Re: RFC GSoC idea: git configuration caching (needs co-mentor!)

[Junio C Hamano <gitster@pobox.com>]

Michael Haggerty <mhagger@alum.mit.edu> writes:

> I just wrote up the idea that fell out of the discussion [1] about the
> other configuration features that I proposed.  As far as I am concerned,
> it can be merged as soon as somebody volunteers as a co-mentor.  The
> idea is embodied in a pull request against the git.github.io repository
> [2]; the text is also appended below for your convenience.
>
> Michael
>
> [1] http://article.gmane.org/gmane.comp.version-control.git/242952
> [2] https://github.com/git/git.github.io/pull/7
>
> ### git configuration API improvements
>
> There are many places in Git that need to read a configuration value.
> Currently, each such site calls `git_config()`, which reads and parses
> the configuration files every time that it is called.  This is
> wasteful, because it results in the configuration files being
> processed multiple times during a single `git` invocation.  It also
> prevents the implementation of potential new features, like adding
> syntax to allow a configuration file to unset a previously-set value.
>
> This goal of this project is to make configuration work as follows:
>
> * Read the configuration from files once and cache the results in an
>   appropriate data structure in memory.
>
> * Change `git_config()` to iterate through the pre-read values in
>   memory rather than re-reading the configuration files.
>
> * Add new API calls that allow the cache to be inquired easily and
>   efficiently.  Rewrite other functions like `git_config_int()` to be
>   cache-aware.

Are you sure about the second sentence of this item is what you
want?

git_config_<type>(name, value) are all about parsing "value" (string
or NULL) as <type>, return the parsed value or complain against a
bad value for "name".  They do not care where these "name" and
"value" come from right now, and there is no reason for them to
start caring about caching.  They will still be the underlying
helper functions the git_config() callbacks will depend on even
after the second item in your list happens.

A set of new API calls would look more like this, I would think:

	extern int git_get_config_string_multi(const char *, int *, const char ***);
	const char **values;
        int num_values;

        if (git_get_config_string_multi("sample.str", &num_values, &values))
        	return -1;
	printf("[sample]\n");
	for (i = 0; i < num_values; i++)
		printf("  str = %s\n", value[i]);
	printf("\n");
	free(values);

with a "singleton" wrapper that may be in essense:

	const char *git_get_config_string(const char *name)
        {
		const char **values, *result;
                int num_values;

	        if (git_get_config_string_multi("sample.str", &num_values, &values))
        		return NULL;
                result = num_values ? values[num_values - 1] : NULL;
                free(values);
		return result;
	}

that implements the "last one wins" semantics.  The real thing would
need to avoid allocation and free overhead.

> * Rewrite callers to use the new API wherever possible.
>
> You will need to consider how to handle other config API entry points
> like `git_config_early()` and `git_config_from_file()`, as well as how
> to invalidate the cache correctly in the case that the configuration
> is changed while `git` is executing.
>
> See
> [this mailing list
> thread](http://article.gmane.org/gmane.comp.version-control.git/242952)
> for some discussion about this and related ideas.
>
>  - Language: C
>  - Difficulty: medium
>  - Possible mentors: Michael Haggerty