New "startup" plugin hook, closes #834

datasette/app.py

@@ -302,6 +302,13 @@ class Datasette:
         self._permission_checks = collections.deque(maxlen=200)
         self._root_token = secrets.token_hex(32)
 
+    async def invoke_startup(self):
+        for hook in pm.hook.startup(datasette=self):
+            if callable(hook):
+                hook = hook()
+            if asyncio.iscoroutine(hook):
+                hook = await hook
+
     def sign(self, value, namespace="default"):
         return URLSafeSerializer(self._secret, namespace).dumps(value)
 

datasette/cli.py

@@ -397,6 +397,9 @@ def serve(
         # Private utility mechanism for writing unit tests
         return ds
 
+    # Run the "startup" plugin hooks
+    asyncio.get_event_loop().run_until_complete(ds.invoke_startup())
+
     # Run async sanity checks - but only if we're not under pytest
     asyncio.get_event_loop().run_until_complete(check_databases(ds))
 

datasette/hookspecs.py

@@ -5,6 +5,11 @@ hookspec = HookspecMarker("datasette")
 hookimpl = HookimplMarker("datasette")
 
 
+@hookspec
+def startup(datasette):
+    "Fires directly after Datasette first starts running"
+
+
 @hookspec
 def asgi_wrapper(datasette):
     "Returns an ASGI middleware callable to wrap our ASGI application with"