Add automated test to detect asyncio usage during module load

Author: jcheng5

.github/workflows/pytest.yaml

@@ -75,9 +75,7 @@ jobs:
       - name: Run unit tests
         if: steps.install.outcome == 'success' && (success() || failure())
         run: |
-          pwd
-          cd tests
-          pytest
+          make test
 
       - name: Type check with pyright
         if: steps.install.outcome == 'success' && (success() || failure())

Makefile

@@ -58,6 +58,7 @@ lint: ## check style with flake8
 	flake8 --show-source --max-line-length=127 shiny tests examples
 
 test: ## run tests quickly with the default Python
+	python3 tests/asyncio_prevent.py
 	pytest
 
 test-all: ## run tests on every Python version with tox

tests/asyncio_prevent.py

@@ -0,0 +1,39 @@
+"""This script is NOT designed to be run as part of a pytest suite; it demands a clean
+Python process because it needs to be run before the shiny module is loaded.
+
+The purpose of this script is to ensure that no shiny module initialization code uses
+the event loop. The running event loop during shiny module startup is different than the
+running event loop during the app's operation (I assume uvicorn creates the latter). The
+creation of, say, an asyncio.Lock during the course of shiny module startup will result
+in race conditions if the lock is used during the app's operation."""
+
+if __name__ == "__main__":
+    import sys
+    import asyncio
+    import importlib
+    from typing import Optional
+
+    if "shiny" in sys.modules:
+        raise RuntimeError(
+            "Bad test: shiny was already loaded, it's important that SpyEventLoopPolicy"
+            " is installed before shiny loads"
+        )
+
+    class SpyEventLoopPolicy(asyncio.DefaultEventLoopPolicy):
+        def get_event_loop(self):
+            raise RuntimeError("get_event_loop called")
+
+        def set_event_loop(self, loop: Optional[asyncio.AbstractEventLoop]):
+            raise RuntimeError("set_event_loop called")
+
+        def new_event_loop(self):
+            raise RuntimeError("new_event_loop called")
+
+    asyncio.set_event_loop_policy(SpyEventLoopPolicy())
+
+    # Doing this instead of "import shiny" so no linter is tempted to remove it
+    importlib.import_module("shiny")
+    sys.stderr.write(
+        "Success; shiny module loading did not attempt to access an asyncio event "
+        "loop\n"
+    )