUF_INIT;
+
 	if (err == EEXIST) {
-		die("Unable to create '%s.lock': %s.\n\n"
+		strbuf_addf(&buf, "Unable to create '%s.lock': %s.\n\n"
 		    "If no other git process is currently running, this probably means a\n"
 		    "git process crashed in this repository earlier. Make sure no other git\n"
 		    "process is running and remove the file manually to continue.",
 		    path, strerror(err));
-	} else {
-		die("Unable to create '%s.lock': %s", path, strerror(err));
-	}
+	} else
+		strbuf_addf(&buf, "Unable to create '%s.lock': %s", path, strerror(err));
+	return strbuf_detach(&buf, NULL);
+}
+
+int unable_to_lock_error(const char *path, int err)
+{
+	char *msg = unable_to_lock_message(path, err);
+	error("%s", msg);
+	free(msg);
+	return -1;
+}
+
+NORETURN void unable_to_lock_index_die(const char *path, int err)
+{
+	die("%s", unable_to_lock_message(path, err));
 }
 
 int hold_lock_file_for_update(struct lock_file *lk, const char *path, int flags)

refs.c

@@ -972,8 +972,10 @@ static int repack_without_ref(const char *refname)
 	if (!found)
 		return 0;
 	fd = hold_lock_file_for_update(&packlock, git_path("packed-refs"), 0);
-	if (fd < 0)
+	if (fd < 0) {
+		unable_to_lock_error(git_path("packed-refs"), errno);
 		return error("cannot delete '%s' from packed refs", refname);
+	}
 
 	for (list = packed_ref_list; list; list = list->next) {
 		char line[PATH_MAX + 100];