Make transitions optional

Transitions, introduced in bazelbuild#1963 can now be disabled by calling bazel with `--@io_bazel_rules_docker//transitions:enable=no`.
Added tests to verify the behaviour.

README.md

@@ -208,6 +208,12 @@ like:
 Unable to load package for //:WORKSPACE: BUILD file not found in any of the following directories.
 ```
 
+* rules_docker uses transitions to build your containers using toolchains the correct
+architecture and operating system. If you run into issues with toolchain resolutions,
+you can disable this behaviour, by adding this to your .bazelrc:
+```
+build --@io_bazel_rules_docker//transitions:enable=false
+```
 ## Using with Docker locally.
 
 Suppose you have a `container_image` target `//my/image:helloworld`:
@@ -635,7 +641,7 @@ nodejs_image(
     name = "nodejs_image",
     entry_point = "@your_workspace//path/to:file.js",
     # npm deps will be put into their own layer
-    data = [":file.js", "@npm//some-npm-dep"],    
+    data = [":file.js", "@npm//some-npm-dep"],
     ...
 )
 ```
@@ -1150,7 +1156,7 @@ load("@io_bazel_rules_docker//toolchains/docker:toolchain.bzl",
 
 docker_toolchain_configure(
   name = "docker_config",
-  # Replace this with a Bazel label to the config.json file. Note absolute or relative 
+  # Replace this with a Bazel label to the config.json file. Note absolute or relative
   # paths are not supported. Docker allows you to specify custom authentication credentials
   # in the client configuration JSON file.
   # See https://docs.docker.com/engine/reference/commandline/cli/#configuration-files

container/image.bzl

@@ -770,7 +770,15 @@ _outputs["config_digest"] = "%{name}.json.sha256"
 _outputs["build_script"] = "%{name}.executable"
 
 def _image_transition_impl(settings, attr):
-    return dicts.add(settings, {
+    if not settings["@io_bazel_rules_docker//transitions:enable"]:
+        # Once bazel < 5.0 is not supported we can return an empty dict here
+        return {
+            "//command_line_option:platforms": settings["//command_line_option:platforms"],
+            "@io_bazel_rules_docker//platforms:image_transition_cpu": "//plaftorms:image_transition_cpu_unset",
+            "@io_bazel_rules_docker//platforms:image_transition_os": "//plaftorms:image_transition_os_unset",
+        }
+
+    return {
         "//command_line_option:platforms": "@io_bazel_rules_docker//platforms:image_transition",
         "@io_bazel_rules_docker//platforms:image_transition_cpu": "@platforms//cpu:" + {
             # Architecture aliases.
@@ -779,11 +787,14 @@ def _image_transition_impl(settings, attr):
             "ppc64le": "ppc",
         }.get(attr.architecture, attr.architecture),
         "@io_bazel_rules_docker//platforms:image_transition_os": "@platforms//os:" + attr.operating_system,
-    })
+    }
 
 _image_transition = transition(
     implementation = _image_transition_impl,
-    inputs = [],
+    inputs = [
+        "@io_bazel_rules_docker//transitions:enable",
+        "//command_line_option:platforms",
+    ],
     outputs = [
         "//command_line_option:platforms",
         "@io_bazel_rules_docker//platforms:image_transition_cpu",

tests/container/BUILD

@@ -30,6 +30,7 @@ load("//contrib:test.bzl", "container_test")
 load(":apple.bzl", "create_banana_directory")
 load(":empty_layers.bzl", "empty_layers")
 load(":pull_info_validation_test.bzl", "pull_info_validation_test")
+load(":transitions.bzl", "templated_file", "transition_test")
 
 package(default_visibility = ["//visibility:public"])
 
@@ -999,3 +1000,80 @@ container_bundle(
         "localhost:5000/image4:latest": "//testdata:with_double_label",
     },
 )
+
+genrule(
+    name = "got_os",
+    outs = ["got_os.txt"],
+    cmd = select({
+        "@platforms//os:windows": "echo windows > \"$@\"",
+        "@platforms//os:linux": "echo linux > \"$@\"",
+        "@platforms//os:macos": "echo macos > \"$@\"",
+    }),
+)
+
+genrule(
+    name = "got_arch",
+    outs = ["got_arch.txt"],
+    cmd = select({
+        "@platforms//cpu:arm64": "echo arm64 > \"$@\"",
+        "@platforms//cpu:aarch64": "echo arm64 > \"$@\"",
+        "@platforms//cpu:x86_64": "echo x86_64 > \"$@\"",
+        "@platforms//cpu:arm": "echo arm > \"$@\"",
+    }),
+)
+
+container_image(
+    name = "transitioned_image",
+    architecture = "arm64",
+    files = [
+        ":got_arch",
+        ":got_os",
+    ],
+    operating_system = "windows",
+)
+
+templated_file(
+    name = "transitions_off",
+    out = "transitions_off.yaml",
+    substitutions =
+        select({
+            "@platforms//os:linux": ["%WANT_OS%=linux"],
+            "@platforms//os:macos": ["%WANT_OS%=macos"],
+        }) + select({
+            "@platforms//cpu:arm64": ["%WANT_ARCH%=arm64"],
+            "@platforms//cpu:aarch64": ["%WANT_ARCH%=arm64"],
+            "@platforms//cpu:x86_64": ["%WANT_ARCH%=x86_64"],
+        }),
+    template = "//tests/container/configs:transitions_off.yaml.tpl",
+)
+
+container_test(
+    name = "_transitions_on_test_base",
+    configs = ["//tests/container/configs:transitions_on.yaml"],
+    driver = "tar",
+    image = ":transitioned_image",
+    # marked manual, because it will only pass if transitions are enabled
+    # enabled or disabled
+    tags = ["manual"],
+)
+
+transition_test(
+    name = "transitions_on_test",
+    actual = ":_transitions_on_test_base",
+    transitions_enabled = True,
+)
+
+container_test(
+    name = "_transitions_off_test_base",
+    configs = ["transitions_off"],
+    driver = "tar",
+    image = ":transitioned_image",
+    # marked manual, because it will only pass if transitions are disabled
+    tags = ["manual"],
+)
+
+transition_test(
+    name = "transitions_off_test",
+    actual = ":_transitions_off_test_base",
+    transitions_enabled = False,
+)

