Restore compatibility with existing execute_write_fn() callbacks

Closes #2691

datasette/database.py

@@ -1,6 +1,7 @@
 import asyncio
 import atexit
 from collections import namedtuple
+import inspect
 import os
 from pathlib import Path
 import janus
@@ -263,15 +264,21 @@ class Database:
     def _wrap_fn_with_hooks(self, fn, request, transaction, track_event):
         from .plugins import pm
 
-        # Wrap fn so it receives track_event if its signature supports it
+        # Wrap fn so it receives track_event if its signature supports it.
+        # Historically fn was called positionally, so any single-parameter
+        # name (conn, connection, db, ...) worked. Preserve that by only
+        # switching to keyword dependency injection when the callback
+        # explicitly opts in by declaring a `track_event` parameter.
         original_fn = fn
 
-        def fn_with_track_event(conn):
-            return call_with_supported_arguments(
-                original_fn, conn=conn, track_event=track_event
-            )
+        if "track_event" in inspect.signature(original_fn).parameters:
 
-        fn = fn_with_track_event
+            def fn_with_track_event(conn):
+                return call_with_supported_arguments(
+                    original_fn, conn=conn, track_event=track_event
+                )
+
+            fn = fn_with_track_event
 
         wrappers = pm.hook.write_wrapper(
             datasette=self.ds,

tests/test_internals_database.py

@@ -539,6 +539,37 @@ async def test_execute_write_fn_exception(db):
         await db.execute_write_fn(write_fn)
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("param_name", ["conn", "connection", "db", "c"])
+async def test_execute_write_fn_accepts_any_single_param_name(db, param_name):
+    # Plugins historically relied on the fact that the callback was invoked
+    # positionally, so any parameter name worked. Preserve that contract.
+    scope = {}
+    exec(
+        "def write_fn({0}):\n"
+        "    return {0}.execute('select 1 + 1').fetchone()[0]".format(param_name),
+        scope,
+    )
+    write_fn = scope["write_fn"]
+    result = await db.execute_write_fn(write_fn)
+    assert result == 2
+
+
+@pytest.mark.asyncio
+async def test_execute_write_fn_with_track_event(db):
+    # When the callback declares track_event it still receives both args
+    # via dependency injection.
+    seen = []
+
+    def write_fn(conn, track_event):
+        seen.append(track_event)
+        return conn.execute("select 1 + 1").fetchone()[0]
+
+    result = await db.execute_write_fn(write_fn)
+    assert result == 2
+    assert len(seen) == 1 and callable(seen[0])
+
+
 @pytest.mark.asyncio
 @pytest.mark.timeout(1)
 async def test_execute_write_fn_connection_exception(tmpdir, app_client):