Support both Python 2 and 3 in tools (i.e. host configuration)

[brandjon]

We need to provide a way for genrules to invoke python 3 binaries as tools. Currently the tools attribute uses the special HOST config, which prevents switching the PY2/PY3 mode. Design doc to come.

Targeting 2018 Q4.

[brandjon]

(P1 for a feature request essentially means "this quarter", so not as urgent as P1 for a bug.)

[brandjon]

This crosses over into Configurability team's domain and I understand they're prioritizing it, so until we understand more I'm marking this P2.

[brandjon]

This is blocked on work by Configurability team to add an "execution transition". Unlike the host transition, this transition will allow for subsequent transitions such as changes to the Python version.

Once that's in, we can add new attributes to rules to use this transition in place of the host transition, e.g. create genrule.exec_tools to replace genrule.tools. Users can incrementally migrate dependencies from tools to exec_tools and likewise for other rules.

Over the long term, the difference between the host configuration and the "execution platform configuration" will decrease, as more things are migrated to use platforms and toolchains rather than ad hoc flags.

[brandjon]

Update: This is being actively worked on by configurability team but there likely won't be anything in the 0.27 release, which is the release that will turn on Python toolchains. See #7899 for migration-related concerns.

The new plan may in fact be to not create an exec_tools attribute but just change genrule's tools attribute to use the execution config instead of host config.

[brandjon]

Latest update is that exec_tools will be added to genrule until blockers for migrating tools to exec config can be worked out. For starlark rules there's already a cfg="exec" value for attribute definitions.

[brandjon]

FYI exec_tools for genrule will be in 0.28 (implemented in d97392f).

[speedskater]

@brandjon Thanks for this hint of using cfg="exec". It safed my day. Do you know if there is a specific reason, why this is not part of the official documentation?

[lberki]

I'll close this since Python 2 support (at least for the native rules) is going away soon.