add config_set API for caching config-like files

Currently `git_config()` uses a callback mechanism and file rereads for
config values. Due to this approach, it is not uncommon for the config
files to be parsed several times during the run of a git program, with
different callbacks picking out different variables useful to themselves.

Add a `config_set`, that can be used to construct an in-memory cache for
config-like files that the caller specifies (i.e., files like `.gitmodules`,
`~/.gitconfig` etc.). Add two external functions `git_configset_get_value`
and `git_configset_get_value_multi` for querying from the config sets.
`git_configset_get_value` follows `last one wins` semantic (i.e. if there
are multiple matches for the queried key in the files of the configset the
value returned will be the last entry in `value_list`).
`git_configset_get_value_multi` returns a list of values sorted in order of
increasing priority (i.e. last match will be at the end of the list). Add
type specific query functions like `git_configset_get_bool` and similar.

Add a default `config_set`, `the_config_set` to cache all key-value pairs
read from usual config files (repo specific .git/config, user wide
~/.gitconfig, XDG config and the global /etc/gitconfig). `the_config_set`
is populated using `git_config()`.

Add two external functions `git_config_get_value` and
`git_config_get_value_multi` for querying in a non-callback manner from
`the_config_set`. Also, add type specific query functions that are
implemented as a thin wrapper around the `config_set` API.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-config.txt

@@ -77,6 +77,86 @@ To read a specific file in git-config format, use
 `git_config_from_file`. This takes the same callback and data parameters
 as `git_config`.
 
+Querying For Specific Variables
+-------------------------------
+
+For programs wanting to query for specific variables in a non-callback
+manner, the config API provides two functions `git_config_get_value`
+and `git_config_get_value_multi`. They both read values from an internal
+cache generated previously from reading the config files.
+
+`int git_config_get_value(const char *key, const char **value)`::
+
+	Finds the highest-priority value for the configuration variable `key`,
+	stores the pointer to it in `value` and returns 0. When the
+	configuration variable `key` is not found, returns 1 without touching
+	`value`. The caller should not free or modify `value`, as it is owned
+	by the cache.
+
+`const struct string_list *git_config_get_value_multi(const char *key)`::
+
+	Finds and returns the value list, sorted in order of increasing priority
+	for the configuration variable `key`. When the configuration variable
+	`key` is not found, returns NULL. The caller should not free or modify
+	the returned pointer, as it is owned by the cache.
+
+`void git_config_clear(void)`::
+
+	Resets and invalidates the config cache.
+
+The config API also provides type specific API functions which do conversion
+as well as retrieval for the queried variable, including:
+
+`int git_config_get_int(const char *key, int *dest)`::
+
+	Finds and parses the value to an integer for the configuration variable
+	`key`. Dies on error; otherwise, stores the value of the parsed integer in
+	`dest` and returns 0. When the configuration variable `key` is not found,
+	returns 1 without touching `dest`.
+
+`int git_config_get_ulong(const char *key, unsigned long *dest)`::
+
+	Similar to `git_config_get_int` but for unsigned longs.
+
+`int git_config_get_bool(const char *key, int *dest)`::
+
+	Finds and parses the value into a boolean value, for the configuration
+	variable `key` respecting keywords like "true" and "false". Integer
+	values are converted into true/false values (when they are non-zero or
+	zero, respectively). Other values cause a die(). If parsing is successful,
+	stores the value of the parsed result in `dest` and returns 0. When the
+	configuration variable `key` is not found, returns 1 without touching
+	`dest`.
+
+`int git_config_get_bool_or_int(const char *key, int *is_bool, int *dest)`::
+
+	Similar to `git_config_get_bool`, except that integers are copied as-is,
+	and `is_bool` flag is unset.
+
+`int git_config_get_maybe_bool(const char *key, int *dest)`::
+
+	Similar to `git_config_get_bool`, except that it returns -1 on error
+	rather than dying.
+
+`int git_config_get_string_const(const char *key, const char **dest)`::
+
+	Allocates and copies the retrieved string into the `dest` parameter for
+	the configuration variable `key`; if NULL string is given, prints an
+	error message and returns -1. When the configuration variable `key` is
+	not found, returns 1 without touching `dest`.
+
+`int git_config_get_string(const char *key, char **dest)`::
+
+	Similar to `git_config_get_string_const`, except that retrieved value
+	copied into the `dest` parameter is a mutable string.
+
+`int git_config_get_pathname(const char *key, const char **dest)`::
+
+	Similar to `git_config_get_string`, but expands `~` or `~user` into
+	the user's home directory when found at the beginning of the path.
+
+See test-config.c for usage examples.
+
 Value Parsing Helpers
 ---------------------
 
@@ -134,6 +214,68 @@ int read_file_with_include(const char *file, config_fn_t fn, void *data)
 `git_config` respects includes automatically. The lower-level
 `git_config_from_file` does not.
 
