Implement pre-build copy

[Emilgardis]

Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom docker image (optional)

Describe your request

From reddit

Make it easy to integrate scripts to the pre-build hook.

Describe why this would be a good inclusion for cross

No response

[Alexhuszagh]

I'm not sure this is needed: I remember being initially very favorable of the idea of supporting other commands in the pre-build hooks or customizing the base image, but in retrospect it seems like the solution we implemented was much better.

Currently, we have:

[target.x86_64-unknown-linux-gnu]
pre-build = [
    "dpkg --add-architecture arm64", 
    "apt-get update && apt-get install --assume-yes libssl-dev:arm64"
]

And my initial proposal would have been:

[target.x86_64-unknown-linux-gnu]
pre-build = [
    "RUN dpkg --add-architecture arm64", 
    "RUN apt-get update && apt-get install --assume-yes libssl-dev:arm64"
]

Which honestly in retrospect is not very legible, and a Dockerfile is better in that case. If we use a copy, will all copies be done before all runs? How will this affect cached builds if another step is added? What else do we need to implement? ENV? ARG? I think just using a Dockerfile or simple pre-build RUN instructions is enough.

The current solution for a Dockerfile is just much nicer for this:

FROM ghcr.io/cross-rs/x86_64-unknown-linux-gnu:main
RUN dpkg --add-architecture arm64
RUN apt-get update && apt-get install --assume-yes libssl-dev:arm64

COPY myscript.sh /
RUN /myscript.sh

[Emilgardis]

We only have one RUN inserted, each value in pre-build is separated by a newline and evaluated with eval

I think it'd be fine to include a way to call a script specifically, maybe that usecase is what should be supported and not a general "copy"

[Alexhuszagh]

We only have one RUN inserted, each value in pre-build is separated by a newline and evaluated with eval

I think it'd be fine to include a way to call a script specifically, maybe that usecase is what should be supported and not a general "copy"

I like that better, and make it clear it would happen after the pre-build run commands, which I believe is best.

[Emilgardis]

maybe simply

[*]
pre-build = "./myscript.sh" # Note: not an array.

don't think we need to support pre-build via array and this new alternative

[Alexhuszagh]

That makes it super simple, and correct. I think that's the correct approach. We should probably do:

• Allow an array or string
• If a string, check if it's a valid file, in which case copy and run it.
• If a string but not a valid file, treat it as a single-element array.
• Convert array to run commands, convert string to COPY + RUN.

[Alexhuszagh]

This was implemented in #910.