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, 86 insertions, 0 deletions

diff --git a/t/t1300-config.sh b/t/t1300-config.sh
index e0dd5d65ce..0f92dfe6fb 100755
--- a/t/t1300-config.sh
+++ b/t/t1300-config.sh
@@ -2059,6 +2059,92 @@ 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' '
+	test_must_fail env GIT_CONFIG_GLOBAL=does-not-exist GIT_CONFIG_SYSTEM=/dev/null git config --global --list &&
+	test_must_fail env GIT_CONFIG_GLOBAL=/dev/null GIT_CONFIG_SYSTEM=does-not-exist git config --system --list &&
+	GIT_CONFIG_GLOBAL=does-not-exist GIT_CONFIG_SYSTEM=does-not-exist git version
+'
+
+test_expect_success 'system override has no effect with GIT_CONFIG_NOSYSTEM' '
+	# `git config --system` has different semantics compared to other
+	# commands as it ignores GIT_CONFIG_NOSYSTEM. We thus test whether the
+	# variable has an effect via a different proxy.
+	cat >alias-config <<-EOF &&
+	[alias]
+		hello-world = !echo "hello world"
+	EOF
+	test_must_fail env GIT_CONFIG_NOSYSTEM=true GIT_CONFIG_SYSTEM=alias-config \
+		git hello-world &&
+	GIT_CONFIG_NOSYSTEM=false GIT_CONFIG_SYSTEM=alias-config \
+		git hello-world >actual &&
+	echo "hello world" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'write to overridden global and system config' '
+	cat >expect <<EOF &&
+[config]
+	key = value
+EOF
+
+	GIT_CONFIG_GLOBAL=write-to-global git config --global config.key value &&
+	test_cmp expect write-to-global &&
+
+	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" '