Adds tests for self-perpetuating tasks (#31)

Just wanted to demonstrate that a task can be scheduled while it is
currently running.  This just uses `docket.add`, but later we could
think about making self-perpetuating tasks first-class citizens.

Closes #7

Author: chrisguidry

tests/test_fundamentals.py

@@ -7,8 +7,8 @@
 
 import logging
 from datetime import datetime, timedelta
-from typing import Annotated, Callable
 from logging import LoggerAdapter
+from typing import Annotated, Callable
 from unittest.mock import AsyncMock
 from uuid import uuid4
 
@@ -473,3 +473,52 @@ async def the_task(a: str, b: str, logger: LoggerAdapter = TaskLogger()):
     assert called
     assert "Task is running" in caplog.text
     assert "docket.task.the_task" in caplog.text
+
+
+async def test_self_perpetuating_immediate_tasks(
+    docket: Docket, worker: Worker, now: Callable[[], datetime]
+):
+    """docket should support self-perpetuating tasks"""
+
+    calls: dict[str, list[int]] = {
+        "first": [],
+        "second": [],
+    }
+
+    async def the_task(start: int, iteration: int, key: str = TaskKey()):
+        calls[key].append(start + iteration)
+        if iteration < 3:
+            await docket.add(the_task, key=key)(start, iteration + 1)
+
+    await docket.add(the_task, key="first")(10, 1)
+    await docket.add(the_task, key="second")(20, 1)
+
+    await worker.run_until_finished()
+
+    assert calls["first"] == [11, 12, 13]
+    assert calls["second"] == [21, 22, 23]
+
+
+async def test_self_perpetuating_scheduled_tasks(
+    docket: Docket, worker: Worker, now: Callable[[], datetime]
+):
+    """docket should support self-perpetuating tasks"""
+
+    calls: dict[str, list[int]] = {
+        "first": [],
+        "second": [],
+    }
+
+    async def the_task(start: int, iteration: int, key: str = TaskKey()):
+        calls[key].append(start + iteration)
+        if iteration < 3:
+            soon = now() + timedelta(milliseconds=100)
+            await docket.add(the_task, key=key, when=soon)(start, iteration + 1)
+
+    await docket.add(the_task, key="first")(10, 1)
+    await docket.add(the_task, key="second")(20, 1)
+
+    await worker.run_until_finished()
+
+    assert calls["first"] == [11, 12, 13]
+    assert calls["second"] == [21, 22, 23]