bazel: Conflicting actions for `cc_proto_library` with transition in Starlark rule dependency

Description of the problem / feature request:

As of 7acf9ea3fea4850d21114d84fbc5b663d105d3c7, Bazel generates conflicting actions when a custom Starlark rule depends on a cc_proto_library target in the exec configuration if there is a transition applied to the library.

Bugs: what’s the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

$ cat WORKSPACE
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "com_google_protobuf",
    sha256 = "7892a35d979304a404400a101c46ce90e85ec9e2a766a86041bb361f626247f5",
    strip_prefix = "protobuf-3.16.0",
    url = "https://github.com/protocolbuffers/protobuf/archive/v3.16.0.tar.gz",
)

load("@com_google_protobuf//:protobuf_deps.bzl", "protobuf_deps")

protobuf_deps()

$ cat BUILD.bazel
load(":rules.bzl", "exec_with_library_dep", "apply_transition")

proto_library(
    name = "proto",
    srcs = ["empty.proto"],
    deps = [],
)

cc_proto_library(
    name = "cc_proto",
    deps = [":proto"],
)

apply_transition(
    name = "cc_proto_with_transition",
    native_target = ":cc_proto",
)

exec_with_library_dep(
    name = "error",
    out = "dummy.gen",
    library = ":cc_proto_with_transition",
)

$ touch empty.proto

$ cat rules.bzl
_COPT_BUILD_SETTING = "//command_line_option:copt"

def _set_foobar_copt(settings, attr):
    return {_COPT_BUILD_SETTING: ["-DFOOBAR"]}

set_foobar_copt = transition(
    implementation = _set_foobar_copt,
    inputs = [],
    outputs = [_COPT_BUILD_SETTING],
)

def _apply_transition_impl(ctx):
    pass

apply_transition = rule(
        implementation = _apply_transition_impl,
        attrs = {
            "native_target": attr.label(
                cfg = set_foobar_copt,
            ),
            "_allowlist_function_transition": attr.label(
                default = "@bazel_tools//tools/whitelists/function_transition_whitelist",
            ),
        },
    )


def _exec_with_library_dep_impl(ctx):
    ctx.actions.write(ctx.outputs.out, "")

exec_with_library_dep = rule(
    _exec_with_library_dep_impl,
    attrs = {
        "library": attr.label(
            mandatory = True,
            cfg = "exec",
        ),
        "out": attr.output(
            mandatory = True,
        ),
    },
)

$ bazel build //:error

You can also clone https://github.com/fmeum/cc_proto_library_transition_issue.

The command will generate the following error:

ERROR: file 'external/com_google_protobuf/_objs/protobuf_lite/int128.o' is generated by these conflicting actions:
Label: @com_google_protobuf//:protobuf_lite
RuleClass: cc_library rule
Configuration: 8d1a3355bd0201255e646e7ddf342ec528414ebbf2f1feab6e6b20aced50487e, 64b9564990ccaa73f2844f556c40131bd590d44ee9288aea2e3f3beb0c1a956b
Mnemonic: CppCompile
Action key: 4a432e1690a4da20c2fe249ae6554cf201d9792e4375c5a945c9962a8f2ec395, 9dc03c391082ba0e9a3e692ac5cc7257e5abab2a8de3eb5226db9bc4a4ba9f50
Progress message: Compiling src/google/protobuf/stubs/int128.cc
PrimaryInput: File:[/home/fhenneke/.cache/bazel/_bazel_fhenneke/3665225be4dbfed855cbc8b2bd45f83a/external/com_google_protobuf[source]]src/google/protobuf/stubs/int128.cc
PrimaryOutput: File:[[<execution_root>]bazel-out/k8-opt-exec-2B5CBBC6-ST-cbcaac683f1b/bin]external/com_google_protobuf/_objs/protobuf_lite/int128.o
Owner information: ConfiguredTargetKey{label=@com_google_protobuf//:protobuf_lite, config=BuildConfigurationValue.Key[8d1a3355bd0201255e646e7ddf342ec528414ebbf2f1feab6e6b20aced50487e]}, ConfiguredTargetKey{label=@com_google_protobuf//:protobuf_lite, config=BuildConfigurationValue.Key[64b9564990ccaa73f2844f556c40131bd590d44ee9288aea2e3f3beb0c1a956b]}
MandatoryInputs: are equal
Outputs: are equal
ERROR: com.google.devtools.build.lib.skyframe.ArtifactConflictFinder$ConflictException: com.google.devtools.build.lib.actions.MutableActionGraph$ActionConflictException: for external/com_google_protobuf/_objs/protobuf_lite/int128.o, previous action: action 'Compiling src/google/protobuf/stubs/int128.cc', attempted action: action 'Compiling src/google/protobuf/stubs/int128.cc'
INFO: Elapsed time: 0.121s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (0 packages loaded, 0 targets configured)

What operating system are you running Bazel on?

Ubuntu 20.10

What’s the output of bazel info release?

The issue reproduces with release 4.0.0 as well as master @ 7caa01fcb69b1b3d5c67476b100b13e82097227d, but not with release 3.7.2.

Using bazel-bisect.sh, I have identified the breaking commit as 7acf9ea3fea4850d21114d84fbc5b663d105d3c7, which enabled exec transitions on proto rules.