[swift-build-dev] Proposal: dependency preheating

[Daniel Dunbar]

I think it would be fine to have a --dependencies or --dependencies-only
flag, and bypass the local sources as much as possible.

I'm not sure exactly how useful it would be yet, but I wouldn't object to a
PR for it as long as it was a maintainable addition (e.g. we need to make
sure it handles the upcoming changes w.r.t. products API w/ additional
work).

 - Daniel

On Fri, Jan 27, 2017 at 6:05 AM, Ankit Aggarwal via swift-build-dev <
swift-build-dev at swift.org> wrote:

> Hi,
>
> I think this makes sense and is very useful. The fetch part does sound
> like a bug and should be an easy fix (hopefully).
>
> Instead of preset we could add a flag to swift build for e.g.
> --dependencies or --deps-only. This would automatically give you all the
> other build options.
> $ swift build --deps-only
>
> This might be a bit tricky to do but should be possible by only reading
> the product target dependencies in the root manifest. What do others think?
>
> On 27-Jan-2017, at 6:48 PM, swizzlr via swift-build-dev <
> swift-build-dev at swift.org> wrote:
>
> Sometimes it is desirable to run a sequence of commands and cache the
> output of those commands as a filesystem snapshot in CI. This is especially
> relevant to swift packages, where compilation times are relatively long,
> but the dependencies don’t change often.
>
> In particular, Docker users will often do something like so (though this
> can be applied to Travis and other CI solutions).
>
> FROM swift
>
> WORKDIR /code
>
> COPY Package.swift /code/
> RUN swift build -c release # This fails because of point 2, below
> # Changes to `Sources` will not trigger refetch or recompilation of
> dependencies
> COPY ./Sources /code/Sources
> RUN swift build -c release
> EXPOSE 8000
> CMD .build/release/App
>
> Unfortunately, the above fails in certain scenarios:
>
> 1. `swift package fetch` will break if the target source layout doesn’t
> match. This bug is described in https://bugs.swift.org/browse/SR-3756
> 2. `swift build` will correctly fail if it cannot find sources matching
> the source layout described in Package.swift. This is a feature, not a bug.
>
> I propose making this workflow easier with `swift package preheat` – a
> command which will fetch ONLY dependencies specified in Package.swift, and
> optionally (with the option —build) compile those dependencies, but not the
> main Package.
>
> This would also be great for bootstrapping a project with a lot of
> dependencies, or with deps that have long compilation times.
>
> This command should also support specifying a build configuration, so that
> we can cache both debug and release builds at the same time to accelerate
> development iteration.
>
> Are there any comments, concerns, objections or +1s to this?
>
> Tom
>
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