This RFC is preparation for config.[ch] to be libified as as part of the
libification effort that Emily described in [1]. One of the first goals is
to read config from a file, but the trouble with how config.c is written
today is that all reading operations rely on global state, so before turning
that into a library, we'd want to make that state non-global.

This series gets us about halfway there; it does enough plumbing for a
workable-but-kinda-ugly library interface, but with a little bit more work,
I think we can get rid of global state in-tree as well. That requires a fair
amount of work though, so I'd like to get thoughts on that before starting
work.

= Description

This series extracts the global config reading state into "struct
config_reader" and plumbs it through the config reading machinery. It's very
similar to how we've plumbed "struct repository" and other 'context objects'
in the past, except:

 * The global state (named "the_reader") for the git process lives in a
   config.c static variable, and not on "the_repository". See 3/6 for the
   rationale.

 * I've stopped short of adding "struct config_reader" to config.h public
   functions, since that would affect non-config.c callers.

If we stop right here, it's quite easy to extend it to a future config-lib.h
without having to adjust the config.h interface:

 * Move the core config reading functionality from config.c to config-lib.c.

 * Have config-lib.h accept "struct config_reader" as an arg.

 * Have config.h call config-lib.h while passing "the_reader".

and I have some WIP patches that do just that [3], but I think they can be
significantly improved if we go a bit further...

= Leftover bits and RFC

With a bit more work on the config machinery, we could make it so that
config reading stops being global even without adjusting non-config.c
callers. The idea is pretty simple: have the config machinery initialize an
internal "struct config_reader" every time we read config and expose that
state to the config callbacks (instead of, in this series, asking the caller
to initialize "struct config_reader" themselves). I believe that only config
callbacks are accessing this state, e.g. because they use the low-level
information (like builtin/config.c printing the filename and scope of the
value) or for error reporting (like git_parse_int() reporting the filename
and line number of the value it failed to parse), and only config callbacks
should be accessing this state anyway.

The catch (aka the reason I stopped halfway through) is that I couldn't find
a way to expose "struct config_reader" state without some fairly big
changes, complexity-wise or LoC-wise, e.g.

 * We could add "struct config_reader" to "config_fn_t", i.e.

   -typedef int (*config_fn_t)(const char *var, const char *val, void
   *data); +typedef int (*config_fn_t)(const struct config_reader *reader,
   const char *var, const char *val, void *data);

   which isn't complex at all, except that there are ~100 config_fn_t
   implementations [3] and a good number of them may never reference
   "reader". If the churn is tolerable, I think this a good way forward.

 * We could create a new kind of "config_fn_t" that accepts "struct
   config_reader", e.g.

   typedef int (*config_fn_t)(const char *var, const char *val, void *data);
   +typedef int (*config_state_fn_t)(const struct config_reader *reader,
   const char *var, const char *val, void *data);

   and only adjust the callers that would actually reference "reader". This
   is less churn, but I couldn't find a great way to do this kind of
   'switching between config callback types' elegantly.

 * We could smuggle "struct config_reader" to callback functions in a way
   that interested callers could see it, but uninterested callers could
   ignore. One trick that Jonathan Tan came up with (though not necessarily
   endorsed) would be to allocate a struct for the config value + "struct
   config_reader", then, interested callers could use "offset_of" to recover
   the "struct config_reader". It's a little hacky, but it's low-churn.

= Questions

 * Is this worth merging without the extra work? There are some cleanups in
   this series that could make it valuable, but there are also some hacks
   (see 4/6) that aren't so great.
 * Is the extra work even worth it?
 * Do any of the ideas seem more promising than the others? Are there other
   ideas I'm missing?

[1]
https://lore.kernel.org/git/CAJoAoZ=Cig_kLocxKGax31sU7Xe4==BGzC__Bg2_pr7krNq6MA@mail.gmail.com
[2]
https://github.com/chooglen/git/compare/config/structify-reading...chooglen:git:config/read-without-globals
[3] This is a rough estimate based on "git grep"-ing callers of the config.h
functions. I vaguely recall callbacks being called "old-style", with the
suggestion that we should replace them with the "new-style" constant time
git_config_get_*() family of functions. That would decrease the number of
config callbacks significantly.

Glen Choo (6):
  config.c: plumb config_source through static fns
  config.c: don't assign to "cf" directly
  config.c: create config_reader and the_reader
  config.c: plumb the_reader through callbacks
  config.c: remove current_config_kvi
  config.c: remove current_parsing_scope

 config.c | 489 ++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 287 insertions(+), 202 deletions(-)

base-commit: dadc8e6dacb629f46aee39bde90b6f09b73722eb

Submitted-As: https://lore.kernel.org/git/pull.1463.git.git.1677631097.gitgitgadget@gmail.com