config: provide a version of git_config with more options

Callers may want to provide a specific version of a file in which to look
for config. Right now this can be done by setting the magic global
config_exclusive_filename variable.  By providing a version of git_config
that takes a filename, we can take a step towards making this magic global
go away.

Furthermore, by providing a more "advanced" interface, we now have a a
natural place to add new options for callers like git-config, which care
about tweaking the specifics of config lookup, without disturbing the
large number of "simple" users (i.e., every other part of git).

The astute reader of this patch may notice that the logic for handling
config_exclusive_filename was taken out of git_config_early, but added
into git_config. This means that git_config_early will no longer respect
config_exclusive_filename.  That's OK, because the only other caller of
git_config_early is check_repository_format_gently, but the only function
which sets config_exclusive_filename is cmd_config, which does not call
check_repository_format_gently (and if it did, it would have been a bug,
anyway, as we would be checking the repository format in the wrong file).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/technical/api-config.txt

@@ -47,6 +47,18 @@ will first feed the user-wide one to the callback, and then the
 repo-specific one; by overwriting, the higher-priority repo-specific
 value is left at the end).
 
+The `git_config_with_options` function lets the caller examine config
+while adjusting some of the default behavior of `git_config`. It should
+almost never be used by "regular" git code that is looking up
+configuration variables. It is intended for advanced callers like
+`git-config`, which are intentionally tweaking the normal config-lookup
+process. It takes one extra parameter:
+
+`filename`::
+If this parameter is non-NULL, it specifies the name of a file to
+parse for configuration, rather than looking in the usual files. Regular
+`git_config` defaults to `NULL`.
+
 There is a special version of `git_config` called `git_config_early`.
 This version takes an additional parameter to specify the repository
 config, instead of having it looked up via `git_path`. This is useful

cache.h

@@ -1113,6 +1113,7 @@ extern int git_config_from_file(config_fn_t fn, const char *, void *);
 extern void git_config_push_parameter(const char *text);
 extern int git_config_from_parameters(config_fn_t fn, void *data);
 extern int git_config(config_fn_t fn, void *);
+extern int git_config_with_options(config_fn_t fn, void *, const char *filename);
 extern int git_config_early(config_fn_t fn, void *, const char *repo_config);
 extern int git_parse_ulong(const char *, unsigned long *);
 extern int git_config_int(const char *, const char *);

config.c

@@ -879,9 +879,6 @@ int git_config_early(config_fn_t fn, void *data, const char *repo_config)
 	int ret = 0, found = 0;
 	const char *home = NULL;
 
-	/* Setting $GIT_CONFIG makes git read _only_ the given config file. */
-	if (config_exclusive_filename)
-		return git_config_from_file(fn, config_exclusive_filename, data);
 	if (git_config_system() && !access(git_etc_gitconfig(), R_OK)) {
 		ret += git_config_from_file(fn, git_etc_gitconfig(),
 					    data);
@@ -917,11 +914,19 @@ int git_config_early(config_fn_t fn, void *data, const char *repo_config)
 	return ret == 0 ? found : ret;
 }
 
-int git_config(config_fn_t fn, void *data)
+int git_config_with_options(config_fn_t fn, void *data,
+			    const char *filename)
 {
 	char *repo_config = NULL;
 	int ret;
 
+	/*
+	 * If we have a specific filename, use it. Otherwise, follow the
+	 * regular lookup sequence.
+	 */
+	if (filename)
+		return git_config_from_file(fn, filename, data);
+
 	repo_config = git_pathdup("config");
 	ret = git_config_early(fn, data, repo_config);
 	if (repo_config)
@@ -929,6 +934,11 @@ int git_config(config_fn_t fn, void *data)
 	return ret;
 }
 
+int git_config(config_fn_t fn, void *data)
+{
+	return git_config_with_options(fn, data, config_exclusive_filename);
+}
+
 /*
  * Find all the stuff for git_config_set() below.
  */