config API: add and use a "git_config_get()" family of functions

We already have the basic "git_config_get_value()" function and its
"repo_*" and "configset" siblings to get a given "key" and assign the
last key found to a provided "value".

But some callers don't care about that value, but just want to use the
return value of the "get_value()" function to check whether the key
exist (or another non-zero return value).

The immediate motivation for this is that a subsequent commit will
need to change all callers of the "*_get_value_multi()" family of
functions. In two cases here we (ab)used it to check whether we had
any values for the given key, but didn't care about the return value.

The rest of the callers here used various other config API functions
to do the same, all of which resolved to the same underlying functions
to provide the answer.

Some of these were using either git_config_get_string() or
git_config_get_string_tmp(), see fe4c750fb13 (submodule--helper: fix a
configure_added_submodule() leak, 2022-09-01) for a recent example. We
can now use a helper function that doesn't require a throwaway
variable.

We could have changed git_configset_get_value_multi() (and then
git_config_get_value() etc.) to accept a "NULL" as a "dest" for all
callers, but let's avoid changing the behavior of existing API
users. Having an "unused" value that we throw away internal to
config.c is cheap.

A "NULL as optional dest" pattern is also more fragile, as the intent
of the caller might be misinterpreted if he were to accidentally pass
"NULL", e.g. when "dest" is passed in from another function.

Another name for this function could have been
"*_config_key_exists()", as suggested in [1]. That would work for all
of these callers, and would currently be equivalent to this function,
as the git_configset_get_value() API normalizes all non-zero return
values to a "1".

But adding that API would set us up to lose information, as e.g. if
git_config_parse_key() in the underlying configset_find_element()
fails we'd like to return -1, not 1.

Let's change the underlying configset_find_element() function to
support this use-case, we'll make further use of it in a subsequent
commit where the git_configset_get_value_multi() function itself will
expose this new return value.

This still leaves various inconsistencies and clobbering or ignoring
of the return value in place. E.g here we're modifying
configset_add_value(), but ever since it was added in [2] we've been
ignoring its "int" return value, but as we're changing the
configset_find_element() it uses, let's have it faithfully ferry that
"ret" along.

Let's also use the "RESULT_MUST_BE_USED" macro introduced in [3] to
assert that we're checking the return value of
configset_find_element().

We're leaving the same change to configset_add_value() for some future
series. Once we start paying attention to its return value we'd need
to ferry it up as deep as do_config_from(), and would need to make
least read_{,very_}early_config() and git_protected_config() return an
"int" instead of "void". Let's leave that for now, and focus on
the *_get_*() functions.

1. 3c8687a73ee (add `config_set` API for caching config-like files, 2014-07-28)
2. https://lore.kernel.org/git/xmqqczadkq9f.fsf@gitster.g/
3. 1e8697b5c4e (submodule--helper: check repo{_submodule,}_init()
   return values, 2022-09-01),

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/config.h b/config.h
index ef9eade..cbd82e5 100644
--- a/config.h
+++ b/config.h
@@ -474,6 +474,13 @@ void git_configset_clear(struct config_set *cs);
  * value in the 'dest' pointer.
  */
 
+/**
+ * git_configset_get() returns negative values on error, see
+ * repo_config_get() below.
+ */
+RESULT_MUST_BE_USED
+int git_configset_get(struct config_set *cs, const char *key);
+
 /*
  * Finds the highest-priority value for the configuration variable `key`
  * and config set `cs`, stores the pointer to it in `value` and returns 0.
@@ -494,6 +501,14 @@ int git_configset_get_pathname(struct config_set *cs, const char *key, const cha
 /* Functions for reading a repository's config */
 struct repository;
 void repo_config(struct repository *repo, config_fn_t fn, void *data);
+
+/**
+ * Run only the discover part of the repo_config_get_*() functions
+ * below, in addition to 1 if not found, returns negative values on
+ * error (e.g. if the key itself is invalid).
+ */
+RESULT_MUST_BE_USED
+int repo_config_get(struct repository *repo, const char *key);
 int repo_config_get_value(struct repository *repo,
 			  const char *key, const char **value);
 const struct string_list *repo_config_get_value_multi(struct repository *repo,
@@ -530,8 +545,15 @@ void git_protected_config(config_fn_t fn, void *data);
  * 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.
+ *
+ * For those git_config_get*() functions that aren't documented,
+ * consult the corresponding repo_config_get*() function's
+ * documentation.
  */
 
+RESULT_MUST_BE_USED
+int git_config_get(const char *key);
+
 /**
  * Finds the highest-priority value for the configuration variable `key`,
  * stores the pointer to it in `value` and returns 0. When the