Replace Janus queue with asyncio.Future

Closes #1752

AI generated patch explanation: https://gisthost.github.io/?e2b8d9c7666e988b5c003ff5e5ef3098

Author: simonw

datasette/database.py

@@ -4,7 +4,6 @@
 import inspect
 import os
 from pathlib import Path
-import janus
 import queue
 import sqlite_utils
 import sys
@@ -330,13 +329,16 @@ def track_event(event):
         else:
             # For non-blocking writes, spawn a background task to
             # dispatch events after the write thread completes
-            task_id, reply_queue = result
+            task_id, reply_future = result
 
             async def _dispatch_events_after_write():
-                write_result = await reply_queue.async_q.get()
-                if not isinstance(write_result, Exception):
-                    for event in pending_events:
-                        await self.ds.track_event(event)
+                try:
+                    await reply_future
+                except Exception:
+                    # if the write failed, don't emit success events
+                    return
+                for event in pending_events:
+                    await self.ds.track_event(event)
 
             asyncio.ensure_future(_dispatch_events_after_write())
             result = task_id
@@ -390,18 +392,15 @@ async def _send_to_write_thread(
             )
             self._write_thread.start()
         task_id = uuid.uuid5(uuid.NAMESPACE_DNS, "datasette.io")
-        reply_queue = janus.Queue()
+        loop = asyncio.get_running_loop()
+        reply_future = loop.create_future()
         self._write_queue.put(
-            WriteTask(fn, task_id, reply_queue, isolated_connection, transaction)
+            WriteTask(fn, task_id, loop, reply_future, isolated_connection, transaction)
         )
         if block:
-            result = await reply_queue.async_q.get()
-            if isinstance(result, Exception):
-                raise result
-            else:
-                return result
+            return await reply_future
         else:
-            return task_id, reply_queue
+            return task_id, reply_future
 
     def _execute_writes(self):
         # Infinite looping thread that protects the single write connection
@@ -422,36 +421,37 @@ def _execute_writes(self):
                     except Exception:
                         pass
                 return
+            exception = None
+            result = None
             if conn_exception is not None:
-                result = conn_exception
-            else:
-                if task.isolated_connection:
-                    isolated_connection = self.connect(write=True)
-                    try:
-                        result = task.fn(isolated_connection)
-                    except Exception as e:
-                        sys.stderr.write("{}\n".format(e))
-                        sys.stderr.flush()
-                        result = e
-                    finally:
-                        isolated_connection.close()
-                        try:
-                            self._all_file_connections.remove(isolated_connection)
-                        except ValueError:
-                            # Was probably a memory connection
-                            pass
-                else:
+                exception = conn_exception
+            elif task.isolated_connection:
+                isolated_connection = self.connect(write=True)
+                try:
+                    result = task.fn(isolated_connection)
+                except Exception as e:
+                    sys.stderr.write("{}\n".format(e))
+                    sys.stderr.flush()
+                    exception = e
+                finally:
+                    isolated_connection.close()
                     try:
-                        if task.transaction:
-                            with conn:
-                                result = task.fn(conn)
-                        else:
+                        self._all_file_connections.remove(isolated_connection)
+                    except ValueError:
+                        # Was probably a memory connection
+                        pass
+            else:
+                try:
+                    if task.transaction:
+                        with conn:
                             result = task.fn(conn)
-                    except Exception as e:
-                        sys.stderr.write("{}\n".format(e))
-                        sys.stderr.flush()
-                        result = e
-            task.reply_queue.sync_q.put(result)
+                    else:
+                        result = task.fn(conn)
+                except Exception as e:
+                    sys.stderr.write("{}\n".format(e))
+                    sys.stderr.flush()
+                    exception = e
+            _deliver_write_result(task, result, exception)
 
     async def execute_fn(self, fn):
         self._check_not_closed()
@@ -892,16 +892,45 @@ def wrapped(conn):
 
 
 class WriteTask:
-    __slots__ = ("fn", "task_id", "reply_queue", "isolated_connection", "transaction")
+    __slots__ = (
+        "fn",
+        "task_id",
+        "loop",
+        "reply_future",
+        "isolated_connection",
+        "transaction",
+    )
 
-    def __init__(self, fn, task_id, reply_queue, isolated_connection, transaction):
+    def __init__(
+        self, fn, task_id, loop, reply_future, isolated_connection, transaction
+    ):
         self.fn = fn
         self.task_id = task_id
-        self.reply_queue = reply_queue
+        self.loop = loop
+        self.reply_future = reply_future
         self.isolated_connection = isolated_connection
         self.transaction = transaction
 