tests/container/configs/BUILD

@@ -12,4 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-exports_files(glob(["*.yaml"]))
+exports_files(glob(["*.yaml"]) + glob(["*.yaml.tpl"]))

tests/container/configs/transitions_off.yaml.tpl

@@ -0,0 +1,9 @@
+schemaVersion: 2.0.0
+
+fileContentTests:
+  - name: "validate architecture"
+    path: "/Files/got_arch.txt"
+    expectedContents: ["%WANT_ARCH%"]
+  - name: "validate os"
+    path: "/Files/got_os.txt"
+    expectedContents: ["%WANT_OS%"]

tests/container/configs/transitions_on.yaml

@@ -0,0 +1,9 @@
+schemaVersion: 2.0.0
+
+fileContentTests:
+  - name: "validate architecture"
+    path: "/Files/got_arch.txt"
+    expectedContents: ["arm64"]
+  - name: "validate os"
+    path: "/Files/got_os.txt"
+    expectedContents: ["windows"]

tests/container/transitions.bzl

@@ -0,0 +1,77 @@
+"""
+Rules for running tests with a specific value of the //transitions:enabled flag
+"""
+
+def _templated_file_impl(ctx):
+    out = ctx.outputs.out
+    ctx.actions.expand_template(
+        template = ctx.file.template,
+        output = out,
+        substitutions = dict([s.split("=", 1) for s in ctx.attr.substitutions]),
+    )
+    return DefaultInfo(
+        files = depset([out]),
+    )
+
+# Replaces substitutions split on `=` in the template. Substitution is a list
+# so that is usable with multiple selects,
+templated_file = rule(
+    attrs = {
+        "template": attr.label(
+            mandatory = True,
+            allow_single_file = True,
+        ),
+        "substitutions": attr.string_list(),
+        "out": attr.output(),
+    },
+    implementation = _templated_file_impl,
+)
+
+def _enable_transition_impl(settings, attr):
+    _ = settings
+    return {"@io_bazel_rules_docker//transitions:enable": attr.transitions_enabled}
+
+_enable_transition = transition(
+    implementation = _enable_transition_impl,
+    inputs = [],
+    outputs = [
+        "@io_bazel_rules_docker//transitions:enable",
+    ],
+)
+
+def _transitioned_test(ctx):
+    source_info = ctx.attr.actual[DefaultInfo]
+
+    # Bazel wants the executable to be generated by this rule, let's oblige by
+    # just copying the actual runner.
+    executable = None
+    if source_info.files_to_run and source_info.files_to_run.executable:
+        executable = ctx.actions.declare_file("{}_{}".format(ctx.file.actual.basename, "on" if ctx.attr.transitions_enabled else "off"))
+        ctx.actions.run_shell(
+            command = "cp {} {}".format(source_info.files_to_run.executable.path, executable.path),
+            inputs = [source_info.files_to_run.executable],
+            outputs = [executable],
+        )
+    return [DefaultInfo(
+        files = depset(ctx.files.actual),
+        runfiles = source_info.default_runfiles.merge(source_info.data_runfiles),
+        executable = executable,
+    )]
+
+# Defines a test that runs with a specific value of the
+# @io_bazel_rules_docker//transitions:enable flag.
+transition_test = rule(
+    attrs = {
+        "actual": attr.label(
+            mandatory = True,
+            allow_single_file = True,
+        ),
+        "transitions_enabled": attr.bool(),
+        "_allowlist_function_transition": attr.label(
+            default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
+        ),
+    },
+    cfg = _enable_transition,
+    test = True,
+    implementation = _transitioned_test,
+)

transitions/BUILD

@@ -0,0 +1,34 @@
+# Copyright 2022 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
+
+package(
+    default_visibility = ["//visibility:public"],
+)
+
+bool_flag(
+    name = "enable",
+    build_setting_default = True,
+)
+
+config_setting(
+    name = "enabled",
+    flag_values = {"@io_bazel_rules_docker//transitions:enable": "true"},
+)
+
+config_setting(
+    name = "disabled",
+    flag_values = {"@io_bazel_rules_docker//transitions:enable": "false"},
+)