Soft-fail when path is not set

Declare an empty repository with empty toolchains and an error message when the path to the NDK was missing.

BUILD.empty.tpl

@@ -0,0 +1,45 @@
+"""
+This module adds no-op repository rules when the Android NDK is not installed.
+"""
+
+package(default_visibility = ["//visibility:public"])
+
+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.
+
+filegroup(
+    name = "files",
+    srcs = [":error_message"],
+)
+
+filegroup(
+    name = "sdk",
+    srcs = [":error_message"],
+)
+
+# Loop over TARGET_SYSTEM_NAMES and define all empty toolchain targets.
+[toolchain(
+    name = "toolchain_%s" % target_system_name,
+    toolchain = ":error_message",
+    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]
+
+genrule(
+    name = "invalid_android_ndk_repository_error",
+    outs = [
+        "error_message",
+    ],
+    cmd = """echo \
+    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. ; \
+    exit 1 """,
+)

rules.bzl

@@ -23,10 +23,24 @@ def _android_ndk_repository_impl(ctx):
     Returns:
         A final dict of configuration attributes and values.
     """
+
+    ctx.template(
+        "target_systems.bzl",
+        ctx.attr._template_target_systems,
+        {
+        },
+        executable = False,
+    )
+
     ndk_path = ctx.attr.path or ctx.os.environ.get("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.")
+        ctx.template(
+            "BUILD.bazel",
+            ctx.attr._template_empty_repository,
+            {},
+            executable = False,
+        )
+        return
 
     if ctx.os.name == "linux":
         clang_directory = "toolchains/llvm/prebuilt/linux-x86_64"
@@ -60,14 +74,6 @@ def _android_ndk_repository_impl(ctx):
         executable = False,
     )
 
-    ctx.template(
-        "target_systems.bzl",
-        ctx.attr._template_target_systems,
-        {
-        },
-        executable = False,
-    )
-
     ctx.template(
         "%s/BUILD.bazel" % clang_directory,
         ctx.attr._template_ndk_clang,
@@ -121,6 +127,7 @@ android_ndk_repository = repository_rule(
         "_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),
+        "_template_empty_repository": attr.label(default = ":BUILD.empty.tpl", allow_single_file = True),
     },
     local = True,
     implementation = _android_ndk_repository_impl,