default_constraint_value should not add a constraint to toolchains #8778
Description
Description of the problem / feature request:
This issue came up when analyzing bazel-contrib/rules_go#2115. Two toolchains are needed to build a target: a Go toolchain, and a Docker toolchain. The target platform is @io_bazel_rules_go//go/toolchain:linux_amd64, which has the following constraints:
@bazel_tools//platforms:linux@bazel_tools//platforms:x86_64@io_bazel_rules_go//go/toolchain:cgo_off(the constraint setting iscgo_constraintin the same package)@io_bazel_rules_go//go/toolchain:not_darwin(the constraint setting isdarwin_constraintin the same package)
cgo_constraint and darwin_constraint have default_constraint_value set to cgo_on and is_darwin, respectively. This is so that when the default target platform is used, these values can be used in select statements. In particular, cgo_on is used to add an additional dependency on a target that requests a C/C++ toolchain.
The problem is that Bazel rejects all registered Docker toolchains for the target platform. These toolchains don't specify any constraints related to cgo_constraint or darwin_constraint, since those settings are irrelevant to Docker. It appears that because of the default_constraint_value attribute in the definition of these settings, the Docker toolchains implicitly require cgo_on and is_darwin. Because of this, Docker toolchains cannot be used:
INFO: ToolchainResolution: Looking for toolchain of type @io_bazel_rules_docker//toolchains/docker:toolchain_type...
INFO: ToolchainResolution: Considering toolchain @docker_config//:toolchain...
INFO: ToolchainResolution: Toolchain constraint @io_bazel_rules_go//go/toolchain:darwin_constraint has value @io_bazel_rules_go//go/toolchain:is_darwin, which does not match value @io_bazel_rules_go//go/toolchain:not_darwin from the target platform @io_bazel_rules_go//go/toolchain:linux_amd64
INFO: ToolchainResolution: Toolchain constraint @io_bazel_rules_go//go/toolchain:cgo_constraint has value @io_bazel_rules_go//go/toolchain:cgo_on, which does not match value @io_bazel_rules_go//go/toolchain:cgo_off from the target platform @io_bazel_rules_go//go/toolchain:linux_amd64
INFO: ToolchainResolution: Rejected toolchain @docker_config//:toolchain, because of target platform mismatch
Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.
This can be reproduced in a standalone example:
WORKSPACE
workspace(name = "toolchain_test")
register_toolchains(
"//:go_toolchain_linux_amd64",
"//:docker_toolchain_linux_amd64",
)
BUILD.bazel
load("//:def.bzl", "go_toolchain", "go_image", "docker_toolchain")
platform(
name = "linux_amd64",
constraint_values = [
"@bazel_tools//platforms:linux",
"@bazel_tools//platforms:x86_64",
":cgo_off",
],
)
constraint_setting(
name = "cgo",
default_constraint_value = ":cgo_on",
)
constraint_value(
name = "cgo_on",
constraint_setting = ":cgo",
)
constraint_value(
name = "cgo_off",
constraint_setting = ":cgo",
)
toolchain_type(
name = "go_toolchain_type",
)
go_toolchain(
name = "go_toolchain_linux_amd64_impl",
)
toolchain(
name = "go_toolchain_linux_amd64",
target_compatible_with = [
"@bazel_tools//platforms:linux",
"@bazel_tools//platforms:x86_64",
":cgo_off",
],
toolchain = ":go_toolchain_linux_amd64_impl",
toolchain_type = ":go_toolchain_type",
)
toolchain_type(
name = "docker_toolchain_type",
)
docker_toolchain(
name = "docker_toolchain_linux_amd64_impl",
)
toolchain(
name = "docker_toolchain_linux_amd64",
target_compatible_with = [
"@bazel_tools//platforms:linux",
"@bazel_tools//platforms:x86_64",
],
toolchain = ":docker_toolchain_linux_amd64_impl",
toolchain_type = ":docker_toolchain_type",
)
go_image(
name = "x",
)
def.bzl
def _go_toolchain_impl(ctx):
return [platform_common.ToolchainInfo()]
go_toolchain = rule(
implementation = _go_toolchain_impl,
)
def _docker_toolchain_impl(ctx):
return [platform_common.ToolchainInfo()]
docker_toolchain = rule(
implementation = _docker_toolchain_impl,
)
def _go_image_impl(ctx):
pass
go_image = rule(
implementation = _go_image_impl,
toolchains = [
"//:go_toolchain_type",
"//:docker_toolchain_type",
],
)
Build with:
bazel build --toolchain_resolution_debug --platforms=//:linux_amd64 //:x
INFO: Invocation ID: 5c4d2935-aadd-4816-aab5-dc3c3ad6f27d
Loading:
Loading: 0 packages loaded
Analyzing: target //:x (1 packages loaded, 0 targets configured)
INFO: ToolchainResolution: Looking for toolchain of type //:docker_toolchain_type...
INFO: ToolchainResolution: Looking for toolchain of type //:go_toolchain_type...
INFO: ToolchainResolution: Considering toolchain //:docker_toolchain_linux_amd64_impl...
INFO: ToolchainResolution: Considering toolchain //:go_toolchain_linux_amd64_impl...
INFO: ToolchainResolution: Toolchain constraint //:cgo has value //:cgo_on, which does not match value //:cgo_off from the target platform //:linux_amd64
INFO: ToolchainResolution: Rejected toolchain //:docker_toolchain_linux_amd64_impl, because of target platform mismatch
INFO: ToolchainResolution: For toolchain type //:go_toolchain_type, possible execution platforms and toolchains: {@bazel_tools//platforms:host_platform -> //:go_toolchain_linux_amd64_impl}
INFO: ToolchainResolution: No toolchains found
ERROR: While resolving toolchains for target //:x: no matching toolchains found for types //:docker_toolchain_type
ERROR: Analysis of target '//:x' failed; build aborted: no matching toolchains found for types //:docker_toolchain_type
INFO: Elapsed time: 1.838s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (8 packages loaded, 45 targets configured)
FAILED: Build did NOT complete successfully (8 packages loaded, 45 targets configured)
What operating system are you running Bazel on?
macOS 10.14.5
What's the output of bazel info release?
release 0.27.1rc1