Support async hooks. Fixes #4207.

examples/addons/nonblocking.py

@@ -1,16 +1,26 @@
 """
-Make events hooks non-blocking.
-
-When event hooks are decorated with @concurrent, they will be run in their own thread, freeing the main event loop.
-Please note that this generally opens the door to race conditions and decreases performance if not required.
+Make events hooks non-blocking using async or @concurrent
 """
+import asyncio
 import time
 
 from mitmproxy.script import concurrent
+from mitmproxy import ctx
 
 
-@concurrent  # Remove this and see what happens
-def request(flow):
+# Hooks can be async, which allows the hook to call async functions and perform async I/O
+# without blocking other requests. This is generally preferred for new addons.
+async def request(flow):
+    ctx.log.info(f"handle request: {flow.request.host}{flow.request.path}")
+    await asyncio.sleep(5)
+    ctx.log.info(f"start  request: {flow.request.host}{flow.request.path}")
+
+
+# Another option is to use @concurrent, which launches the hook in its own thread.
+# Please note that this generally opens the door to race conditions and decreases performance if not required.
+# Rename the function below to request(flow) to try it out.
+@concurrent  # Remove this to make it synchronous and see what happens
+def request_concurrent(flow):
     # This is ugly in mitmproxy's UI, but you don't want to use mitmproxy.ctx.log from a different thread.
     print(f"handle request: {flow.request.host}{flow.request.path}")
     time.sleep(5)