cache (new feature): add '-j' (--jobs=NUM) to control concurrency

[salewski]

The fix for issue #80 introduced the ability for the ads-github-cache program to update concurrently the individual pages within "paged collections". The current implementation, however, is unconstrained in the sense that it simply spawns a background process for each page of a given collection. It would be nice to be able to control the number of such background processes via a command line option.

For similarity to other familiar tools, the syntax should take the form:

    -j jobs, --jobs=NUM

Since concurrency is the default, the NUM argument is required.

[salewski]

This issue has been fixed for ads-github-tools-0.3.4 (released 2022-10-16).

The blurb about it from our 'NEWS' file:

*** issue #81: "cache (new feature): add '-j' (--jobs=NUM) to control concurrency"

    https://github.com/salewski/ads-github-tools/issues/81

    The 'ads-github-cache' program now accepts a new '-j NUM' (--jobs=NUM)
    command line option to control the number of outstanding requests that
    will be made to the GitHub v3 API endpoints when obtaining data for
    objects represented as "paged collections".

    By default, the number is twice the number of CPUs found to be available
    at runtime, but with a hard cap of 20 (to avoid making an excessive number
    of requests on machines that happen to have a larger number of CPUs).

    If provided, the user-specified number will be used unconditionally,
    without any cap (assuming the machine has enough resources
    available). Please use this feature responsibly.

    Note that values of NUM must be an integer greater than zero (the program
    will reject other values and exit with an error status).

    If the value 1 is provided for NUM, the effect will be to eliminate
    concurrent requests.

    This change introduces a new minor utility to the ads-github-tools
    project: 'ads-github-nproc'. This new program simply prints on stdout the
    number of CPUs available to the current process. It provides a common
    interface to the rest of the 'ads-github-tools' programs that may need
    that data. To accomplish its task, however, the program will attempt to
    use a number of platform specific techniques. The code is adapted from the
    'AX_COUNT_CPUS' macro from the GNU Autoconf Archive. See
    ads-github-nproc(1) and the comments in the code for original authorship
    of the macro, as well a licensing that is inherited from the original m4
    macro source file (though still GPL compatible).