[PATCH/RFC] More friendly message when locking the index fails.

[Matthieu Moy <Matthieu.Moy@imag.fr>]

Just saying that index.lock exists doesn't tell the user _what_ to do
to fix the problem. We should give an indication that it's normally
safe to delete index.lock after making sure git isn't running here.
---
> I'll write a FAQ entry on the wiki with answers, and that would
> probably be a good idea to give indication to the user directly in the
> error message too, otherwise, the problem can be blocking for
> beginners.

Something along the lines of this patch maybe?

 builtin-update-index.c |   11 +++++++++--
 lockfile.c             |    9 ++++++++-
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/builtin-update-index.c b/builtin-update-index.c
index 5604977..ab8ab8f 100644
--- a/builtin-update-index.c
+++ b/builtin-update-index.c
@@ -742,8 +742,15 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 		if (newfd < 0) {
 			if (refresh_flags & REFRESH_QUIET)
 				exit(128);
-			die("unable to create '%s.lock': %s",
-			    get_index_file(), strerror(lock_error));
+			if (lock_error == EEXIST) {
+				die("Unable to create '%s.lock': %s\n"
+				    "This probably means a git process crashed in this repository earlier.\n"
+				    "Make sure no other git process is running and remove the file manually.",
+				    get_index_file(), strerror(lock_error));
+			} else {
+				die("Unable to create '%s.lock': %s",
+				    get_index_file(), strerror(lock_error));
+			}
 		}
 		if (write_cache(newfd, active_cache, active_nr) ||
 		    commit_locked_index(lock_file))
diff --git a/lockfile.c b/lockfile.c
index 021c337..bb59f7e 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -159,7 +159,14 @@ int hold_lock_file_for_update(struct lock_file *lk, const char *path, int flags)
 {
 	int fd = lock_file(lk, path, flags);
 	if (fd < 0 && (flags & LOCK_DIE_ON_ERROR))
-		die("unable to create '%s.lock': %s", path, strerror(errno));
+		if (errno == EEXIST) {
+			die("Unable to create '%s.lock': %s\n"
+			    "This probably means a git process crashed in this repository earlier.\n"
+			    "Make sure no other git process is running and remove the file manually.",
+			    path, strerror(errno));
+		} else {
+			die("Unable to create '%s.lock': %s", path, strerror(errno));
+		}
 	return fd;
 }
 
-- 
1.6.1.2.322.g01114.dirty