boost_build: derive b2's -j from resource_size - #1577
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b2 reads none of the parallelism environment variables the framework sets (CMAKE_BUILD_PARALLEL_LEVEL, GNUMAKEFLAGS, MESON_NUM_PROCESSES, NINJA_JOBS), and modern b2 defaults -j to every detected CPU thread. So a boost_build target with a resource_size reserved N cpus from the Bazel scheduler and then forked as many compilers as the machine had cores -- the reservation bought nothing, and Bazel kept scheduling other actions alongside it. Derive the job count from the same resource_size that produces the resource_set, and put it on the b2 command line ahead of user_options so an explicit user -j still wins. Targets without a resource_size are unchanged. The arithmetic moves into parallelism_for/get_resource_parallelism so the env vars and the command line flag come from one place and cannot drift. boost_build.bzl had no stardoc target, so its attributes were not rendered anywhere; add one.
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b2 reads none of the parallelism environment variables the framework sets (CMAKE_BUILD_PARALLEL_LEVEL, GNUMAKEFLAGS, MESON_NUM_PROCESSES, NINJA_JOBS), and modern b2 defaults -j to every detected CPU thread. So a boost_build target with a resource_size reserved N cpus from the Bazel scheduler and then forked as many compilers as the machine had cores -- the reservation bought nothing, and Bazel kept scheduling other actions alongside it.
Derive the job count from the same resource_size that produces the resource_set, and put it on the b2 command line ahead of user_options so an explicit user -j still wins. Targets without a resource_size are unchanged.
The arithmetic moves into parallelism_for/get_resource_parallelism so the env vars and the command line flag come from one place and cannot drift.
boost_build.bzl had no stardoc target, so its attributes were not rendered anywhere; add one.