Tags to execution requirements propagation in Starlark

[buchgr]

Every target in Bazel supports specifying a tags attribute:

cc_binary(
  ...
  tags = ["local"]
)

While tags can be used for a variety of use cases (i.e. query), Bazel also defines a set of tags that have special meaning for how it executes an action. For example, no-sandbox tells Bazel to run this action without a sandboxing strategy and no-remote tells it to not run the action on a remote execution system.

The issue is that a user sets tags on a target and not on an action. So a rule implementation needs to intentionally pass the tags forward to every action it creates (as an execution requirement).

def _impl(ctx):
    ctx.actions.run(
       ...
       execution_requirements = {tag:"" for tag in ctx.attr.tags.keys()},
    )

foo = rule(
    implementation = _impl,
    attrs = {
      ...
    },
)

If a rule doesn't add execution_requirements = ctx.attr.tags.keys() or similar to each action it creates then a user tagging a target as no-remote will have no effect for execution.

We need to answer the following questions:

• Is it desirable for Starlark rules to by default implicitly pass along a defined set of tags (i.e. the ones supported by Bazel) as an execution requirement to every action?
• Would enabling this by default be a backwards incompatible change?
• What are the semantics when a rule also defines their own execution requirements?

cc: @laurentlb @cparsons @nlopezgi @agoulti @jmmv

[benjaminp]

An interesting special case today is that ctx.actions.run_shell will copy the rule tags over into the execution requirements iff you pass a string command.

[ishikhman]

Design doc link

[ishikhman]

was rolled back

[arlyon]

@ishikhman is this change applied or is this currently still in progress? It's not clear if it has been rolled back or not.

[ishikhman]

@arlyon it will be included into our next release, 0.29, which will happen in the beginning of August.
The change is already in the HEAD though, feel free ti try it out!
Just add a flag --incompatible_allow_tags_propagation to your build.

Note1: flag will be flipped to be true by default in September's release.
Note2: at the moment the change affects only Starlark rules, change for the native rules is coming.
See #8830 for details.