build: let a build config declare host after a cross build - #7237

A build config that declares an MRuby::CrossBuild before its own
MRuby::Build.new('host') cannot be built. No defines, no toolchain settings,
nothing but the order:

MRuby::CrossBuild.new('cross') do |conf|
  conf.toolchain
  conf.gem :core => "mruby-bin-mruby"
  conf.test_runner.command = 'env'
end

MRuby::Build.new('host') do |conf|
  conf.toolchain
  conf.gem :core => 'mruby-bin-mrbc'
  conf.gem :core => 'mruby-bin-mruby'
end

$ rake
rake aborted!
Don't know how to build task 'build/host/lib/libmruby.a' (See the list of available tasks with `rake --tasks`)
Did you mean?  build/host/bin/mruby
               build/cross/lib/libmruby.a
tasks/presym.rake:5:in 'block (2 levels) in <top (required)>'

Swapping the two declarations builds fine.

Why

CrossBuild#initialize settles the build it borrows mrbc from as the target
is declared, and where the config has written no host yet it generates one
under that name, deliberately crippled:

        MRuby::Build.new('host') do |conf|
          conf.toolchain
          conf.build_mrbc_exec
          conf.disable_libmruby
        end

Build#initialize initialises a target only when the name is free, and reopens
it otherwise. Every instance variable, @enable_libmruby included, sits inside
that unless:

      unless current = MRuby.targets[@name]
        ...
        @enable_libmruby = true
        ...
        MRuby.targets[@name] = current = self
      end

      MRuby::Build.current = current
      begin
        current.instance_eval(&block)

So the config's own host block does not run on a fresh build. It runs on the
generated one, which already carries disable_libmruby and a gem list of its
own. libmruby_enabled? stays false, no libmruby.a rule is defined for
host, and tasks/presym.rake asks for one.

The failure is loud, so nothing is silently miscompiled, but the message names
a missing rake task and points at presym.rake, which says nothing about
declaration order. No in-tree config is affected:
build_config/IntelEdison.rb is the only one that declares both, and it
declares host first.

What this does

Ask the question once, from the Rakefile, where the config has been read whole.
MRuby.resolve_mrbc_hosts binds every cross build before any gem is set up, so
a host written after a cross build is as visible as one written before it,
and the defines the answer turns on are still the ones the config alone has
written, which is what #7230 rests on.

A build generated because no declared host will do is now internal and named
after the target that asked for it, so it can no longer take a name the build
config wants. Cross targets that agree on MRB_NO_FLOAT share one, so a config
with several of them still builds mrbc once.

The name is all that moves. build/host is still where a generated mrbc is
written, since a config that declares no host claims that directory for
nothing else:

Two things follow for a config of cross builds alone. No host target is left
for rake install and rake install_bin to name, so they fall back to every
target the config did declare. And where build/host is taken, by a declared
host or by a first generated build that answers otherwise on floats, the
generated build sits under the target that asked for it, so tasks/presym.rake
reads mrbc_build to leave those objects to the build that owns them: without
it the cross build scanned 13 files belonging to its own mrbc build, 62
against 49.

Verified

This also composes with #7236, which records what built an output so that a
build directory two configs share is rebuilt rather than reused. The two are
independent: one is about the name a build takes, the other about the directory
it writes into. Applied together they merge without conflict, the config above
builds, and a build/host written by a config of cross builds alone is
reported and rebuilt when an ordinary build comes back to it, 2123 assertions
and 0 KO in both directions.

Environment

No C source changes, so .text is unaffected in every build.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JJf1R2cytz2gAnJ2CgpGp1

Summary by CodeRabbit

• New Features

  • Improved cross-build support by automatically selecting compatible mrbc tooling.
  • Added support for externally provided mrbc executables.
  • Reuses compatible generated build tools to streamline build setup.

• Bug Fixes

  • Installation now works when no host build is defined, falling back to available target artifacts.
  • Added safeguards against conflicting generated build-tool names.