builtin.h: update documentation

The documentation for the builtin API was moved from the technical
documentation and into a comment in builtin.h by ec14d4ecb5 (builtin.h: take
over documentation from api-builtin.txt, 2017-08-02). This documentation
wasn't updated as part of the major overhaul to include a repository struct
in 9b1cb5070f (builtin: add a repository parameter for builtin functions,
2024-09-13).

There was a brief update regarding the move from *.txt to *.adoc by
e8015223c7 (builtin.h: *.txt -> *.adoc fixes, 2025-03-03).

I noticed that there was quite a bit missing from the old documentation,
which is still visible on git-scm.com [1].

[1] https://github.com/git/git-scm.com/issues/2124

This change updates the documentation in the following ways:

 1. Updates the cmd_foo() prototype to include a repository.
 2. Adds some newlines to have uniformity in the list of flags.
 3. Adds a description of the NO_PARSEOPT flag.
 4. Describes the tests that perform checks on all builtins, which may trip
    up a contributor working on a new builtin.

I double-checked these instructions against a toy example in my local branch
to be sure that it was complete.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 25 insertions, 1 deletions

diff --git a/builtin.h b/builtin.h
index 1b35565fbd..e5e16ecaa6 100644
--- a/builtin.h
+++ b/builtin.h
@@ -17,7 +17,8 @@
  * . Define the implementation of the built-in command `foo` with
  *   signature:
  *
- *	int cmd_foo(int argc, const char **argv, const char *prefix);
+ *	int cmd_foo(int argc, const char **argv,
+ *		    const char *prefix, struct repository *repo);
  *
  * . Add the external declaration for the function to `builtin.h`.
  *
@@ -29,12 +30,14 @@
  * where options is the bitwise-or of:
  *
  * `RUN_SETUP`:
+ *
  *	If there is not a Git directory to work on, abort.  If there
  *	is a work tree, chdir to the top of it if the command was
  *	invoked in a subdirectory.  If there is no work tree, no
  *	chdir() is done.
  *
  * `RUN_SETUP_GENTLY`:
+ *
  *	If there is a Git directory, chdir as per RUN_SETUP, otherwise,
  *	don't chdir anywhere.
  *
@@ -57,6 +60,12 @@
  *	more informed decision, e.g., by ignoring `pager.<cmd>` for
  *	certain subcommands.
  *
+ * `NO_PARSEOPT`:
+ *
+ *	Most Git builtins use the parseopt library for parsing options.
+ *	This flag indicates that a custom parser is used and thus the
+ *	builtin would not appear in 'git --list-cmds=parseopt'.
+ *
  * . Add `builtin/foo.o` to `BUILTIN_OBJS` in `Makefile`.
  *
  * Additionally, if `foo` is a new command, there are 4 more things to do:
@@ -69,6 +78,21 @@
  *
  * . Add an entry for `/git-foo` to `.gitignore`.
  *
+ * As you work on implementing your builtin, be mindful that the
+ * following tests will check different aspects of the builtin's
+ * readiness and adherence to matching the documentation:
+ *
+ * * t0012-help.sh checks that the builtin can handle -h, which comes
+ *   automatically with the parseopt API.
+ *
+ * * t0450-txt-doc-vs-help.sh checks that the -h help output matches the
+ *   SYNOPSIS in the documentation for the builtin.
+ *
+ * * t1517-outside-repo.sh checks that the builtin can handle -h when
+ *   run outside of the context of a repository. Note that this test
+ *   requires that the usage has a space after the builtin name, so some
+ *   minimum description of options is required.
+ *
  *
  * How a built-in is called
  * ------------------------