Re: 2.20.0 - Undocumented change in submodule update wrt # parallel jobs

[Stefan Beller <sbeller@google.com>]

On Thu, Dec 13, 2018 at 6:17 AM Junio C Hamano <gitster@pobox.com> wrote:
>
> Sjon Hortensius <sjon@parse.nl> writes:
>
> > When switching to 2.20 our `git submodule update' (which clones
> > through ssh) broke because our ssh-server rejected the ~20
> > simultaneous connections the git-client makes. This seems to be caused
> > by a (possibly unintended) change in
> > https://github.com/git/git/commit/90efe595c53f4bb1851371344c35eff71f604d2b
> > which removed the default of max_jobs=1
> >
> > While this can easily be fixed by configuring submodule.fetchJobs I
> > think this change should be documented - or reverted back to it's
> > previous default of 1
>
> The commit in question does not look like it _wanted_ to change the
> default; rather, it appears to me that it wanted to be bug-to-bug
> compatible with the original, and any such change of behaviour is
> entirely unintended.

Indeed.

> I think the attached may be sufficient to change the default
> max_jobs back to 1.

I think so, too. I can wrap it into a commit with a proper message.

>
> By the way, is there a place where we document that the default
> value for fetchjobs, when unconfigured, is 1?

`man git config`

    submodule.fetchJobs
           Specifies how many submodules are fetched/cloned at the
           same time. A positive integer allows up to that number of
           submodules fetched in parallel. A value of 0 will give some
           reasonable default. If unset, it defaults to 1.

and that seems to be the only place, other places only reference
this place:

    Documentation$ git grep submodule.fetch
    config/submodule.txt:66:submodule.fetchJobs::
    git-clone.txt:259:      Defaults to the `submodule.fetchJobs` option.
    git-submodule.txt:408:  Defaults to the `submodule.fetchJobs` option.

The behavior of that seems to have been there since the beginning of
a028a1930c (fetching submodules: respect `submodule.fetchJobs`
config option, 2016-02-29)


> If we are not making
> such a concrete promise, then I would think it is OK to update the
> default without any fanfare, as long as we have good reasons to do
> so.  For this particular one, however, as I already said, I do not
> think we wanted to change the default to unlimited or anything like
> that, so...

We definitely want the diff below as a proper patch.

>  builtin/submodule--helper.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
> index 789d00d87d..e8cdf84f1c 100644
> --- a/builtin/submodule--helper.c
> +++ b/builtin/submodule--helper.c
> @@ -1552,7 +1552,7 @@ struct submodule_update_clone {
>  #define SUBMODULE_UPDATE_CLONE_INIT {0, MODULE_LIST_INIT, 0, \
>         SUBMODULE_UPDATE_STRATEGY_INIT, 0, 0, -1, STRING_LIST_INIT_DUP, 0, \
>         NULL, NULL, NULL, \
> -       NULL, 0, 0, 0, NULL, 0, 0, 0}
> +       NULL, 0, 0, 0, NULL, 0, 0, 1}
>
>
>  static void next_submodule_warn_missing(struct submodule_update_clone *suc,