Generate empty repo when path is not set

.bazelci/presubmit.yml

@@ -31,11 +31,32 @@ cpu_features: &cpu_features
   build_targets:
     - "//:all"
 
+hello_world_host_cc: &hello_world_host_cc
+  working_directory: tests/cc_binary
+  build_flags:
+    - "--incompatible_disallow_empty_glob"
+  build_targets:
+    - "//:all"
+
+hello_world_ndk_cc: &hello_world_ndk_cc
+  working_directory: tests/cc_binary
+  build_flags:
+    - "--incompatible_disallow_empty_glob"
+    - "--incompatible_enable_android_toolchain_resolution"
+    - "--platforms=//:arm64-v8a"
+  build_targets:
+    - "//:all"
+
 ubuntu2004: &ubuntu2004
   platform: ubuntu2004
   environment:
     ANDROID_NDK_HOME: /opt/android-ndk-r25b
 
+ubuntu2004-no-ndk: &ubuntu2004-no-ndk
+  platform: ubuntu2004
+  environment:
+    ANDROID_NDK_HOME: ""
+
 macos: &macos
   platform: macos
   environment:
@@ -67,10 +88,17 @@ tasks:
     name: Basic Example Bzlmod
     <<: [*windows, *basic_example_bzlmod]
 
-
   cpu_features_ubuntu2004:
     name: CPU Features
     <<: [*ubuntu2004, *cpu_features]
   cpu_features_macos:
     name: CPU Features
     <<: [*macos, *cpu_features]
+
+  hello_world_host_cc_without_ndk:
+    name: Host Hello World without Android NDK
+    <<: [*ubuntu2004-no-ndk, *hello_world_host_cc]
+
+  hello_world_android_cc_without_ndk:
+    name: Host Hello World without Android NDK (should fail)
+    <<: [*ubuntu2004-no-ndk, *hello_world_ndk_cc]

BUILD.empty.tpl

@@ -0,0 +1,58 @@
+"""
+This module adds no-op repository rules when the Android NDK is not installed.
+"""
+
+package(default_visibility = ["//visibility:public"])
+
+load(":dummy_cc_toolchain.bzl", "dummy_cc_config", "dummy_cc_toolchain")
+load("//:target_systems.bzl", "CPU_CONSTRAINT", "TARGET_SYSTEM_NAMES")
+
+# android_ndk_repository was used without a valid Android NDK being set.
+# Either the path attribute of android_ndk_repository or the ANDROID_NDK_HOME
+# environment variable must be set.
+# This is a minimal BUILD file to allow non-Android builds to continue.
+
+# Loop over TARGET_SYSTEM_NAMES and define all empty toolchain targets.
+[toolchain(
+    name = "toolchain_%s" % target_system_name,
+    toolchain = ":dummy_android_ndk_toolchain_cc",
+    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
+    target_compatible_with = [
+        "@platforms//os:android",
+        CPU_CONSTRAINT[target_system_name],
+    ],
+) for target_system_name in TARGET_SYSTEM_NAMES]
+
+cc_toolchain(
+    name = "dummy_android_ndk_toolchain_cc",
+    all_files = ":invalid_android_ndk_repository_error",
+    compiler_files = ":invalid_android_ndk_repository_error",
+    dwp_files = ":invalid_android_ndk_repository_error",
+    linker_files = ":invalid_android_ndk_repository_error",
+    objcopy_files = ":invalid_android_ndk_repository_error",
+    strip_files = ":invalid_android_ndk_repository_error",
+    supports_param_files = 0,
+    toolchain_config = ":cc_toolchain_config",
+    toolchain_identifier = "dummy_wasm32_cc",
+)
+
+dummy_cc_config(
+    name = "cc_toolchain_config",
+)
+
+cc_library(
+    name = "cpufeatures",
+    data = [":error_message"],
+)
+
+genrule(
+    name = "invalid_android_ndk_repository_error",
+    outs = [
+        "error_message",
+    ],
+    cmd = """echo
+    echo rules_android_ndk was used without a valid Android NDK being set: \
+    either the path attribute of android_ndk_repository or the ANDROID_NDK_HOME environment variable must be set.
+    echo
+    exit 1""",
+)

dummy_cc_toolchain.bzl.tpl

@@ -0,0 +1,28 @@
+def _dummy_cc_toolchain_impl(_ctx):
+    # The `all_files` attribute is referenced by rustc_compile_action().
+    return [platform_common.ToolchainInfo(all_files = depset([]))]
+
+dummy_cc_toolchain = rule(
+    implementation = _dummy_cc_toolchain_impl,
+    attrs = {},
+)
+
+# dummy values from https://bazel.build/tutorials/ccp-toolchain-config#configuring_the_c_toolchain
+def _config_impl(ctx):
+    return cc_common.create_cc_toolchain_config_info(
+        ctx = ctx,
+        toolchain_identifier = "dummy-android-ndk-toolchain-cc",
+        host_system_name = "unknown",
+        target_system_name = "unknown",
+        target_cpu = "unknown",
+        target_libc = "unknown",
+        compiler = "unknown",
+        abi_version = "unknown",
+        abi_libc_version = "unknown",
+    )
+
+dummy_cc_config = rule(
+    implementation = _config_impl,
+    attrs = {},
+    provides = [CcToolchainConfigInfo],
+)