label_flag cannot refer to a platform

[keybuk]

Description of the bug:

When attempting to use label_flag to select a platform via a command-line flag, the following error occurs:

ERROR: BUILD.bazel:14:14: Target :with_secondary_platform was referenced as a platform, but does not provide PlatformInfo

Which category does this issue belong to?

Configurability

What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

To support microcontroller packages with multiple cores, we can pass the platform for the primary/startup core on the command line with --platforms; but can't make the platform for the secondary/co-processor core configurable in the same way:

Would like to be able to build with:

bazel build \
    --platforms=@//:primary_platform \
    --@//:with_secondary_platform=@//:secondary_platform \
    @//:primary_binary

load("@rules_platform//platform_data:defs.bzl", "platform_data")

# Platform for primary core specified with --platforms=@//:primary_platform
platform(
    name = "primary_platform",
    constraint_values = [
        "@platforms//cpu:armv8-m",
    ],
)

cc_binary(
    name = "primary_binary",
    target_compatible_with = [
        "@platforms//cpu:armv8-m",
    ],
    # Primary core binary builds and links secondary core binary via transition.
    deps = ["secondary_firmware"],
)

platform_data(
    name = "secondary_firmware",
    # We want the platform for the secondary code binary to be configurable,
    # e.g. with --@//:with_secondary_platform=@//:secondary_platform
    platform = ":with_secondary_platform",
    target = ":secondary_binary",
)

label_flag(
    name = "with_secondary_platform",
    # Set default to anything.
    build_setting_default = "@platforms//host",
)

platform(
    name = "secondary_platform",
    constraint_values = [
        "@platforms//cpu:armv6-m",
    ],
)

cc_binary(
    name = "secondary_binary",
    target_compatible_with = [
        "@platforms//cpu:armv6-m",
    ],
)

Which operating system are you running Bazel on?

Linux

What is the output of bazel info release?

release 8.1.0

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response

[katre]

Hmm, interesting.

The core problem is technical: when the target :secondary_firnmware is analyzed, the configuration has the flag --platforms=//:with_secondary_platform. However, to resolve the label_flag, it also needs to be analyzed, with a --platforms flag that points to itself, leading to a cycle and a failure.

What you really need is to resolve the label_flag before you set the --platforms flag in the transition. I would suggest writing a custom rule and transition that does exactly that. The code for platform_data is actually pretty simple, can you try giving that a shot?

I'm not sure this is something Bazel can easily do automatically, the core issue is that the ordering of analysis is backwards in your example.

[keybuk]

I can definitely modify platform_data to suit our needs, but I don’t know how to go about resolving the label_flag within the rule implementation. Do you have an example or sample code that I can reference to get started?

[fmeum]

You probably don't even need a label_flag: You can use a string_flag instead, make it an implicit dependency of your rule and then access it in a custom transition to set --platforms to the value of the flag.

[keybuk]

Whether I use a label_flag or string_flag, the transition implementation function always gets the name of the label_flag or string_flag, not the value of the flag

[fmeum]

That doesn't seem right, could you post your code?

[keybuk]

@fmeum sure, here's the BUILD.bazel:

load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
load(":defs.bzl", "platform_target")

string_flag(
    name = "target_platform",
    build_setting_default = "@//platforms:host",
)

platform_target(
    name = "my_target",
    binary = "//anything",  # any cc_binary
)

and defs.bzl:

load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")

def _platform_target_transition_impl(settings, attr):
    print("platform: " + repr(attr._platform))
    return {
        "//command_line_option:platforms": str(attr._platform),
    }

_platform_target_transition = transition(
    implementation = _platform_target_transition_impl,
    inputs = ["//:target_platform"],
    outputs = [
        "//command_line_option:platforms",
    ],
)

def _platform_target_impl(ctx):
    print("Impl")
    print("- binary: " + repr(ctx.executable.binary))
    print("- platform: " + repr(ctx.attr._platform))
    return DefaultInfo(
        files = depset(ctx.files.binary),
    )

platform_target = rule(
    implementation = _platform_target_impl,
    cfg = _platform_target_transition,
    attrs = {
        "binary": attr.label(
            mandatory = True,
            executable = True,
            cfg = "target",
        ),
        "_platform": attr.label(default = Label("//test:target_platform")),
        "_allowlist_function_transition": attr.label(
            default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
        ),
    },
)

Then bazel build --@//:target_platform=//any/platform @//:my_target

DEBUG: defs.bzl:4:10: platform: Label("//test:target_platform")
ERROR: BUILD.bazel:9:16: Target //test:target_platform was referenced as a platform, but does not provide PlatformInfo
ERROR: Analysis of target '//:my_target' failed; build aborted

See how in the transition impl, only the label name is available, not the value. There's no way to get providers here either

[fmeum]

Oh, I see. Does attr._platform not have BuildSettingInfo available? You may have to switch to a transition on a dependency edge rather than a transition on the rule to get access to providers.