Add cross-platform NDK repository support

BUILD

@@ -1 +1,7 @@
-# Do not remove, this empty BUILD file is necessary for _android_ndk_repository_impl in rules.bzl.
+# Note that this BUILD file is necessary for `android_ndk_repository` in `rules.bzl`.
+
+exports_files([
+    "LICENSE",
+] + glob([
+    "*.tpl",
+]))

BUILD.ndk_clang.tpl

@@ -1,5 +1,6 @@
 """Declarations for the NDK's Clang directory."""
 
+load("@rules_cc//cc/toolchains:cc_toolchain_suite.bzl", "cc_toolchain_suite")
 load("@rules_cc//cc/toolchains:cc_toolchain.bzl", "cc_toolchain")
 load("@@{repository_name}//:ndk_cc_toolchain_config.bzl", "ndk_cc_toolchain_config_rule")
 load("//:target_systems.bzl", "TARGET_SYSTEM_NAMES")
@@ -52,7 +53,7 @@ filegroup(
             "lib64/**/*",
             "lib/**/*",
         ],
-        # Need to allow_empty here because previous NDK versions had 
+        # Need to allow_empty here because previous NDK versions had
         # "lib" & "lib64" directories but recent ones only have "lib".
         allow_empty = True,
     ),

BUILD.ndk_root.tpl

@@ -19,6 +19,7 @@ alias(
         "@platforms//os:android",
         CPU_CONSTRAINT[target_system_name],
     ],
+    exec_compatible_with = {exec_compatible_with},
     toolchain = "//{clang_directory}:cc_toolchain_%s" % target_system_name,
     toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
 ) for target_system_name in TARGET_SYSTEM_NAMES]

extension.bzl

@@ -14,7 +14,7 @@
 
 """A bzlmod extension for loading the NDK."""
 
-load(":rules.bzl", "android_ndk_repository")
+load(":rules.bzl", "DEFAULT_API_LEVEL", "android_ndk_repository")
 
 def _android_ndk_repository_extension_impl(module_ctx):
     root_modules = [m for m in module_ctx.modules if m.is_root and m.tags.configure]
@@ -36,12 +36,19 @@ def _android_ndk_repository_extension_impl(module_ctx):
         **kwargs
     )
 
+_CONFIGURE_TAG_CLASS = tag_class(attrs = {
+    "api_level": attr.int(
+        doc = "The minimum Android API level to target.",
+        default = DEFAULT_API_LEVEL,
+    ),
+    "path": attr.string(
+        doc = "The path to the local Android NDK installation. If not set, ANDROID_NDK_HOME environment variable is used.",
+    ),
+})
+
 android_ndk_repository_extension = module_extension(
     implementation = _android_ndk_repository_extension_impl,
     tag_classes = {
-        "configure": tag_class(attrs = {
-            "path": attr.string(),
-            "api_level": attr.int(),
-        }),
+        "configure": _CONFIGURE_TAG_CLASS,
     },
 )

rules.bzl

@@ -14,30 +14,64 @@
 
 """A repository rule for integrating the Android NDK."""
 
-def _android_ndk_repository_impl(ctx):
+DEFAULT_API_LEVEL = 31
+
+_EXEC_CONSTRAINTS = {
+    "darwin-arm64": [
+        "@platforms//os:macos",
+        "@platforms//cpu:aarch64",
+    ],
+    "darwin-x86_64": [
+        "@platforms//os:macos",
+        "@platforms//cpu:x86_64",
+    ],
+    "linux-x86_64": [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+    "windows-x86_64": [
+        "@platforms//os:windows",
+        "@platforms//cpu:x86_64",
+    ],
+}
+
+def _get_clang_resource_dir(ctx, clang_directory, is_windows):
+    clang_resource_dir = getattr(ctx.attr, "clang_resource_dir", None)
+    if clang_resource_dir:
+        return clang_resource_dir
+
+    result = ctx.execute([clang_directory + "/bin/clang", "--print-resource-dir"])
+    if result.return_code != 0:
+        fail("Failed to execute clang: %s" % result.stderr)
+    stdout = result.stdout.strip()
+    if is_windows:
+        stdout = stdout.replace("\\", "/")
+    return stdout.split(clang_directory)[1].strip("/")
+
+def _android_ndk_repository_common(ctx, ndk_path):
     """Install the Android NDK files.
 
