Tools are fetched for all platforms

[alexeagle]

Follow-up to #12, this time I'll try to be more helpful in resolving it.

cd examples/target-determinator
bazel build :all

results in waiting for three copies of target-determinator to download.

[alexeagle]

#14 proposes a simple way to test this using bazel's downloader config. Here's a sample failure when the CI job ran (on the main build of my fork, since this PR doesn't pick up ci.yaml changes from untrusted sources)

https://github.com/alexeagle/rules_multitool/actions/runs/8197661135/job/22419952253

[alexeagle]

I'll try again to explain the design that fixes this problem. Sorry it's pretty challenging - I did try to make a PR here but didn't have enough time.

At the top-level is a repository macro that creates one "hub" repo, and then also one "platform" repo per supported platform.

external/rules_multitool.hub
external/rules_multitool.linux-x86_64
...

The "hub" repo must not fetch any tools. It will contain the

toolchain(
        name = "target-determinator_{os}_{cpu}_toolchain".format(name=name, os=os, cpu=cpu),
        toolchain = ":target-determinator_{os}_{cpu}_toolchain_info".format(name=name, os=os, cpu=cpu),
        toolchain_type = _TOOLCHAIN_TYPE,
        exec_compatible_with = [
            "@platforms//cpu:{cpu}".format(cpu=cpu),
            "@platforms//os:{os}".format(os=os),
        ],
        target_compatible_with = [
            "@platforms//cpu:{cpu}".format(cpu=cpu),
            "@platforms//os:{os}".format(os=os),
        ],
    )

for all platforms. These may be freely registered (either in MODULE.bazel for bzlmod or in the repository macro)

the toolchain attribute is the special sauce. See https://bazel.build/rules/deploying#registering_toolchains:

in the analysis phase Bazel needs to analyze all toolchain targets that are registered. Bazel will not need to analyze all targets referenced by toolchain.toolchain attribute.

So this is the attribute that should be changed to point into the "platform" repos, for example it should look like

"@rules_multitool.{os}-{cpu}//tools:target-determinator_toolchain_info".format(os=os, cpu=cpu)

Bazel won't try to run any of the downloads for the @rules_multitool.{os}-{cpu} repo if it didn't select the toolchain based on the *_compatible_with attributes. So, in the implementation of the "platform" repos you are free to do whatever fetches for that platform.

Come chat me on Bazel Slack if I can be more helpful to explain it?