fix: py_proto_library: external runfiles

Previously, the import path within the runfiles was only correct for the
case --legacy_external_runfiles=True (which copied the runfiles into
`$RUNFILES/<main repo>/external/<external repo>/<path>` in addition to
`$RUNFILES/<external repo>/<path>`. This flag was flipped to False in
Bazel 8.0.0.

Fixes #2515.

Added a regression test, and tested locally against the minimal
reproducer in that issue.

CHANGELOG.md

@@ -70,6 +70,7 @@ Unreleased changes template.
 * (pypi) Using {bzl:obj}`pip_parse.experimental_requirement_cycles` and
   {bzl:obj}`pip_parse.use_hub_alias_dependencies` together now works when
   using WORKSPACE files.
+* (py_proto_library) Fix import paths in Bazel 8.
 
 [pep-695]: https://peps.python.org/pep-0695/
 

examples/bzlmod/MODULE.bazel

@@ -272,5 +272,11 @@ local_path_override(
     path = "other_module",
 )
 
+bazel_dep(name = "foo_external", version = "")
+local_path_override(
+    module_name = "foo_external",
+    path = "py_proto_library/foo_external",
+)
+
 # example test dependencies
 bazel_dep(name = "rules_shell", version = "0.3.0", dev_dependency = True)

examples/bzlmod/py_proto_library/BUILD.bazel

@@ -1,3 +1,4 @@
+load("@bazel_skylib//rules:native_binary.bzl", "native_test")
 load("@rules_python//python:py_test.bzl", "py_test")
 
 py_test(
@@ -16,3 +17,12 @@ py_test(
         "//py_proto_library/example.com/another_proto:message_proto_py_pb2",
     ],
 )
+
+# Regression test for https://github.com/bazelbuild/rules_python/issues/2515
+#
+# This test failed before https://github.com/bazelbuild/rules_python/pull/2516
+# when ran with --legacy_external_runfiles=False (default in Bazel 8.0.0).
+native_test(
+    name = "external_import_test",
+    src = "@foo_external//:py_binary_with_proto",
+)

examples/bzlmod/py_proto_library/foo_external/BUILD.bazel

@@ -0,0 +1,22 @@
+
+load("@rules_python//python:proto.bzl", "py_proto_library")
+load("@rules_python//python:py_binary.bzl", "py_binary")
+
+package(default_visibility = ["//visibility:public"])
+
+proto_library(
+    name = "proto_lib",
+    srcs = ["nested/foo/my_proto.proto"],
+    strip_import_prefix = "/nested/foo",
+)
+
+py_proto_library(
+    name = "a_proto",
+    deps = [":proto_lib"],
+)
+
+py_binary(
+    name = "py_binary_with_proto",
+    srcs = ["py_binary_with_proto.py"],
+    deps = [":a_proto"],
+)

examples/bzlmod/py_proto_library/foo_external/MODULE.bazel

@@ -0,0 +1,8 @@
+module(
+    name = "foo_external",
+    version = "0.0.1",
+)
+
+bazel_dep(name = "rules_python", version = "1.0.0")
+bazel_dep(name = "protobuf", version = "28.2", repo_name = "com_google_protobuf")
+bazel_dep(name = "rules_proto", version = "7.0.2")

examples/bzlmod/py_proto_library/foo_external/nested/foo/my_proto.proto

@@ -0,0 +1,6 @@
+syntax = "proto3";
+
+package my_proto;
+
+message MyMessage {
+}

examples/bzlmod/py_proto_library/foo_external/py_binary_with_proto.py

@@ -0,0 +1,5 @@
+import sys
+
+if __name__ == "__main__":
+    import my_proto_pb2
+    sys.exit(0)

python/private/proto/py_proto_library.bzl

@@ -98,7 +98,13 @@ def _py_proto_aspect_impl(target, ctx):
             proto_root = proto_root[len(ctx.bin_dir.path) + 1:]
 
         plugin_output = ctx.bin_dir.path + "/" + proto_root
-        proto_root = ctx.workspace_name + "/" + proto_root
+
+        # Import path within the runfiles tree
+        proto_import_path = ""
+        if proto_root.startswith("external/"):
+            proto_import_path = proto_root[len("external") + 1:]
+        else:
+            proto_import_path = ctx.workspace_name + "/" + proto_root
 
         proto_common.compile(
             actions = ctx.actions,
@@ -130,7 +136,7 @@ def _py_proto_aspect_impl(target, ctx):
                 # _virtual_imports. But it's undesirable otherwise, because it
                 # will put the repo root at the top of the PYTHONPATH, ahead of
                 # directories added through `imports` attributes.
-                [proto_root] if "_virtual_imports" in proto_root else [],
+                [proto_import_path] if "_virtual_imports" in proto_import_path else [],
                 transitive = [dep[PyInfo].imports for dep in api_deps] + [dep.imports for dep in deps],
             ),
             runfiles_from_proto_deps = runfiles_from_proto_deps,