fix: support debian multiarch with local toolchains (#3100)

Apparently, there is a "multiarch" style of Python installations, where
the shared
libraries can be in a sub-directory. The best docs I could find about
this are
https://wiki.debian.org/Python/MultiArch.

To fix, looking at the `MULTIARCH` sysconfig var tells what
subdirectory, if any
should be looked in.

Along the way, fix a changelog issue reference url.

Fixes #3099

Author: rickeylev

CHANGELOG.md

@@ -85,10 +85,12 @@ END_UNRELEASED_TEMPLATE
   ([#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`).
+  ([#3043](https://github.com/bazel-contrib/rules_python/issues/3043)).
 * (pypi) The pipstar `defaults` configuration now supports any custom platform
   name.
 * Multi-line python imports (e.g. with escaped newlines) are now correctly processed by Gazelle.
+* (toolchains) `local_runtime_repo` works with multiarch Debian with Python 3.8
+  ([#3099](https://github.com/bazel-contrib/rules_python/issues/3099)).
 
 {#v0-0-0-added}
 ### Added

python/private/get_local_runtime_info.py

@@ -35,7 +35,15 @@
     # of settings.
     # https://stackoverflow.com/questions/47423246/get-pythons-lib-path
     # For now, it seems LIBDIR has what is needed, so just use that.
+    # See also: MULTIARCH
     "LIBDIR",
+    # On Debian, with multiarch enabled, prior to Python 3.10, `LIBDIR` didn't
+    # tell the location of the libs, just the base directory. The `MULTIARCH`
+    # sysconfig variable tells the subdirectory within it with the libs.
+    # See:
+    # https://wiki.debian.org/Python/MultiArch
+    # https://git.launchpad.net/ubuntu/+source/python3.12/tree/debian/changelog#n842
+    "MULTIARCH",
     # The versioned libpythonX.Y.so.N file. Usually?
     # It might be a static archive (.a) file instead.
     "INSTSONAME",

python/private/local_runtime_repo.bzl

@@ -126,6 +126,7 @@ def _local_runtime_repo_impl(rctx):
     # In some cases, the same value is returned for multiple keys. Not clear why.
     shared_lib_names = {v: None for v in shared_lib_names}.keys()
     shared_lib_dir = info["LIBDIR"]
+    multiarch = info["MULTIARCH"]
 
     # The specific files are symlinked instead of the whole directory
     # because it can point to a directory that has more than just
@@ -135,6 +136,11 @@ def _local_runtime_repo_impl(rctx):
     for name in shared_lib_names:
         origin = rctx.path("{}/{}".format(shared_lib_dir, name))
 
+        # If the origin doesn't exist, try the multiarch location, in case
+        # it's an older Python / Debian release.
+        if not origin.exists and multiarch:
+            origin = rctx.path("{}/{}/{}".format(shared_lib_dir, multiarch, name))
+
         # The reported names don't always exist; it depends on the particulars
         # of the runtime installation.
         if origin.exists: