Transition container image target platform

Transition to the target platform associated with the `architecture` and
`operating_system` attributes on container image rules.

This change allows for container image rules to build the correct binary for
the target platform, regardless of the host platform. Container image rules
would require the use of the `target_compatible_with` attribute to prevent
mismatching host and target platforms building dependencies incorrectly.
Additionally, hosts which did not match the target platform had to explicitly
specify the target platform with the `--platforms` command-line option.

This change fixes the aforementioned issues and
bazelbuild#690.

Massive thank you to @joneshf for the initial source. It has been adapted to
automatically select the target platform associated with the container image,
as opposed to always using `@io_bazel_rules_go//go/toolchain:linux_amd64`.

lang/image.bzl

@@ -34,8 +34,8 @@ def _binary_name(ctx):
     # /app/foo/bar/baz/blah
     return "/".join([
         ctx.attr.directory,
-        ctx.attr.binary.label.package,
-        ctx.attr.binary.label.name,
+        ctx.attr.binary[0].label.package,
+        ctx.attr.binary[0].label.name,
     ])
 
 def _runfiles_dir(ctx):
@@ -148,7 +148,7 @@ def _app_layer_impl(ctx, runfiles = None, emptyfiles = None):
     parent_parts = _get_layers(ctx, ctx.attr.name, ctx.attr.base)
     filepath = _final_file_path if ctx.attr.binary else layer_file_path
     emptyfilepath = _final_emptyfile_path if ctx.attr.binary else _layer_emptyfile_path
-    dep = ctx.attr.dep or ctx.attr.binary
+    dep = (ctx.attr.dep or ctx.attr.binary)[0]
     top_layer = ctx.attr.binary and not ctx.attr.dep
 
     if ctx.attr.create_empty_workspace_dir:
@@ -239,6 +239,27 @@ def _app_layer_impl(ctx, runfiles = None, emptyfiles = None):
         null_cmd = args == [],
     )
 
+def _image_transition_impl(settings, attr):
+    _ignore = (settings, attr)
+
+    # Architecture aliases.
+    architecture = {
+        "386": "x86_32",
+        "amd64": "x86_64",
+        "ppc64le": "ppc",
+    }.get(attr.architecture, attr.architecture)
+    return {
+        "//command_line_option:platforms": "//platforms:{}_{}".format(attr.operating_system, architecture),
+    }
+
+image_transition = transition(
+    implementation = _image_transition_impl,
+    inputs = [],
+    outputs = [
+        "//command_line_option:platforms",
+    ],
+)
+
 image = struct(
     attrs = dicts.add(_container.image.attrs, {
         # The base image on which to overlay the dependency layers.
@@ -250,7 +271,7 @@ image = struct(
         # the runfiles dir.
         "binary": attr.label(
             executable = True,
-            cfg = "target",
+            cfg = image_transition,
         ),
         # Set this to true to create an empty workspace directory under the
         # app directory specified as the 'directory' attribute.
@@ -263,11 +284,14 @@ image = struct(
         # The dependency whose runfiles we're appending.
         # If not specified, then the layer will be treated as the top layer,
         # and all remaining deps of "binary" will be added under runfiles.
-        "dep": attr.label(),
+        "dep": attr.label(cfg = image_transition),
         "directory": attr.string(default = "/app"),
         "entrypoint": attr.string_list(default = []),
         "legacy_run_behavior": attr.bool(default = False),
         "workdir": attr.string(default = ""),
+        "_allowlist_function_transition": attr.label(
+            default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
+        ),
     }),
     outputs = _container.image.outputs,
     toolchains = ["@io_bazel_rules_docker//toolchains/docker:toolchain_type"],

platforms/BUILD

@@ -56,3 +56,52 @@ platform(
     ],
     parents = ["@buildkite_config//config:platform"],
 )
+
+# All OSs known to Bazel.
+#
+# curl -Lfs https://raw.githubusercontent.com/bazelbuild/platforms/main/os/BUILD | grep -Eo 'name = "\w+"' | grep -Eo '"\w+"' | grep -Ev 'srcs|os|none' | tr -d '"' | sort -u -f | xargs -I '{}' echo '"@platforms//os:{}",'
+_OS_CONSTRAINTS = (
+    "@platforms//os:android",
+    "@platforms//os:freebsd",
+    "@platforms//os:linux",
+    "@platforms//os:netbsd",
+    "@platforms//os:openbsd",
+    "@platforms//os:qnx",
+    "@platforms//os:wasi",
+    "@platforms//os:windows",
+)
+
+# All CPUs known to Bazel.
+#
+# curl -Lfs https://raw.githubusercontent.com/bazelbuild/platforms/main/cpu/BUILD | grep -Eo 'name = "\w+"' | grep -Eo '"\w+"' | grep -Ev 'cpu|srcs' | tr -d '"' | sort -u -f | xargs -I '{}' echo '"@platforms//cpu:{}",'
+_CPU_CONSTRAINTS = (
+    "@platforms//cpu:aarch64",
+    "@platforms//cpu:arm",
+    "@platforms//cpu:arm64",
+    "@platforms//cpu:arm64e",
+    "@platforms//cpu:arm64_32",
+    "@platforms//cpu:armv7",
+    "@platforms//cpu:armv7k",
+    "@platforms//cpu:i386",
+    "@platforms//cpu:mips64",
+    "@platforms//cpu:ppc",
+    "@platforms//cpu:riscv32",
+    "@platforms//cpu:riscv64",
+    "@platforms//cpu:s390x",
+    "@platforms//cpu:wasm32",
+    "@platforms//cpu:wasm64",
+    "@platforms//cpu:x86_32",
+    "@platforms//cpu:x86_64",
+)
+
+# Register all known Bazel platform combinations for use with transitions.
+[platform(
+    name = "{}_{}".format(
+        os.rsplit(":", 1)[1],
+        cpu.rsplit(":", 1)[1],
+    ),
+    constraint_values = [
+        os,
+        cpu,
+    ],
+) for os in _OS_CONSTRAINTS for cpu in _CPU_CONSTRAINTS]