img_tool: define explicit cross compiled img tool binaries for different exec platforms

Author: malt3

img_tool/MODULE.bazel

@@ -12,6 +12,7 @@ bazel_dep(name = "bazel_lib", version = "3.0.0-beta.1")
 bazel_dep(name = "bazel_skylib", version = "1.7.1")
 bazel_dep(name = "gazelle", version = "0.45.0")
 bazel_dep(name = "googleapis", version = "0.0.0-20251003-2193a2bf")
+bazel_dep(name = "platforms", version = "0.0.11")
 bazel_dep(name = "protobuf", version = "29.5")
 bazel_dep(name = "rules_go", version = "0.57.0")
 bazel_dep(name = "rules_proto", version = "7.1.0")

img_tool/cmd/img/BUILD.bazel

@@ -30,6 +30,27 @@ go_binary(
     visibility = ["//visibility:public"],
 )
 
+platform_tuples = [
+    ("linux", "amd64"),
+    ("linux", "arm64"),
+    ("darwin", "amd64"),
+    ("darwin", "arm64"),
+    ("windows", "amd64"),
+    ("windows", "arm64"),
+]
+
+[
+    go_binary(
+        name = "img_{}_{}".format(os, arch),
+        embed = [":img_lib"],
+        goarch = arch,
+        goos = os,
+        pure = "on",
+        visibility = ["//toolchain:__subpackages__"],
+    )
+    for os, arch in platform_tuples
+]
+
 build_test(
     name = "test",
     targets = [":img"],

img_tool/toolchain/BUILD.bazel

@@ -5,18 +5,29 @@ You need to register them explicitly if you want to build from source.
 """
 
 load("@rules_img//img:image_toolchain.bzl", "TOOLCHAIN_TYPE", "image_toolchain")
+load(":def.bzl", "goarch_to_constraint", "goos_to_constraint", "platform_tuples")
 
 package(
     default_visibility = ["//visibility:public"],
 )
 
-image_toolchain(
-    name = "image_toolchain",
-    tool_exe = "//cmd/img",
-)
+[
+    image_toolchain(
+        name = "image_toolchain_{}_{}".format(os, arch),
+        tool_exe = "//cmd/img:img_{}_{}".format(os, arch),
+    )
+    for os, arch in platform_tuples
+]
 
-toolchain(
-    name = "toolchain",
-    toolchain = ":image_toolchain",
-    toolchain_type = TOOLCHAIN_TYPE,
-)
+[
+    toolchain(
+        name = "toolchain_{}_{}".format(os, arch),
+        exec_compatible_with = [
+            goos_to_constraint(os),
+            goarch_to_constraint(arch),
+        ],
+        toolchain = ":image_toolchain_{}_{}".format(os, arch),
+        toolchain_type = TOOLCHAIN_TYPE,
+    )
+    for os, arch in platform_tuples
+]

img_tool/toolchain/def.bzl

@@ -0,0 +1,49 @@
+"""Toolchain platform definitions and constraint mapping utilities.
+
+This module provides utilities for mapping between Go platform identifiers
+(GOOS/GOARCH) and Bazel platform constraints.
+"""
+
+# List of supported (OS, architecture) platform tuples
+platform_tuples = [
+    ("linux", "amd64"),
+    ("linux", "arm64"),
+    ("darwin", "amd64"),
+    ("darwin", "arm64"),
+    ("windows", "amd64"),
+    ("windows", "arm64"),
+]
+
+def goos_to_constraint(goos):
+    """Converts a Go OS identifier to a Bazel platform constraint.
+
+    Args:
+        goos: A string representing the Go OS identifier (e.g., "linux", "darwin", "windows")
+
+    Returns:
+        A string representing the Bazel platform constraint label for the OS
+    """
+    if goos == "linux":
+        return "@platforms//os:linux"
+    elif goos == "darwin":
+        return "@platforms//os:macos"
+    elif goos == "windows":
+        return "@platforms//os:windows"
+    else:
+        fail("Unknown goos: {}".format(goos))
+
+def goarch_to_constraint(goarch):
+    """Converts a Go architecture identifier to a Bazel platform constraint.
+
+    Args:
+        goarch: A string representing the Go architecture identifier (e.g., "amd64", "arm64")
+
+    Returns:
+        A string representing the Bazel platform constraint label for the CPU architecture
+    """
+    if goarch == "amd64":
+        return "@platforms//cpu:x86_64"
+    elif goarch == "arm64":
+        return "@platforms//cpu:aarch64"
+    else:
+        fail("Unknown goarch: {}".format(goarch))