Expose as_executable in ctx.host_fragments.cpp

[mboes]

Description of the feature request:

ctx.host_fragments.cpp already contains fields such as compiler_executable, nm_executable etc pointing to the locations of said executable detected by Bazel. We propose to add ctx.host_fragments.cpp.as_executable to that set for the assembler.

Feature requests: what underlying problem are you trying to solve with this feature?

Older versions of GCC call an as executable that it expects to be on the path. In hermetic builds with no default user environment, (i.e. with no /usr/bin in PATH), we expose in the PATH a folder with symlinks to exactly those executables that are necessary for the build. But it's hard to do if Bazel doesn't tell us where the assembler is. It already tells us where gcc, ar, nm etc are. So it shouldn't be very onerous to expose as as well.

What operating system are you running Bazel on?

ubuntu 16.04

What's the output of bazel info release?

0.15.0

Have you found anything relevant by searching the web?

Nothing in the issue tracker.

Any other information, logs, or outputs that you want to share?

cc @iphydf

[hlopko]

New C++ toolchain Skylark API is already submitted at HEAD, so it will be available in the next release. Then you can get path to ar by:

load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain")
load("@bazel_tools//tools/build_defs/cc:action_names.bzl", "CPP_LINK_STATIC_LIBRARY_ACTION_NAME")

cc_toolchain = find_cpp_toolchain(ctx)
feature_configuration = cc_common.configure_features(
    cc_toolchain = cc_toolchain,
    requested_features = ctx.features,
    unsupported_features = ctx.disabled_features)
archive_variables = cc_common.create_link_build_variables(
    feature_configuration = feature_configuration,
    cc_toolchain = cc_toolchain,
    is_using_linker = false)
archiver = cc_common.get_tool_for_action(
    feature_configuration = feature_configuration,
    action_name = CPP_LINK_STATIC_LIBRARY_ACTION_NAME)
archive_command_line = cc_common.get_command_line(
    feature_configuration = feature_configuration,
    action_name = CPP_LINK_STATIC_LIBRARY_ACTION_NAME,
    variables = archive_variables)

[hlopko]

#4571 is the tracking bug. Therefore I'll close this issue. Thanks!