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2019-11-11-target-platform-constraints.md

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86 lines (68 loc) · 3.32 KB
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created last updated status reviewers title authors discussion thread
2019-11-11
2019-12-17
Implemented
aragos
cparsons
Exposing Target Platform Constraints
katre

Abstract

Rule authors occasionally need information about the target platform in a rule implementation. This can be achieved currently using either custom toolchains or by using implicit attributes with a select() call. This proposal aims to allow rule authors the information they need in a simpler manner.

Background

Every configured target has a target platform that the outputs are compatible with. This target platform may be set (either by default or by a command-line flag) when the build begins, or may be changed as a result of a configuration transition.

A current workaround to determine the target platform is to check ctx.configuration.host_path_separator, or to add an implicit attribute that selects on a relevant platform constraint. However, both of these are inexact and add additional overhead for rule authors.

Current Status

The target platform information, as a PlatformInfo provider, is currently available during rule implementation (and can be accessed by native rules), via the ToolchainContext. However, the ToolchainContextApi itself only exposes toolchain info providers via a map-like API, which allows code such as:

toolchain = ctx.toolchains['//tools/cpp:toolchain_type']

Proposal: A check method that takes a constraint value label

We can add a method (on SkylarkRuleContextApi) to check whether a given constraint value is present. The value passed would need to be a valid ConstraintValueInfo provider, obtained by an implicit depdency on the constraint value definition that is to be checked. This ensures both that the constraint value is valid, and that the rule author has the correct visibility available to use the constraint value.

def _impl(ctx):
  windows_constraint = ctx.attr._windows_constraint[platform_common.ConstraintValueInfo]
  if ctx.target_platform_has_constraint(windows_constraint):
    platform_separator = '\\'


my_rule = rule(
  implementation = _impl,
  attrs = {
    ...
    '_windows_constraint': attr.label(default = '@bazel_platforms//os:windows'),
  },
)

This version requires an additional implicit attribute from rule authors to use, but the overhead is low and the correctness guarantees are worthwhile.

The implementation can check the type and then call a similar method in ConstraintCollection to perform the actual check.

Backward-compatibility

This is a new feature being added to SkylarkRuleContextApi, so there are no backwards compatibility concerns.