Merge branch 'ps/config-global-override' into next

Replace GIT_CONFIG_NOSYSTEM mechanism to decline from reading the
system-wide configuration file with GIT_CONFIG_SYSTEM that lets
users specify from which file to read the system-wide configuration
(setting it to an empty file would essentially be the same as
setting NOSYSTEM), and introduce GIT_CONFIG_GLOBAL to override the
per-user configuration in $HOME/.gitconfig.

* ps/config-global-override:
  config: allow overriding of global and system configuration
  config: unify code paths to get global config paths
  config: rename `git_etc_config()`

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.

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. If `GIT_CONFIG_SYSTEM` is set, the
+	system config file defined at build time (usually `/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

builtin/config.c

@@ -671,9 +671,9 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 	}
 
 	if (use_global_config) {
-		char *user_config = expand_user_path("~/.gitconfig", 0);
-		char *xdg_config = xdg_config_home("config");
+		char *user_config, *xdg_config;
 
+		git_global_config(&user_config, &xdg_config);
 		if (!user_config)
 			/*
 			 * It is unknown if HOME/.gitconfig exists, so
@@ -695,7 +695,7 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		}
 	}
 	else if (use_system_config) {
-		given_config_source.file = git_etc_gitconfig();
+		given_config_source.file = git_system_config();
 		given_config_source.scope = CONFIG_SCOPE_SYSTEM;
 	} else if (use_local_config) {
 		given_config_source.file = git_pathdup("config");

config.c

@@ -1830,12 +1830,26 @@ static int git_config_from_blob_ref(config_fn_t fn,
 	return git_config_from_blob_oid(fn, name, &oid, data);
 }
 
-const char *git_etc_gitconfig(void)
+char *git_system_config(void)
 {
-	static const char *system_wide;
-	if (!system_wide)
-		system_wide = system_path(ETC_GITCONFIG);
-	return system_wide;
+	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_out, char **xdg_out)
+{
+	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;
 }
 
 /*
@@ -1869,8 +1883,9 @@ static int do_git_config_sequence(const struct config_options *opts,
 				  config_fn_t fn, void *data)
 {
 	int ret = 0;
-	char *xdg_config = xdg_config_home("config");
-	char *user_config = expand_user_path("~/.gitconfig", 0);
+	char *system_config = git_system_config();
+	char *xdg_config = NULL;
+	char *user_config = NULL;
 	char *repo_config;
 	enum config_scope prev_parsing_scope = current_parsing_scope;
 
@@ -1882,13 +1897,14 @@ static int do_git_config_sequence(const struct config_options *opts,
 		repo_config = NULL;
 
 	current_parsing_scope = CONFIG_SCOPE_SYSTEM;
-	if (git_config_system() && !access_or_die(git_etc_gitconfig(), R_OK,
-						  opts->system_gently ?
-						  ACCESS_EACCES_OK : 0))
-		ret += git_config_from_file(fn, git_etc_gitconfig(),
-					    data);
+	if (git_config_system() && system_config &&
+	    !access_or_die(system_config, R_OK,
+			   opts->system_gently ? ACCESS_EACCES_OK : 0))
+		ret += git_config_from_file(fn, system_config, data);
 
 	current_parsing_scope = CONFIG_SCOPE_GLOBAL;
+	git_global_config(&user_config, &xdg_config);
+
 	if (xdg_config && !access_or_die(xdg_config, R_OK, ACCESS_EACCES_OK))
 		ret += git_config_from_file(fn, xdg_config, data);
 
@@ -1913,6 +1929,7 @@ static int do_git_config_sequence(const struct config_options *opts,
 		die(_("unable to parse command-line config"));
 
 	current_parsing_scope = prev_parsing_scope;
+	free(system_config);
 	free(xdg_config);
 	free(user_config);
 	free(repo_config);

config.h

@@ -318,7 +318,6 @@ int git_config_rename_section(const char *, const char *);
 int git_config_rename_section_in_file(const char *, const char *, const char *);
 int git_config_copy_section(const char *, const char *);
 int git_config_copy_section_in_file(const char *, const char *, const char *);
-const char *git_etc_gitconfig(void);
 int git_env_bool(const char *, int);
 unsigned long git_env_ulong(const char *, unsigned long);
 int git_config_system(void);
@@ -327,6 +326,9 @@ int config_error_nonbool(const char *);
 #define config_error_nonbool(s) (config_error_nonbool(s), const_error())
 #endif
 
+char *git_system_config(void);
+void git_global_config(char **user, char **xdg);
+
 int git_config_parse_parameter(const char *, config_fn_t fn, void *data);
 
 enum config_scope current_config_scope(void);

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" '