Bazel 1.0.0 compatibility about toolchains_llvm

[siddharthab]

Thanks for the report. I will try to get to it over this weekend.

[luxe]

more specifically, something like this fails:

/home/laptop/.cache/bazel/_bazel_laptop/install/a4696acfbd71b79376c811717f94d1ac/_embedded_binaries/linux-sandbox -t 15 -w /home/laptop/.cache/bazel/_bazel_laptop/9e74040f84b92184f3149fef1e79ff7d/sandbox/linux-sandbox/123/execroot/__main__ -w /tmp -w /dev/shm -D -- external/llvm_toolchain/bin/llvm-ar @bazel-out/host/bin/external/boost/libchrono.a-2.params

The external path(external/llvm_toolchain/bin/llvm-ar) is no longer there. Somehow no longer copied into the sandbox.

[luxe]

I think we need to add an ar action in cc_toolchain_config.bzl
https://docs.bazel.build/versions/master/cc-toolchain-config-reference.html#ar-actions

[luxe]

It seems the execroot occasionally doesn't include the external/ folder where the llvm tools are (clang and ar), but I've seen it show up for things like clang++

temporary hack: --strategy=CppCompile=standalone --strategy=CppLink=standalone.
It leaves the sandbox which is undesirable.

[luxe]

This ticket explains the issue well.
bazelbuild/bazel#8531

I've solved the problem with:
--incompatible_use_specific_tool_files=false

To fix this, we could probably change native.cc_toolchain to use ar_files
https://github.com/grailbio/bazel-toolchain/blob/f2d0aeca399e70297414341d228c7be460b4d6a0/toolchain/internal/configure.bzl#L126

But at the same time, the docs say ar_files is currently not used.
https://docs.bazel.build/versions/master/be/c-cpp.html#cc_library

[siddharthab]

Thank you for the investigation. Your help made this an easy task for me.