config: move documentation to config.h

Move the documentation from Documentation/technical/api-config.txt into
config.h as it's easier for the developers to find the usage information
beside the code instead of looking for it in another doc file, also
documentation/technical/api-config.txt is removed because the information
it has is now redundant and it'll be hard to keep it up to date and
syncronized with the documentation in config.h

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Reviewed-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.h

@@ -4,6 +4,22 @@
 #include "hashmap.h"
 #include "string-list.h"
 
+
+/**
+ * The config API gives callers a way to access Git configuration files
+ * (and files which have the same syntax).
+ *
+ * General Usage
+ * -------------
+ *
+ * Config files are parsed linearly, and each variable found is passed to a
+ * caller-provided callback function. The callback function is responsible
+ * for any actions to be taken on the config option, and is free to ignore
+ * some options. It is not uncommon for the configuration to be parsed
+ * several times during the run of a Git program, with different callbacks
+ * picking out different variables useful to themselves.
+ */
+
 struct object_id;
 
 /* git_config_parse_key() returns these negated: */
@@ -71,9 +87,34 @@ struct config_options {
 	} error_action;
 };
 
+/**
+ * A config callback function takes three parameters:
+ *
+ * - the name of the parsed variable. This is in canonical "flat" form: the
+ *   section, subsection, and variable segments will be separated by dots,
+ *   and the section and variable segments will be all lowercase. E.g.,
+ *   `core.ignorecase`, `diff.SomeType.textconv`.
+ *
+ * - the value of the found variable, as a string. If the variable had no
+ *   value specified, the value will be NULL (typically this means it
+ *   should be interpreted as boolean true).
+ *
+ * - a void pointer passed in by the caller of the config API; this can
+ *   contain callback-specific data
+ *
+ * A config callback should return 0 for success, or -1 if the variable
+ * could not be parsed properly.
+ */
 typedef int (*config_fn_t)(const char *, const char *, void *);
+
 int git_default_config(const char *, const char *, void *);
+
+/**
+ * Read a specific file in git-config format.
+ * This function takes the same callback and data parameters as `git_config`.
+ */
 int git_config_from_file(config_fn_t fn, const char *, void *);
+
 int git_config_from_file_with_options(config_fn_t fn, const char *,
 				      void *,
 				      const struct config_options *);
@@ -88,34 +129,157 @@ void git_config_push_parameter(const char *text);
 int git_config_from_parameters(config_fn_t fn, void *data);
 void read_early_config(config_fn_t cb, void *data);
 void read_very_early_config(config_fn_t cb, void *data);
+
+/**
+ * Most programs will simply want to look up variables in all config files
+ * that Git knows about, using the normal precedence rules. To do this,
+ * call `git_config` with a callback function and void data pointer.
+ *
+ * `git_config` will read all config sources in order of increasing
+ * priority. Thus a callback should typically overwrite previously-seen
+ * entries with new ones (e.g., if both the user-wide `~/.gitconfig` and
+ * repo-specific `.git/config` contain `color.ui`, the config machinery
+ * 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).
+ */
 void git_config(config_fn_t fn, void *);
+
+/**
+ * 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 two extra parameters:
+ *
+ * - `config_source`
+ * If this parameter is non-NULL, it specifies the source to parse for
+ * configuration, rather than looking in the usual files. See `struct
+ * git_config_source` in `config.h` for details. Regular `git_config` defaults
+ * to `NULL`.
+ *
+ * - `opts`
+ * Specify options to adjust the behavior of parsing config files. See `struct
+ * config_options` in `config.h` for details. As an example: regular `git_config`
+ * sets `opts.respect_includes` to `1` by default.
+ */
 int config_with_options(config_fn_t fn, void *,
