remote: add meaningful exit code on missing/existing

Change the exit code for the likes of "git remote add/rename" to exit
with 2 if the remote in question doesn't exist, and 3 if it
does. Before we'd just die() and exit with the general 128 exit code.

This changes the output message from e.g.:

    fatal: remote origin already exists.

To:

    error: remote origin already exists.

Which I believe is a feature, since we generally use "fatal" for the
generic errors, and "error" for the more specific ones with a custom
exit code, but this part of the change may break code that already
relies on stderr parsing (not that we ever supported that...).

The motivation for this is a discussion around some code in GitLab's
gitaly which wanted to check this, and had to parse stderr to do so:
https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2695

It's worth noting as an aside that a method of checking this that
doesn't rely on that is to check with "git config" whether the value
in question does or doesn't exist. That introduces a TOCTOU race
condition, but on the other hand this code (e.g. "git remote add")
already has a TOCTOU race.

We go through the config.lock for the actual setting of the config,
but the pseudocode logic is:

    read_config();
    check_config_and_arg_sanity();
    save_config();

So e.g. if a sleep() is added right after the remote_is_configured()
check in add() we'll clobber remote.NAME.url, and add another (usually
duplicate) remote.NAME.fetch entry (and other values, depending on
invocation).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/Documentation/git-remote.txt b/Documentation/git-remote.txt
index ea73386c81..31c29c9b31 100644
--- a/Documentation/git-remote.txt
+++ b/Documentation/git-remote.txt
@@ -203,6 +203,17 @@ The remote configuration is achieved using the `remote.origin.url` and
 `remote.origin.fetch` configuration variables.  (See
 linkgit:git-config[1]).
 
+EXIT STATUS
+-----------
+
+On success, the exit status is `0`.
+
+When subcommands such as 'add', 'rename', and 'remove' can't find the
+remote in question, the exit status is `2`. When the remote already
+exists, the exit status is `3`.
+
+On any other error, the exit status may be any other non-zero value.
+
 EXAMPLES
 --------