Use of current toolchain in bazel rule - static-libraries

Question

I've use the below bazel rule to build static libraries with bazel:
def _cc_static_library_impl(ctx):
cc_deps = [dep[CcInfo] for dep in ctx.attr.deps]
libraries = []
for cc_dep in cc_deps:
for link_input in cc_dep.linking_context.linker_inputs.to_list():
for library in link_input.libraries:
libraries += library.pic_objects
args = ["r", ctx.outputs.out.path] + [f.path for f in libraries]
ctx.actions.run(
inputs = libraries,
outputs = [ctx.outputs.out],
executable = "/usr/bin/ar",
arguments = args,
)
return [DefaultInfo()]
cc_static_library = rule(
implementation = _cc_static_library_impl,
attrs = {
"deps": attr.label_list(providers = [CcInfo]),
},
outputs = {"out": "lib%{name}.a"},
)
How can I extract the command to use from the current toolchain instead of using the hardcoded /usr/bin/ar? I've based the rule on what I've found on the internet and I have very limited knowledge about this. This example seems to do something related:
https://github.com/bazelbuild/rules_cc/blob/main/examples/my_c_archive/my_c_archive.bzl

Answer 1

This is the relevant part of my_c_archive:
archiver_path = cc_common.get_tool_for_action(
feature_configuration = feature_configuration,
action_name = CPP_LINK_STATIC_LIBRARY_ACTION_NAME,
)
That gets you the path, and then you need to add cc_toolchain.all_files to your ctx.actions.run call, so it ends up looking like this:
ctx.actions.run(
inputs = depset(
direct = [libraries],
transitive = [
cc_toolchain.all_files,
],
),
outputs = [ctx.outputs.out],
executable = archiver_path,
arguments = args,
)
However, you'll also notice that my_c_archive builds up a command line and environment variables to call the archiver with. A simple toolchain won't have anything to pass in either of those, but a more complex one may not work correctly without adding them (for example, passing -m32 or setting PATH).
Part of the starlark implementation of cc_import in _create_archive_action in cc_import.bzl is a good place to start for handling all the intricacies. It creates an action to produce a static library from a set of object files, with flexibility to work with many toolchains.