kleaf: Skip declare_host_tools() if using defaults.

If using the default list of host tools, simply
skip the declare_host_tools() call.

This simplifies the Kleaf's MODULE.bazel, or the
root MODULE.bazel when using Kleaf as the root module.

Test: TH
Bug: 328465178
Change-Id: I8225bdabc378e459f70d10a768aea0d15cb4160b

2 files changed, 20 insertions, 13 deletions

diff --git a/kleaf/bzlmod/bazel.MODULE.bazel b/kleaf/bzlmod/bazel.MODULE.bazel
index 7f65f1c..8ea802b 100644
--- a/kleaf/bzlmod/bazel.MODULE.bazel
+++ b/kleaf/bzlmod/bazel.MODULE.bazel
@@ -56,7 +56,6 @@ new_kleaf_local_repository(
 
 kernel_toolchain_ext = use_extension("//build/kernel/kleaf:kernel_toolchain_ext.bzl", "kernel_toolchain_ext")
 kernel_toolchain_ext.declare_toolchain_constants(toolchain_constants = "//common:build.config.constants")
-kernel_toolchain_ext.declare_host_tools()
 use_repo(kernel_toolchain_ext, "kernel_toolchain_info")
 use_repo(kernel_toolchain_ext, "kleaf_clang_toolchain")
 use_repo(kernel_toolchain_ext, "kleaf_host_tools")
diff --git a/kleaf/impl/declare_host_tools.bzl b/kleaf/impl/declare_host_tools.bzl
index 649c593..a4b207f 100644
--- a/kleaf/impl/declare_host_tools.bzl
+++ b/kleaf/impl/declare_host_tools.bzl
@@ -19,6 +19,18 @@ visibility([
     "//",  # for root MODULE.bazel
 ])
 
+# Superset of all tools we need from host.
+# For the subset of host tools we typically use for a kernel build,
+# see //build/kernel:hermetic-tools.
+_DEFAULT_HOST_TOOLS = [
+    "bash",
+    "perl",
+    "rsync",
+    "sh",
+    # For BTRFS (b/292212788)
+    "find",
+]
+
 def _kleaf_host_tools_repo_impl(repository_ctx):
     repository_ctx.file("WORKSPACE", """\
 workspace(name = "{}")
@@ -52,6 +64,9 @@ def _declare_repos(module_ctx, tag_name):
         for declared in getattr(module.tags, tag_name):
             host_tools += declared.host_tools
 
+    if not host_tools:
+        host_tools = _DEFAULT_HOST_TOOLS
+
     kleaf_host_tools_repo(
         name = "kleaf_host_tools",
         host_tools = host_tools,
@@ -61,18 +76,11 @@ _tag_class = tag_class(
     doc = "Declares a list of host tools to be symlinked in the extension `kleaf_host_tools.",
     attrs = {
         "host_tools": attr.string_list(
-            doc = "List of host tools",
-            # Superset of all tools we need from host.
-            # For the subset of host tools we typically use for a kernel build,
-            # see //build/kernel:hermetic-tools.
-            default = [
-                "bash",
-                "perl",
-                "rsync",
-                "sh",
-                # For BTRFS (b/292212788)
-                "find",
-            ],
+            doc = """List of host tools.
+
+                If `declare_host_tools` is not called anywhere, or only called
+                with empty `host_tools`, the default is `{}`.
+            """.format(repr(_DEFAULT_HOST_TOOLS)),
         ),
     },
 )