[swift-build-dev] Proposal: dependency preheating

[Ankit Aggarwal]

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 <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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