 			struct git_config_source *config_source,
 			const struct config_options *opts);
+
+/**
+ * Value Parsing Helpers
+ * ---------------------
+ *
+ * The following helper functions aid in parsing string values
+ */
+
 int git_parse_ssize_t(const char *, ssize_t *);
 int git_parse_ulong(const char *, unsigned long *);
+
+/**
+ * Same as `git_config_bool`, except that it returns -1 on error rather
+ * than dying.
+ */
 int git_parse_maybe_bool(const char *);
+
+/**
+ * Parse the string to an integer, including unit factors. Dies on error;
+ * otherwise, returns the parsed result.
+ */
 int git_config_int(const char *, const char *);
+
 int64_t git_config_int64(const char *, const char *);
+
+/**
+ * Identical to `git_config_int`, but for unsigned longs.
+ */
 unsigned long git_config_ulong(const char *, const char *);
+
 ssize_t git_config_ssize_t(const char *, const char *);
+
+/**
+ * Same as `git_config_bool`, except that integers are returned as-is, and
+ * an `is_bool` flag is unset.
+ */
 int git_config_bool_or_int(const char *, const char *, int *);
+
+/**
+ * Parse a string into a boolean value, 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, the return value is the result.
+ */
 int git_config_bool(const char *, const char *);
+
+/**
+ * Allocates and copies the value string into the `dest` parameter; if no
+ * string is given, prints an error message and returns -1.
+ */
 int git_config_string(const char **, const char *, const char *);
+
+/**
+ * Similar to `git_config_string`, but expands `~` or `~user` into the
+ * user's home directory when found at the beginning of the path.
+ */
 int git_config_pathname(const char **, const char *, const char *);
+
 int git_config_expiry_date(timestamp_t *, const char *, const char *);
 int git_config_color(char *, const char *, const char *);
 int git_config_set_in_file_gently(const char *, const char *, const char *);
+
+/**
+ * write config values to a specific config file, takes a key/value pair as
+ * parameter.
+ */
 void git_config_set_in_file(const char *, const char *, const char *);
+
 int git_config_set_gently(const char *, const char *);
+
+/**
+ * write config values to `.git/config`, takes a key/value pair as parameter.
+ */
 void git_config_set(const char *, const char *);
+
 int git_config_parse_key(const char *, char **, int *);
 int git_config_key_is_valid(const char *key);
 int git_config_set_multivar_gently(const char *, const char *, const char *, int);
 void git_config_set_multivar(const char *, const char *, const char *, int);
 int git_config_set_multivar_in_file_gently(const char *, const char *, const char *, const char *, int);
+
+/**
+ * takes four parameters:
+ *
+ * - the name of the file, as a string, to which key/value pairs will be written.
+ *
+ * - the name of key, as a string. This is in canonical "flat" form: the section,
+ *   subsection, and variable segments will be separated by dots, and the section
+ *   and variable segments will be all lowercase.
+ *   E.g., `core.ignorecase`, `diff.SomeType.textconv`.
+ *
+ * - the value of the variable, as a string. If value is equal to NULL, it will
+ *   remove the matching key from the config file.
+ *
+ * - the value regex, as a string. It will disregard key/value pairs where value
+ *   does not match.
+ *
+ * - a multi_replace value, as an int. If value is equal to zero, nothing or only
+ *   one matching key/value is replaced, else all matching key/values (regardless
+ *   how many) are removed, before the new pair is written.
+ *
+ * It returns 0 on success.
+ */
 void git_config_set_multivar_in_file(const char *, const char *, const char *, const char *, int);
+
+/**
+ * rename or remove sections in the config file
+ * parameters `old_name` and `new_name`
+ * If NULL is passed through `new_name` parameter,
+ * the section will be removed from the config file.
+ */
 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 *);
@@ -142,6 +306,30 @@ enum config_scope current_config_scope(void);
 const char *current_config_origin_type(void);
 const char *current_config_name(void);
 