     Args:
         ctx: An implementation context.
+        ndk_path: The path to the ndk
 
     Returns:
         A final dict of configuration attributes and values.
     """
-    ndk_path = ctx.attr.path or ctx.getenv("ANDROID_NDK_HOME", None)
-    if not ndk_path:
-        fail("Either the ANDROID_NDK_HOME environment variable or the " +
-             "path attribute of android_ndk_repository must be set.")
-    if ndk_path.startswith("$WORKSPACE_ROOT"):
-        ndk_path = str(ctx.workspace_root) + ndk_path.removeprefix("$WORKSPACE_ROOT")
-
     is_windows = False
     executable_extension = ""
-    if ctx.os.name == "linux":
+    exec_compatible_with = None
+    platform = ctx.os.name
+    if hasattr(ctx.attr, "platform"):
+        platform = ctx.attr.platform
+        exec_compatible_with = _EXEC_CONSTRAINTS[platform]
+
+    if platform.startswith("linux"):
         clang_directory = "toolchains/llvm/prebuilt/linux-x86_64"
-    elif ctx.os.name == "mac os x":
+    elif platform.startswith(("mac", "darwin")):
         # Note: darwin-x86_64 does indeed contain fat binaries with arm64 slices, too.
         clang_directory = "toolchains/llvm/prebuilt/darwin-x86_64"
-    elif ctx.os.name.startswith("windows"):
+    elif platform.startswith("windows"):
         clang_directory = "toolchains/llvm/prebuilt/windows-x86_64"
         is_windows = True
         executable_extension = ".exe"
@@ -48,15 +82,9 @@ def _android_ndk_repository_impl(ctx):
 
     _create_symlinks(ctx, ndk_path, clang_directory, sysroot_directory)
 
-    api_level = ctx.attr.api_level or 31
+    api_level = ctx.attr.api_level or DEFAULT_API_LEVEL
 
-    result = ctx.execute([clang_directory + "/bin/clang", "--print-resource-dir"])
-    if result.return_code != 0:
-        fail("Failed to execute clang: %s" % result.stderr)
-    stdout = result.stdout.strip()
-    if is_windows:
-        stdout = stdout.replace("\\", "/")
-    clang_resource_directory = stdout.split(clang_directory)[1].strip("/")
+    clang_resource_directory = _get_clang_resource_dir(ctx, clang_directory, is_windows)
 
     # Use a label relative to the workspace from which this repository rule came
     # to get the workspace name.
@@ -67,6 +95,7 @@ def _android_ndk_repository_impl(ctx):
         ctx.attr._template_ndk_root,
         {
             "{clang_directory}": clang_directory,
+            "{exec_compatible_with}": repr(exec_compatible_with),
         },
         executable = False,
     )
@@ -83,11 +112,11 @@ def _android_ndk_repository_impl(ctx):
         "%s/BUILD.bazel" % clang_directory,
         ctx.attr._template_ndk_clang,
         {
-            "{repository_name}": repository_name,
             "{api_level}": str(api_level),
             "{clang_resource_directory}": clang_resource_directory,
-            "{sysroot_directory}": sysroot_directory,
             "{executable_extension}": executable_extension,
+            "{repository_name}": repository_name,
+            "{sysroot_directory}": sysroot_directory,
         },
         executable = False,
     )
@@ -124,16 +153,77 @@ def _create_symlinks(ctx, ndk_path, clang_directory, sysroot_directory):
     # TODO(#32): Remove this hack
     ctx.symlink(ndk_path + "sources", "ndk/sources")
 
