Fix environment_vars not reaching ffmpeg processes (#24635)

* start the forkserver after the config applies environment_vars

The forkserver started as a side effect of creating the multiprocessing Manager at the top of main(), before the config loaded, so every worker process and the ffmpeg it launched inherited the environment from before environment_vars were applied. Variables like LIBVA_DRIVER_NAME set in config.yml never reached ffmpeg. The Manager now uses the fork context, so the forkserver does not start until the first worker process.

* fix test

frigate/__main__.py

@@ -15,7 +15,9 @@ from frigate.util.config import find_config_file
 
 
 def main() -> None:
-    manager = mp.Manager()
+    # Fork this manager so the forkserver does not start until the first
+    # worker process, after the config has applied environment_vars.
+    manager = mp.get_context("fork").Manager()
     faulthandler.enable()
 
     # Setup the logging thread

frigate/test/test_process.py

@@ -0,0 +1,81 @@
+import multiprocessing as mp
+import os
+import sys
+import unittest
+from types import SimpleNamespace
+from unittest.mock import patch
+
+import frigate.__main__ as frigate_main
+import frigate.log
+from frigate.config import FrigateConfig
+from frigate.util.process import FrigateProcess
+
+ENV_KEY = "FRIGATE_TEST_PROCESS_ENV"
+
+
+class EnvReportingProcess(FrigateProcess):
+    def __init__(self, stop_event, result_queue):
+        super().__init__(stop_event, 0, name="env_reporter", daemon=True)
+        self.result_queue = result_queue
+
+    def run(self) -> None:
+        self.pre_run_setup()
+        self.result_queue.put(os.environ.get(ENV_KEY))
+
+
+def _run_main_and_report(result_queue) -> None:
+    """Run frigate's main() with the config load and app faked out.
+
+    Runs in a fresh interpreter so no earlier test has started the forkserver.
+    """
+    mp.set_start_method("forkserver", force=True)
+
+    def load(**kwargs):
+        os.environ[ENV_KEY] = "applied"
+        return SimpleNamespace()
+
+    def setup_logging(manager):
+        frigate.log.log_listener = SimpleNamespace(queue=manager.Queue())
+
+    class FakeApp:
+        def __init__(self, config, manager, stop_event):
+            self.stop_event = stop_event
+
+        def start(self):
+            child_queue = mp.Queue()
+            process = EnvReportingProcess(self.stop_event, child_queue)
+            process.start()
+            result_queue.put(child_queue.get(timeout=30))
+            process.join(timeout=10)
+
+    try:
+        with (
+            patch.object(sys, "argv", ["frigate"]),
+            patch.object(FrigateConfig, "load", load),
+            patch("frigate.__main__.setup_logging", setup_logging),
+            patch("frigate.__main__.FrigateApp", FakeApp),
+        ):
+            frigate_main.main()
+    except Exception as err:
+        result_queue.put(f"error: {err!r}")
+
+
+class TestWorkerEnvironment(unittest.TestCase):
+    def test_env_applied_by_config_load_reaches_worker_processes(self):
+        ctx = mp.get_context("spawn")
+        result_queue = ctx.Queue()
+        runner = ctx.Process(target=_run_main_and_report, args=(result_queue,))
+        runner.start()
+
+        try:
+            self.assertEqual(result_queue.get(timeout=60), "applied")
+        finally:
+            runner.join(timeout=10)
+
+            if runner.is_alive():
+                runner.terminate()
+                runner.join()
+
+
+if __name__ == "__main__":
+    unittest.main()