+/**
+ * Include Directives
+ * ------------------
+ *
+ * By default, the config parser does not respect include directives.
+ * However, a caller can use the special `git_config_include` wrapper
+ * callback to support them. To do so, you simply wrap your "real" callback
+ * function and data pointer in a `struct config_include_data`, and pass
+ * the wrapper to the regular config-reading functions. For example:
+ *
+ * -------------------------------------------
+ * int read_file_with_include(const char *file, config_fn_t fn, void *data)
+ * {
+ * struct config_include_data inc = CONFIG_INCLUDE_INIT;
+ * inc.fn = fn;
+ * inc.data = data;
+ * return git_config_from_file(git_config_include, file, &inc);
+ * }
+ * -------------------------------------------
+ *
+ * `git_config` respects includes automatically. The lower-level
+ * `git_config_from_file` does not.
+ *
+ */
 struct config_include_data {
 	int depth;
 	config_fn_t fn;
@@ -169,6 +357,33 @@ int parse_config_key(const char *var,
 		     const char **subsection, int *subsection_len,
 		     const char **key);
 
+/**
+ * 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
+ */
+
 struct config_set_element {
 	struct hashmap_entry ent;
 	char *key;
@@ -197,16 +412,47 @@ struct config_set {
 	struct configset_list list;
 };
 
+/**
+ * Initializes the config_set `cs`.
+ */
 void git_configset_init(struct config_set *cs);
+
+/**
+ * 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_add_file(struct config_set *cs, const char *filename);
+
+/**
+ * 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.
+ */
 const struct string_list *git_configset_get_value_multi(struct config_set *cs, const char *key);
+
+/**
+ * Clears `config_set` structure, removes all saved variable-value pairs.
+ */
 void git_configset_clear(struct config_set *cs);
 
 /*
  * These functions return 1 if not found, and 0 if found, leaving the found
  * value in the 'dest' pointer.
  */
+
+/*
+ * 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.
+ */
 int git_configset_get_value(struct config_set *cs, const char *key, const char **dest);
+
 int git_configset_get_string_const(struct config_set *cs, const char *key, const char **dest);
 int git_configset_get_string(struct config_set *cs, const char *key, char **dest);
 int git_configset_get_int(struct config_set *cs, const char *key, int *dest);
@@ -240,17 +486,94 @@ int repo_config_get_maybe_bool(struct repository *repo,
 int repo_config_get_pathname(struct repository *repo,
 			     const char *key, const char **dest);
 
+/**
+ * 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.
+ */
+
+/**
+ * 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.
+ */
 int git_config_get_value(const char *key, const char **value);
+
+/**
+ * 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.
+ */
 const struct string_list *git_config_get_value_multi(const char *key);
+
+/**
+ * Resets and invalidates the config cache.
+ */
 void git_config_clear(void);
+
+/**
+ * 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(const char *key, const 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_string(const char *key, char **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_int(const char *key, int *dest);
+
+/**
+ * Similar to `git_config_get_int` but for unsigned longs.
+ */
 int git_config_get_ulong(const char *key, unsigned long *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(const char *key, 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_bool_or_int(const char *key, int *is_bool, int *dest);
+
+/**
+ * Similar to `git_config_get_bool`, except that it returns -1 on error
+ * rather than dying.
+ */
 int git_config_get_maybe_bool(const char *key, int *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.
+ */
 int git_config_get_pathname(const char *key, const char **dest);
+
 int git_config_get_index_threads(int *dest);
 int git_config_get_untracked_cache(void);
 int git_config_get_split_index(void);
@@ -270,7 +593,19 @@ struct key_value_info {
 	enum config_scope scope;
 };
 
+/**
+ * First prints the error message specified by the caller in `err` and then
+ * dies printing the line number and the file name of the highest priority
+ * value for the configuration variable `key`.
+ */
 NORETURN void git_die_config(const char *key, const char *err, ...) __attribute__((format(printf, 2, 3)));
+
+/**
+ * Helper function which formats the die error message according to the
+ * parameters entered. Used by `git_die_config()`. It can be used by callers
+ * handling `git_config_get_value_multi()` to print the correct error message
+ * for the desired value.
+ */
 NORETURN void git_die_config_linenr(const char *key, const char *filename, int linenr);
 
 #define LOOKUP_CONFIG(mapping, var) \