config: allow overriding of global and system configuration

In order to have git run in a fully controlled environment without any
misconfiguration, it may be desirable for users or scripts to override
global- and system-level configuration files. We already have a way of
doing this, which is to unset both HOME and XDG_CONFIG_HOME environment
variables and to set `GIT_CONFIG_NOGLOBAL=true`. This is quite kludgy,
and unsetting the first two variables likely has an impact on other
executables spawned by such a script.

The obvious way to fix this would be to introduce `GIT_CONFIG_NOGLOBAL`
as an equivalent to `GIT_CONFIG_NOSYSTEM`. But in the past, it has
turned out that this design is inflexible: we cannot test system-level
parsing of the git configuration in our test harness because there is no
way to change its location, so all tests run with `GIT_CONFIG_NOSYSTEM`
set.

Instead of doing the same mistake with `GIT_CONFIG_NOGLOBAL`, introduce
two new variables `GIT_CONFIG_GLOBAL` and `GIT_CONFIG_SYSTEM`:

    - If unset, git continues to use the usual locations.

    - If set to a specific path, we skip reading the normal
      configuration files and instead take the path. By setting the path
      to `/dev/null`, no configuration will be loaded for the respective
      level.

This implements the usecase where we want to execute code in a sanitized
environment without any potential misconfigurations via `/dev/null`, but
is more flexible and allows for more usecases than simply adding
`GIT_CONFIG_NOGLOBAL`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/config.c b/config.c
index 8900d96cb6..8d4dbe1310 100644
--- a/config.c
+++ b/config.c
@@ -1846,13 +1846,24 @@ static int git_config_from_blob_ref(config_fn_t fn,
 
 char *git_system_config(void)
 {
+	char *system_config = xstrdup_or_null(getenv("GIT_CONFIG_SYSTEM"));
+	if (system_config)
+		return system_config;
 	return system_path(ETC_GITCONFIG);
 }
 
-void git_global_config(char **user_config, char **xdg_config)
+void git_global_config(char **user_out, char **xdg_out)
 {
-	*user_config = expand_user_path("~/.gitconfig", 0);
-	*xdg_config = xdg_config_home("config");
+	char *user_config = xstrdup_or_null(getenv("GIT_CONFIG_GLOBAL"));
+	char *xdg_config = NULL;
+
+	if (!user_config) {
+		user_config = expand_user_path("~/.gitconfig", 0);
+		xdg_config = xdg_config_home("config");
+	}
+
+	*user_out = user_config;
+	*xdg_out = xdg_config;
 }
 
 /*