+_COMMON_ATTR = {
+    "api_level": attr.int(
+        doc = "The minimum Android API level to target.",
+        default = DEFAULT_API_LEVEL,
+    ),
+    "_build": attr.label(
+        default = Label("//:BUILD"),
+        allow_single_file = True,
+    ),
+    "_template_ndk_clang": attr.label(
+        default = Label("//:BUILD.ndk_clang.tpl"),
+        allow_single_file = True,
+    ),
+    "_template_ndk_root": attr.label(
+        default = Label("//:BUILD.ndk_root.tpl"),
+        allow_single_file = True,
+    ),
+    "_template_ndk_sysroot": attr.label(
+        default = Label(":BUILD.ndk_sysroot.tpl"),
+        allow_single_file = True,
+    ),
+    "_template_target_systems": attr.label(
+        default = Label("//:target_systems.bzl.tpl"),
+        allow_single_file = True,
+    ),
+}
+
+def _exec_configuration_android_ndk_repository_impl(ctx):
+    ndk_path = ctx.path(Label(ctx.attr.anchor)).dirname
+
+    return _android_ndk_repository_common(ctx, ndk_path)
+
+exec_configuration_android_ndk_repository = repository_rule(
+    doc = "A repository rule that integrates the Android NDK from a workspace. Uses an anchor label to locate the NDK and requires the host platform and Clang resource directory to be specified. For local NDK installations, use android_ndk_repository instead.",
+    implementation = _exec_configuration_android_ndk_repository_impl,
+    attrs = _COMMON_ATTR | {
+        "anchor": attr.string(
+            doc = "A label to a file in the NDK directory. The directory containing this file is used as the NDK root path.",
+            mandatory = True,
+        ),
+        "clang_resource_dir": attr.string(
+            doc = "The Clang resource directory path. Pass an empty string to auto-detect by running clang --print-resource-dir.",
+            mandatory = True,
+        ),
+        "platform": attr.string(
+            doc = "The execution platform for the NDK toolchain (e.g., 'linux-x86_64', 'darwin-arm64', 'windows-x86_64'). Determines which prebuilt toolchain directory is used.",
+            values = _EXEC_CONSTRAINTS.keys(),
+            mandatory = True,
+        ),
+    },
+)
+
+def _android_ndk_repository_impl(ctx):
+    ndk_path = ctx.attr.path or ctx.getenv("ANDROID_NDK_HOME", None)
+    if not ndk_path:
+        fail("Either the ANDROID_NDK_HOME environment variable or the " +
+             "path attribute of android_ndk_repository must be set.")
+
+    if ndk_path.startswith("$WORKSPACE_ROOT"):
+        ndk_path = str(ctx.workspace_root) + ndk_path.removeprefix("$WORKSPACE_ROOT")
+
+    return _android_ndk_repository_common(ctx, ndk_path)
+
 android_ndk_repository = repository_rule(
-    attrs = {
-        "path": attr.string(),
-        "api_level": attr.int(),
-        "_build": attr.label(default = ":BUILD", allow_single_file = True),
-        "_template_ndk_root": attr.label(default = ":BUILD.ndk_root.tpl", allow_single_file = True),
-        "_template_target_systems": attr.label(default = ":target_systems.bzl.tpl", allow_single_file = True),
-        "_template_ndk_clang": attr.label(default = ":BUILD.ndk_clang.tpl", allow_single_file = True),
-        "_template_ndk_sysroot": attr.label(default = ":BUILD.ndk_sysroot.tpl", allow_single_file = True),
+    doc = "A repository rule that integrates the Android NDK from a local path. Uses ANDROID_NDK_HOME environment variable or the path attribute. This is the rule used by the bzlmod extension.",
+    implementation = _android_ndk_repository_impl,
+    attrs = _COMMON_ATTR | {
+        "path": attr.string(
+            doc = "The path to the local Android NDK installation. If not set, ANDROID_NDK_HOME environment variable is used. May start with $WORKSPACE_ROOT to reference the workspace root.",
+        ),
     },
+    environ = ["ANDROID_NDK_HOME"],
     local = True,
-    implementation = _android_ndk_repository_impl,
 )