[CI] Add /rerun-test --changed to rerun every test file a PR modifies (#37618)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-authored-by: Alison Shao <a.shao@wustl.edu>

test/registered/unit/tools/test_slash_command_handler.py

@@ -1,4 +1,4 @@
-"""Tests for declarative slash-command test groups."""
+"""Tests for slash-command test selection: declarative groups and `--changed`."""
 
 import importlib.util
 import json
@@ -7,7 +7,7 @@ import sys
 import tempfile
 import unittest
 from pathlib import Path
-from types import ModuleType
+from types import ModuleType, SimpleNamespace
 from unittest.mock import patch
 
 from sglang.test.ci.ci_register import register_cpu_ci
@@ -95,5 +95,105 @@ class TestConfiguredTestGroups(CustomTestCase):
             os.chdir(previous_cwd)
 
 
+class TestChangedTestFiles(CustomTestCase):
+    def test_only_dispatchable_changed_test_files_are_selected(self):
+        """`--changed` feeds the dispatcher directly, so every path it returns must
+        be runnable: no deleted tests, helpers, source, `manual/` files, or
+        multimodal `test_*.py` that collect nothing. A pure move is dropped only
+        when it leaves dispatch alone."""
+        handler = _load_handler()
+        mm = handler.MULTIMODAL_TEST_DIR
+        pr = SimpleNamespace(
+            get_files=lambda: [
+                SimpleNamespace(
+                    filename=name,
+                    status=status,
+                    changes=changes,
+                    previous_filename=previous,
+                )
+                for name, status, changes, previous in [
+                    (
+                        "test/registered/unit/mem_cache/test_radix_cache_unit.py",
+                        "modified",
+                        4,
+                        None,
+                    ),
+                    ("test/registered/core/test_srt_endpoint.py", "removed", 12, None),
+                    ("test/registered/unit/mem_cache/helpers.py", "modified", 2, None),
+                    ("python/sglang/srt/mem_cache/radix_cache.py", "modified", 7, None),
+                    ("test/manual/test_not_registered.py", "added", 20, None),
+                    (
+                        "test/registered/spec/test_moved_untouched.py",
+                        "renamed",
+                        0,
+                        "test/registered/core/test_moved_untouched.py",
+                    ),
+                    (
+                        "test/registered/spec/test_moved_into_ci.py",
+                        "renamed",
+                        0,
+                        "test/manual/test_moved_into_ci.py",
+                    ),
+                    (
+                        f"{mm}/2_gpu/test_moved_pool.py",
+                        "renamed",
+                        0,
+                        f"{mm}/unit/test_moved_pool.py",
+                    ),
+                    (
+                        "test/registered/spec/test_moved_and_edited.py",
+                        "renamed",
+                        9,
+                        "test/registered/core/test_moved_and_edited.py",
+                    ),
+                    (f"{mm}/server/test_server_common.py", "modified", 3, None),
+                    (f"{mm}/server/test_server_utils.py", "modified", 3, None),
+                    (f"{mm}/unit/manual/test_fp4_linear.py", "modified", 3, None),
+                    # Absent from the checkout, as a fork-added file is; kept so
+                    # resolve_test_file() reports `File not found` for it.
+                    (
+                        f"{mm}/server/test_server_added_by_fork.py",
+                        "added",
+                        30,
+                        None,
+                    ),
+                ]
+            ]
+        )
+        with tempfile.TemporaryDirectory() as tmp:
+            root = Path(tmp) / mm
+            (root / "server").mkdir(parents=True)
+            (root / "unit" / "manual").mkdir(parents=True)
+            (root / "2_gpu").mkdir(parents=True)
+            (root / "server" / "test_server_common.py").write_text(
+                "def test_diffusion_generation():\n    pass\n"
+            )
+            (root / "server" / "test_server_utils.py").write_text(
+                "def build_server():\n    return None\n"
+            )
+            (root / "unit" / "manual" / "test_fp4_linear.py").write_text(
+                "class TestFp4Linear:\n    def test_it(self):\n        pass\n"
+            )
+            (root / "2_gpu" / "test_moved_pool.py").write_text(
+                "def test_two_gpu():\n    pass\n"
+            )
+            previous_cwd = os.getcwd()
+            try:
+                os.chdir(tmp)
+                self.assertEqual(
+                    handler.changed_test_files(pr),
+                    [
+                        f"{mm}/2_gpu/test_moved_pool.py",
+                        f"{mm}/server/test_server_added_by_fork.py",
+                        f"{mm}/server/test_server_common.py",
+                        "test/registered/spec/test_moved_and_edited.py",
+                        "test/registered/spec/test_moved_into_ci.py",
+                        "test/registered/unit/mem_cache/test_radix_cache_unit.py",
+                    ],
+                )
+            finally:
+                os.chdir(previous_cwd)
+
+
 if __name__ == "__main__":
     unittest.main()