+Custom Configsets
+-----------------
+
+A `config_set` can be used to construct an in-memory cache for
+config-like files that the caller specifies (i.e., files like `.gitmodules`,
+`~/.gitconfig` etc.). For example,
+
+---------------------------------------
+struct config_set gm_config;
+git_configset_init(&gm_config);
+int b;
+/* we add config files to the config_set */
+git_configset_add_file(&gm_config, ".gitmodules");
+git_configset_add_file(&gm_config, ".gitmodules_alt");
+
+if (!git_configset_get_bool(gm_config, "submodule.frotz.ignore", &b)) {
+	/* hack hack hack */
+}
+
+/* when we are done with the configset */
+git_configset_clear(&gm_config);
+----------------------------------------
+
+Configset API provides functions for the above mentioned work flow, including:
+
+`void git_configset_init(struct config_set *cs)`::
+
+	Initializes the config_set `cs`.
+
+`int git_configset_add_file(struct config_set *cs, const char *filename)`::
+
+	Parses the file and adds the variable-value pairs to the `config_set`,
+	dies if there is an error in parsing the file. Returns 0 on success, or
+	-1 if the file does not exist or is inaccessible. The user has to decide
+	if he wants to free the incomplete configset or continue using it when
+	the function returns -1.
+
+`int git_configset_get_value(struct config_set *cs, const char *key, const char **value)`::
+
+	Finds the highest-priority value for the configuration variable `key`
+	and config set `cs`, stores the pointer to it in `value` and returns 0.
+	When the configuration variable `key` is not found, returns 1 without
+	touching `value`. The caller should not free or modify `value`, as it
+	is owned by the cache.
+
+`const struct string_list *git_configset_get_value_multi(struct config_set *cs, const char *key)`::
+
+	Finds and returns the value list, sorted in order of increasing priority
+	for the configuration variable `key` and config set `cs`. When the
+	configuration variable `key` is not found, returns NULL. The caller
+	should not free or modify the returned pointer, as it is owned by the cache.
+
+`void git_configset_clear(struct config_set *cs)`::
+
+	Clears `config_set` structure, removes all saved variable-value pairs.
+
+In addition to above functions, the `config_set` API provides type specific
+functions in the vein of `git_config_get_int` and family but with an extra
+parameter, pointer to struct `config_set`.
+They all behave similarly to the `git_config_get*()` family described in
+"Querying For Specific Variables" above.
+
 Writing Config Files
 --------------------
 

cache.h

@@ -1351,6 +1351,38 @@ extern int parse_config_key(const char *var,
 			    const char **subsection, int *subsection_len,
 			    const char **key);
 
+struct config_set {
+	struct hashmap config_hash;
+	int hash_initialized;
+};
+
+extern void git_configset_init(struct config_set *cs);
+extern int git_configset_add_file(struct config_set *cs, const char *filename);
+extern int git_configset_get_value(struct config_set *cs, const char *key, const char **value);
+extern const struct string_list *git_configset_get_value_multi(struct config_set *cs, const char *key);
+extern void git_configset_clear(struct config_set *cs);
+extern int git_configset_get_string_const(struct config_set *cs, const char *key, const char **dest);
+extern int git_configset_get_string(struct config_set *cs, const char *key, char **dest);
+extern int git_configset_get_int(struct config_set *cs, const char *key, int *dest);
+extern int git_configset_get_ulong(struct config_set *cs, const char *key, unsigned long *dest);
+extern int git_configset_get_bool(struct config_set *cs, const char *key, int *dest);
+extern int git_configset_get_bool_or_int(struct config_set *cs, const char *key, int *is_bool, int *dest);
+extern int git_configset_get_maybe_bool(struct config_set *cs, const char *key, int *dest);
+extern int git_configset_get_pathname(struct config_set *cs, const char *key, const char **dest);
+
+extern int git_config_get_value(const char *key, const char **value);
+extern const struct string_list *git_config_get_value_multi(const char *key);
+extern void git_config_clear(void);
+extern void git_config_iter(config_fn_t fn, void *data);
+extern int git_config_get_string_const(const char *key, const char **dest);
+extern int git_config_get_string(const char *key, char **dest);
+extern int git_config_get_int(const char *key, int *dest);
+extern int git_config_get_ulong(const char *key, unsigned long *dest);
+extern int git_config_get_bool(const char *key, int *dest);
+extern int git_config_get_bool_or_int(const char *key, int *is_bool, int *dest);
+extern int git_config_get_maybe_bool(const char *key, int *dest);
+extern int git_config_get_pathname(const char *key, const char **dest);
+
 extern int committer_ident_sufficiently_given(void);
 extern int author_ident_sufficiently_given(void);