 
+def _deliver_write_result(task, result, exception):
+    # Called from the write thread. Delivers the result back to the
+    # awaiting coroutine on its event loop via call_soon_threadsafe.
+    def _set():
+        if task.reply_future.done():
+            # Awaiter was cancelled; nothing to do.
+            return
+        if exception is not None:
+            task.reply_future.set_exception(exception)
+        else:
+            task.reply_future.set_result(result)
+
+    try:
+        task.loop.call_soon_threadsafe(_set)
+    except RuntimeError:
+        # Event loop has been closed; the awaiter is gone.
+        pass
+
+
 class QueryInterrupted(Exception):
     def __init__(self, e, sql, params):
         self.e = e

docs/changelog.rst

@@ -4,6 +4,14 @@
 Changelog
 =========
 
+.. _unreleased:
+
+Unreleased
+----------
+
+- Dropped Janus as a dependency, previously used to manage the write queue. This should not have any impact on plugin developers or end-users. (:issue:`1752`)
+
+
 .. _v1_0_a29:
 
 1.0a29 (2026-05-12)

pyproject.toml

@@ -32,7 +32,6 @@ dependencies = [
     "pluggy>=1.0",
     "uvicorn>=0.11",
     "aiofiles>=0.4",
-    "janus>=0.6.2",
     "PyYAML>=5.3",
     "mergedeep>=1.1.1",
     "itsdangerous>=1.1",

tests/test_internals_database.py

@@ -2,9 +2,12 @@
 Tests for the datasette.database.Database class
 """
 
+import asyncio
+from types import SimpleNamespace
 from datasette.app import Datasette
 from datasette.database import Database, Results, MultipleValues
 from datasette.database import DatasetteClosedError
+from datasette.database import _deliver_write_result
 from datasette.utils.sqlite import sqlite3, sqlite_version
 from datasette.utils import Column
 import pytest
@@ -590,6 +593,37 @@ def write_fn(conn):
     app_client.ds.remove_database("immutable-db")
 
 
+@pytest.mark.asyncio
+async def test_deliver_write_result_leaves_done_future_alone():
+    loop = asyncio.get_running_loop()
+    reply_future = loop.create_future()
+    reply_future.set_result("original")
+    task = SimpleNamespace(loop=loop, reply_future=reply_future)
+
+    # The write thread can finish after the caller has stopped waiting for the
+    # result. Delivery should notice that the future is already resolved and
+    # leave the caller's outcome alone instead of raising InvalidStateError.
+    _deliver_write_result(task, "replacement", None)
+    await asyncio.sleep(0)
+
+    assert reply_future.result() == "original"
+
+
+@pytest.mark.asyncio
+async def test_deliver_write_result_ignores_closed_loop():
+    closed_loop = asyncio.new_event_loop()
+    closed_loop.close()
+    reply_future = asyncio.get_running_loop().create_future()
+    task = SimpleNamespace(loop=closed_loop, reply_future=reply_future)
+
+    # If the event loop that submitted the write has gone away, the write
+    # thread should drop the result rather than crash while reporting back to
+    # that closed loop.
+    _deliver_write_result(task, "result", None)
+
+    assert not reply_future.done()
+
+
 def table_exists(conn, name):
     return bool(
         conn.execute(

tests/test_write_wrapper.py

@@ -2,6 +2,7 @@
 Tests for the write_wrapper plugin hook.
 """
 
+import asyncio
 from dataclasses import dataclass
 from datasette.app import Datasette
 from datasette.events import Event
@@ -633,8 +634,6 @@ def my_write(conn, track_event):
     assert task_id is not None
 
     # Give the background task time to complete
-    import asyncio
-
     for _ in range(50):
         if ds._tracked_events:
             break
@@ -644,6 +643,30 @@ def my_write(conn, track_event):
     assert ds._tracked_events[0].message == "non-blocking"
 
 
+@pytest.mark.asyncio
+async def test_track_event_with_block_false_discarded_on_exception(
+    ds_with_event_tracking,
+):
+    """Events queued by a non-blocking write are discarded if the write fails."""
+    ds = ds_with_event_tracking
+    db = ds.get_database("test")
+
+    def my_write(conn, track_event):
+        track_event(DummyEvent(actor=None, message="should not fire"))
+        raise ValueError("deliberate error")
+
+    task_id = await db.execute_write_fn(my_write, block=False)
+    assert task_id is not None
+
+    # A following blocking write proves the failed non-blocking task has
+    # completed; one more loop turn lets its event-dispatch task observe the
+    # exception and exit.
+    await db.execute_write_fn(lambda conn: conn.execute("select 1"))
+    await asyncio.sleep(0)
+
+    assert ds._tracked_events == []
+
+
 # --- Tests for RenameTableEvent detection ---