fix(local-toolchains): don't watch non-existent include directory (#3048)

Apparently, Macs can mis-report their include directory.

Since includes are only needed if C extensions are built, skip watching
the directory
if it doesn't exist.

Work around for
https://github.com/bazel-contrib/rules_python/issues/3043

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Ignas Anikevicius <240938+aignas@users.noreply.github.com>

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d8dda48..da22192 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -79,6 +79,9 @@
 * (runfiles) The pypi runfiles package now includes `py.typed` to indicate it
   supports type checking
   ([#2503](https://github.com/bazel-contrib/rules_python/issues/2503)).
+* (toolchains) `local_runtime_repo` now checks if the include directory exists
+  before attempting to watch it, fixing issues on macOS with system Python
+  ({gh-issue}`3043`).
 
 {#v0-0-0-added}
 ### Added

python/private/local_runtime_repo.bzl

diff --git a/python/private/local_runtime_repo.bzl b/python/private/local_runtime_repo.bzl
index ec0643e..3b4b4c0 100644
--- a/python/private/local_runtime_repo.bzl
+++ b/python/private/local_runtime_repo.bzl
@@ -99,7 +99,15 @@
     interpreter_path = info["base_executable"]
 
     # NOTE: Keep in sync with recursive glob in define_local_runtime_toolchain_impl
-    repo_utils.watch_tree(rctx, rctx.path(info["include"]))
+    include_path = rctx.path(info["include"])
+
+    # The reported include path may not exist, and watching a non-existant
+    # path is an error. Silently skip, since includes are only necessary
+    # if C extensions are built.
+    if include_path.exists and include_path.is_dir:
+        repo_utils.watch_tree(rctx, include_path)
+    else:
+        pass
 
     # The cc_library.includes values have to be non-absolute paths, otherwise
     # the toolchain will give an error. Work around this error by making them