[PATCH v2 3/3] config: allow overriding of global and system configuration

[Patrick Steinhardt <ps@pks.im>]

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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_NOSYSTEM`
as an equivalent to `GIT_CONFIG_NOGLOBAL`. 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. This path must
      exist and be readable to ensure that the user didn't typo.

    - If set to `/dev/null`, we do not load either global- or
      system-level configuration at all.

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>
---
 Documentation/git-config.txt |  5 +++
 Documentation/git.txt        | 10 +++++
 config.c                     | 34 ++++++++++++++--
 t/t1300-config.sh            | 75 ++++++++++++++++++++++++++++++++++++
 4 files changed, 120 insertions(+), 4 deletions(-)

diff --git a/Documentation/git-config.txt b/Documentation/git-config.txt
index 4b4cc5c5e8..5cddadafd2 100644
--- a/Documentation/git-config.txt
+++ b/Documentation/git-config.txt
@@ -340,6 +340,11 @@ GIT_CONFIG::
 	Using the "--global" option forces this to ~/.gitconfig. Using the
 	"--system" option forces this to $(prefix)/etc/gitconfig.
 
+GIT_CONFIG_GLOBAL::
+GIT_CONFIG_SYSTEM::
+	Take the configuration from the given files instead from global or
+	system-level configuration. See linkgit:git[1] for details.
+
 GIT_CONFIG_NOSYSTEM::
 	Whether to skip reading settings from the system-wide
 	$(prefix)/etc/gitconfig file. See linkgit:git[1] for details.
diff --git a/Documentation/git.txt b/Documentation/git.txt
index 3a9c44987f..2129629296 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -670,6 +670,16 @@ for further details.
 	If this environment variable is set to `0`, git will not prompt
 	on the terminal (e.g., when asking for HTTP authentication).
 
+`GIT_CONFIG_GLOBAL`::
+`GIT_CONFIG_SYSTEM`::
+	Take the configuration from the given files instead from global or
+	system-level configuration files. The files must exist and be readable
+	by the current user. If `GIT_CONFIG_SYSTEM` is set, `/etc/gitconfig`
+	will not be read. Likewise, if `GIT_CONFIG_GLOBAL` is set, neither
+	`$HOME/.gitconfig` nor `$XDG_CONFIG_HOME/git/config` will be read. Can
+	be set to `/dev/null` to skip reading configuration files of the
+	respective level.
+
 `GIT_CONFIG_NOSYSTEM`::
 	Whether to skip reading settings from the system-wide
 	`$(prefix)/etc/gitconfig` file.  This environment variable can
diff --git a/config.c b/config.c
index 6af0244085..9dfc4a79f7 100644
--- a/config.c
+++ b/config.c
@@ -1847,8 +1847,22 @@ static int git_config_from_blob_ref(config_fn_t fn,
 const char *git_system_config(void)
 {
 	static const char *system_wide;
-	if (!system_wide)
-		system_wide = system_path(ETC_GITCONFIG);
+
+	if (!system_wide) {
+		system_wide = xstrdup_or_null(getenv("GIT_CONFIG_SYSTEM"));
+		if (system_wide) {
+			/*
+			 * If GIT_CONFIG_SYSTEM is set, it overrides the
+			 * /etc/gitconfig. Furthermore, the file must exist in
+			 * order to prevent any typos by the user.
+			 */
+			if (access(system_wide, R_OK))
+				die(_("cannot access '%s'"), system_wide);
+		} else {
+			system_wide = system_path(ETC_GITCONFIG);
+		}
+	}
+
 	return system_wide;
 }
 
@@ -1857,8 +1871,20 @@ void git_global_config(const char **user, const char **xdg)
 	static const char *user_config, *xdg_config;
 
 	if (!user_config) {
-		user_config = expand_user_path("~/.gitconfig", 0);
-		xdg_config = xdg_config_home("config");
+		user_config = xstrdup_or_null(getenv("GIT_CONFIG_GLOBAL"));
+		if (user_config) {
+			/*
+			 * If GIT_CONFIG_GLOBAL is set, then it overrides both
+			 * the ~/.gitconfig and the XDG configuration file.
+			 * Furthermore, the file must exist in order to prevent
+			 * any typos by the user.
+			 */
+			if (access(user_config, R_OK))
+				die(_("cannot access '%s'"), user_config);
+		} else {
+			user_config = expand_user_path("~/.gitconfig", 0);
+			xdg_config = xdg_config_home("config");
+		}
 	}
 
 	*user = user_config;
diff --git a/t/t1300-config.sh b/t/t1300-config.sh
index e0dd5d65ce..5498ca32b0 100755
--- a/t/t1300-config.sh
+++ b/t/t1300-config.sh
@@ -2059,6 +2059,81 @@ test_expect_success '--show-scope with --show-origin' '
 	test_cmp expect output
 '
 
+test_expect_success 'override global and system config' '
+	test_when_finished rm -f "$HOME"/.config/git &&
+
+	cat >"$HOME"/.gitconfig <<-EOF &&
+	[home]
+		config = true
+	EOF
+	mkdir -p "$HOME"/.config/git &&
+	cat >"$HOME"/.config/git/config <<-EOF &&
+	[xdg]
+		config = true
+	EOF
+	cat >.git/config <<-EOF &&
+	[local]
+		config = true
+	EOF
+	cat >custom-global-config <<-EOF &&
+	[global]
+		config = true
+	EOF
+	cat >custom-system-config <<-EOF &&
+	[system]
+		config = true
+	EOF
+
+	cat >expect <<-EOF &&
+	global	xdg.config=true
+	global	home.config=true
+	local	local.config=true
+	EOF
+	git config --show-scope --list >output &&
+	test_cmp expect output &&
+
+	sane_unset GIT_CONFIG_NOSYSTEM &&
+
+	cat >expect <<-EOF &&
+	system	system.config=true
+	global	global.config=true
+	local	local.config=true
+	EOF
+	GIT_CONFIG_SYSTEM=custom-system-config GIT_CONFIG_GLOBAL=custom-global-config \
+		git config --show-scope --list >output &&
+	test_cmp expect output &&
+
+	cat >expect <<-EOF &&
+	local	local.config=true
+	EOF
+	GIT_CONFIG_SYSTEM=/dev/null GIT_CONFIG_GLOBAL=/dev/null git config --show-scope --list >output &&
+	test_cmp expect output
+'
+
+test_expect_success 'override global and system config with missing file' '
+	sane_unset GIT_CONFIG_NOSYSTEM &&
+	test_must_fail env GIT_CONFIG_GLOBAL=does-not-exist git version &&
+	test_must_fail env GIT_CONFIG_SYSTEM=does-not-exist git version &&
+	GIT_CONFIG_NOSYSTEM=true GIT_CONFIG_SYSTEM=does-not-exist git version
+'
+
+test_expect_success 'write to overridden global and system config' '
+	cat >expect <<EOF &&
+[config]
+	key = value
+EOF
+
+	test_must_fail env GIT_CONFIG_GLOBAL=write-to-global git config --global config.key value &&
+	touch write-to-global &&
+	GIT_CONFIG_GLOBAL=write-to-global git config --global config.key value &&
+	test_cmp expect write-to-global &&
+
+	test_must_fail env GIT_CONFIG_SYSTEM=write-to-system git config --system config.key value &&
+	touch write-to-system &&
+	GIT_CONFIG_SYSTEM=write-to-system git config --system config.key value &&
+	test_cmp expect write-to-system
+'
+
 for opt in --local --worktree
 do
 	test_expect_success "$opt requires a repo" '
-- 
2.31.1


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