From 0bf0ca0b85244ca3c78a78f6eb7ada396f8660aa Mon Sep 17 00:00:00 2001
From: Jonathan Slenders <jonathan@slenders.be>
Date: Mon, 22 Mar 2021 15:48:33 +0100
Subject: [PATCH 1/2] Erase background_tasks list in
 `Application.cancel_and_wait_for_background_tasks`.

---
 prompt_toolkit/application/application.py | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/prompt_toolkit/application/application.py b/prompt_toolkit/application/application.py
index 4385d03c9..1a7edf722 100644
--- a/prompt_toolkit/application/application.py
+++ b/prompt_toolkit/application/application.py
@@ -5,6 +5,7 @@
 import sys
 import threading
 import time
+import warnings
 from asyncio import (
     AbstractEventLoop,
     CancelledError,
@@ -285,6 +286,8 @@ def __init__(
         self.loop: Optional[AbstractEventLoop] = None
         self.context: Optional[contextvars.Context] = None
 
+        self.background_tasks: List[Task[None]] = []
+
         #: Quoted insert. This flag is set if we go into quoted insert mode.
         self.quoted_insert = False
 
@@ -425,11 +428,8 @@ def reset(self) -> None:
         # returning, and when we have multiple buffers, we clearly want the
         # content in the other buffers to remain unchanged between several
         # calls of `run`. (And the same is true for the focus stack.)
-
         self.exit_style = ""
 
-        self.background_tasks: List[Task[None]] = []
-
         self.renderer.reset()
         self.key_processor.reset()
         self.layout.reset()
@@ -937,15 +937,23 @@ async def cancel_and_wait_for_background_tasks(self) -> None:
         (If we had nurseries like Trio, this would be the `__aexit__` of a
         nursery.)
         """
-        for task in self.background_tasks:
+        tasks_to_be_cancelled = self.background_tasks
+        self.background_tasks = []
+
+        for task in tasks_to_be_cancelled:
             task.cancel()
 
-        for task in self.background_tasks:
+        for task in tasks_to_be_cancelled:
             try:
                 await task
             except CancelledError:
                 pass
 
+        if self.background_tasks:
+            warnings.warn(
+                "New background tasks were created while prompt_toolkit application terminates."
+            )
+
     async def _poll_output_size(self) -> None:
         """
         Coroutine for polling the terminal dimensions.

From 0f62d9edb87aa82c800b54b8f531bba3de2f154a Mon Sep 17 00:00:00 2001
From: Jonathan Slenders <jonathan@slenders.be>
Date: Mon, 22 Mar 2021 15:49:34 +0100
Subject: [PATCH 2/2] Don't allow creation of background tasks in
 DummyApplication.

---
 prompt_toolkit/application/dummy.py | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/prompt_toolkit/application/dummy.py b/prompt_toolkit/application/dummy.py
index 1124f7026..40abe4310 100644
--- a/prompt_toolkit/application/dummy.py
+++ b/prompt_toolkit/application/dummy.py
@@ -46,3 +46,8 @@ async def run_system_command(
 
     def suspend_to_background(self, suspend_group: bool = True) -> None:
         raise NotImplementedError
+
+    def create_background_task(
+        self, coroutine: "Awaitable[None]"
+    ) -> "asyncio.Task[None]":
+        raise NotImplementedError