allow suppressing of global and system config

The GIT_CONFIG_NOGLOBAL and GIT_CONFIG_NOSYSTEM environment
variables are magic undocumented switches that can be used
to ensure a totally clean environment. This is necessary for
running reliable tests, since those config files may contain
settings that change the outcome of tests.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-config.c

@@ -79,9 +79,10 @@ static int get_value(const char* key_, const char* regex_)
 		local = getenv(CONFIG_LOCAL_ENVIRONMENT);
 		if (!local)
 			local = repo_config = xstrdup(git_path("config"));
-		if (home)
+		if (git_config_global() && home)
 			global = xstrdup(mkpath("%s/.gitconfig", home));
-		system_wide = git_etc_gitconfig();
+		if (git_config_system())
+			system_wide = git_etc_gitconfig();
 	}
 
 	key = xstrdup(key_);

cache.h

@@ -589,6 +589,9 @@ extern int git_config_set_multivar(const char *, const char *, const char *, int
 extern int git_config_rename_section(const char *, const char *);
 extern const char *git_etc_gitconfig(void);
 extern int check_repository_format_version(const char *var, const char *value);
+extern int git_env_bool(const char *, int);
+extern int git_config_system(void);
+extern int git_config_global(void);
 
 #define MAX_GITNAME (1000)
 extern char git_default_email[MAX_GITNAME];

config.c

@@ -492,6 +492,22 @@ const char *git_etc_gitconfig(void)
 	return system_wide;
 }
 
+int git_env_bool(const char *k, int def)
+{
+	const char *v = getenv(k);
+	return v ? git_config_bool(k, v) : def;
+}
+
+int git_config_system(void)
+{
+	return !git_env_bool("GIT_CONFIG_NOSYSTEM", 0);
+}
+
+int git_config_global(void)
+{
+	return !git_env_bool("GIT_CONFIG_NOGLOBAL", 0);
+}
+
 int git_config(config_fn_t fn)
 {
 	int ret = 0;
@@ -504,7 +520,7 @@ int git_config(config_fn_t fn)
 	 * config file otherwise. */
 	filename = getenv(CONFIG_ENVIRONMENT);
 	if (!filename) {
-		if (!access(git_etc_gitconfig(), R_OK))
+		if (git_config_system() && !access(git_etc_gitconfig(), R_OK))
 			ret += git_config_from_file(fn, git_etc_gitconfig());
 		home = getenv("HOME");
 		filename = getenv(CONFIG_LOCAL_ENVIRONMENT);
@@ -512,7 +528,7 @@ int git_config(config_fn_t fn)
 			filename = repo_config = xstrdup(git_path("config"));
 	}
 
-	if (home) {
+	if (git_config_global() && home) {
 		char *user_config = xstrdup(mkpath("%s/.gitconfig", home));
 		if (!access(user_config, R_OK))
 			ret = git_config_from_file(fn, user_config);