config: add ctx arg to config_fn_t

Add a new "const struct config_context *ctx" arg to config_fn_t to hold
additional information about the config iteration operation.
config_context has a "struct key_value_info kvi" member that holds
metadata about the config source being read (e.g. what kind of config
source it is, the filename, etc). In this series, we're only interested
in .kvi, so we could have just used "struct key_value_info" as an arg,
but config_context makes it possible to add/adjust members in the future
without changing the config_fn_t signature. We could also consider other
ways of organizing the args (e.g. moving the config name and value into
config_context or key_value_info), but in my experiments, the
incremental benefit doesn't justify the added complexity (e.g. a
config_fn_t will sometimes invoke another config_fn_t but with a
different config value).

In subsequent commits, the .kvi member will replace the global "struct
config_reader" in config.c, making config iteration a global-free
operation. It requires much more work for the machinery to provide
meaningful values of .kvi, so for now, merely change the signature and
call sites, pass NULL as a placeholder value, and don't rely on the arg
in any meaningful way.

Most of the changes are performed by
contrib/coccinelle/config_fn_ctx.pending.cocci, which, for every
config_fn_t:

- Modifies the signature to accept "const struct config_context *ctx"
- Passes "ctx" to any inner config_fn_t, if needed
- Adds UNUSED attributes to "ctx", if needed

Most config_fn_t instances are easily identified by seeing if they are
called by the various config functions. Most of the remaining ones are
manually named in the .cocci patch. Manual cleanups are still needed,
but the majority of it is trivial; it's either adjusting config_fn_t
that the .cocci patch didn't catch, or adding forward declarations of
"struct config_context ctx" to make the signatures make sense.

The non-trivial changes are in cases where we are invoking a config_fn_t
outside of config machinery, and we now need to decide what value of
"ctx" to pass. These cases are:

- trace2/tr2_cfg.c:tr2_cfg_set_fl()

  This is indirectly called by git_config_set() so that the trace2
  machinery can notice the new config values and update its settings
  using the tr2 config parsing function, i.e. tr2_cfg_cb().

- builtin/checkout.c:checkout_main()

  This calls git_xmerge_config() as a shorthand for parsing a CLI arg.
  This might be worth refactoring away in the future, since
  git_xmerge_config() can call git_default_config(), which can do much
  more than just parsing.

Handle them by creating a KVI_INIT macro that initializes "struct
key_value_info" to a reasonable default, and use that to construct the
"ctx" arg.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 9 insertions, 3 deletions

diff --git a/trace2/tr2_cfg.c b/trace2/tr2_cfg.c
index 78cfc15d52..83bc4fd109 100644
--- a/trace2/tr2_cfg.c
+++ b/trace2/tr2_cfg.c
@@ -99,7 +99,8 @@ struct tr2_cfg_data {
 /*
  * See if the given config key matches any of our patterns of interest.
  */
-static int tr2_cfg_cb(const char *key, const char *value, void *d)
+static int tr2_cfg_cb(const char *key, const char *value,
+		      const struct config_context *ctx UNUSED, void *d)
 {
 	struct strbuf **s;
 	struct tr2_cfg_data *data = (struct tr2_cfg_data *)d;
@@ -142,8 +143,12 @@ void tr2_list_env_vars_fl(const char *file, int line)
 void tr2_cfg_set_fl(const char *file, int line, const char *key,
 		    const char *value)
 {
+	struct key_value_info kvi = KVI_INIT;
+	struct config_context ctx = {
+		.kvi = &kvi,
+	};
 	struct tr2_cfg_data data = { file, line };
 
 	if (tr2_cfg_load_patterns() > 0)
-		tr2_cfg_cb(key, value, &data);
+		tr2_cfg_cb(key, value, &ctx, &data);
 }
diff --git a/trace2/tr2_sysenv.c b/trace2/tr2_sysenv.c
index 069786cb92..f26ec95ab4 100644
--- a/trace2/tr2_sysenv.c
+++ b/trace2/tr2_sysenv.c
@@ -57,7 +57,8 @@ static struct tr2_sysenv_entry tr2_sysenv_settings[] = {
 };
 /* clang-format on */
 
-static int tr2_sysenv_cb(const char *key, const char *value, void *d)
+static int tr2_sysenv_cb(const char *key, const char *value,
+			 const struct config_context *ctx UNUSED, void *d)
 {
 	int k;