Re: [PATCH] fetch: add "--parallel", which fetches all remotes in parallel

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Junio,

On Thu, 18 Jul 2019, Junio C Hamano wrote:

> Palmer Dabbelt <palmer@sifive.com> writes:
>
> > * As I was writing the documentation I found "fetch --jobs".  It seems
> >   like I should use that instead of adding a new argrument, but I wasn't
> >   sure.
>
> Why not?  What makes you feel it is a bad idea to follow that
> pattern?
>
> Ah, --jobs that is taken already is right now too tied to fetches
> that happen in submodules, which arguably was a design mistake.
>
>     -j::
>     --jobs=<n>::
>             Number of parallel children to be used for fetching
>             submodules.  Each will fetch from different submodules,
>             such that fetching many submodules will be faster. By
>             default submodules will be fetched one at a time.
>
> The simplest endgame would be to replace "submodule" with
> "repository" in the above description, perhaps like
>
> 	Number of parallel jobs to be used for fetching from
> 	multiple repositories (both fetching with "--multiple" from
> 	multiple repositories, and also fetching updated contents
> 	for submodules).  By default, fetching from multiple
> 	repositories and submodules is done one at a time.
>
> and nobody would have complained if the system were like so from the
> beginning.  Existing users, however, may want extra flexibility, and
> would complain loudly if we did the above, in which case, we may
> have to
>
>  - introduce --fetch-jobs=<n> for what you are adding;
>
>  - introduce --submodule-fetch-jobs=<n> as a synonym for existing
>    --jobs=<n> and deprecate the current use of --jobs=<n>;
>
>  - eventually repurpose --jobs=<n> as a short-hand to give both
>    --fetch-jobs and --submoduje-fetch-jobs at the same time.

Given that the relevant code looks like this:

        if (remote) {
                if (filter_options.choice || repository_format_partial_clone)
                        fetch_one_setup_partial(remote);
                result = fetch_one(remote, argc, argv, prune_tags_ok);
        } else {
                if (filter_options.choice)
                        die(_("--filter can only be used with the remote "
                              "configured in extensions.partialclone"));
                /* TODO should this also die if we have a previous partial-clone? */
                result = fetch_multiple(&list);
        }

        if (!result && (recurse_submodules != RECURSE_SUBMODULES_OFF)) {
                struct argv_array options = ARGV_ARRAY_INIT;

                add_options_to_argv(&options);
                result = fetch_populated_submodules(the_repository,
                                                    &options,
                                                    submodule_prefix,
                                                    recurse_submodules,
                                                    recurse_submodules_default,
                                                    verbosity < 0,
                                                    max_children);
                argv_array_clear(&options);
        }

i.e. the `fetch_multiple()` call is _strictly_ before the call to
`fetch_populated_submodules()`, I would contend that this level of
separation does not serve anybody but a fan of the complicators' gloves.

You would also find yourself in quite a pickle if you wanted to explain
to a user why those `--fetch-jobs` and `--submodule-fetch-jobs` are
separate options _and_ why they repeat the command name in the option
name. And if it is hard to explain, there is usually a better design
choice in the first place.

In other words, I would be much more in favor of `--jobs` _also_
extending to the `fetch_multiple()` call, not just the
`fetch_populated_submodules()` call.

> > @@ -1456,12 +1459,15 @@ static void add_options_to_argv(struct argv_array *argv)
> >
> >  }
> >
> > -static int fetch_multiple(struct string_list *list)
> > +static int fetch_multiple(struct string_list *list, int i)
> >  {
> > -	int i, result = 0;
>
> 'i' is perfectly a good name for a local variable that is used for
> loop control purposes, but makes a horrible name for a parameter.
>
> Existing 'list' is not any better either---we know it is a list by
> its type already, the name should say what the list is about, what
> it represents.  But having a horribly named parameter already is not
> a good reason to make the code even worse.
>
> And as you said, recursion makes the code structure harder to follow
> here.  Keeping an array of --jobs=<n> cmd structures, looping to
> fill them by starting, doing wait() to reap any of the started ones
> that first exits to refill the slot just opened, etc. would be easier
> to see if done in a loop, I think.

I have to admit that I'd _much_ rather see the strategy of
`fetch_populated_submodules()` emulated, where it uses the
`run_processes_parallel_tr2()` function to perform the actual fetches.

That buys us a lot of advantages:

- the recursion problems mentioned in the original mail will _not_ be
  any issue,
- the progress won't be garbled,
- we can automatically restrict the maximal number of parallel fetches.

It should be super easy to get started with this, simply by moving the
`list` and the `i` variables into a `struct` that is then passed to
`run_processes_parallel_tr2()`, then implementing the three callbacks.

Those callbacks can take inspiration from `submodule.c`'s
`get_next_submodule()`, `fetch_start_failure()`, and `fetch_finish()`
functions, but rest assured that the `fetch_multiple()` ones will look
_a lot_ simpler.

Ciao,
Dscho

P.S.: It would probably also make sense to extend this contribution by a
second patch that teaches `git remote update` a `--jobs` option.