How to use select inside a skylark rule?

[Ming Zhao]

I defined a selector inside my custom build rule:

build_config = select({
'//tools/cpp:asan_enabled': 'asan',
'//tools/cpp:msan_enabled': 'msan',
'//tools/cpp:tsan_enabled': 'tsan',
'//tools/cpp:ubsan_enabled': 'ubsan',
'//conditions:default': None,
})

But I have no idea how to access the value of build_config. I tried
print(dir(build_config)) but it has no method. Maybe I can only access
it in the macro?

[Han-Wen Nienhuys]

a snippet would be useful to understand what you are trying to do.

without looking, I imagine you are creating a rule (as opposed to a
macro), and would expect the rule to have a attribute, eg.

bla = rule(myfunc, attr = {"flavor": select({ .. }))

in myfunc, you should have access to ctx.attr.flavor, which will vary
based on the configuration you are using.

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--
Han-Wen Nienhuys
Google Munich
han...@google.com

[Ming Zhao]

I think I tried that, but I got the following error:

$ bazel build pkg:lw-ats
ERROR: /usr/local/home/mzhao/s/m3/main/tools/build_rules/pkg_deb.bzl:89:11:
Illegal argument: expected <String, Builder> type for 'attrs' but got
<string, SelectorList> instead.
ERROR: /usr/local/home/mzhao/s/m3/main/pkg/BUILD:1: Extension file
'tools/build_rules/pkg_deb.bzl' has errors.
ERROR: error loading package 'pkg': Extension file
'tools/build_rules/pkg_deb.bzl' has errors.

I ended up creating another level of indirection by wrapping it with
another macro, as you can see in my skylark rule here:
https://github.com/mzhaom/trunk/blob/master/tools/build_rules/pkg_deb.bzl

[Han-Wen Nienhuys]

Hmm, let's ask our BUILD/skylark guru.

[Laurent Le Brun]

Hi,

Use select only in the loading phase. It will be evaluated before the
implementation function is called.
Providing a way to evaluate a select within analysis phase is a
feature request (cc Greg who may comment on whether it's a good idea
or not).

--
Laurent

[Greg Estren]

I assume your failed attempt was to specify the select directly within the pkg_deb_ rule class definition, right? Like:

pkg_deb_ = rule(

  _pkg_deb_impl,

  attrs = {

    ...

     'build_config': select({...}),

  })

Shouldn't that be fully resolvable before the analysis phase begins? That syntax looks like that should work to me, although we may have to tidy up Skyframe to make it work. 

[Greg Estren]

Err: Skyframe --> Skylark.

[Greg Estren]

Issue filed at https://github.com/google/bazel/issues/287