qt TaskThread: implement cancellation of tasks, for cleaner shutdown

fixes https://github.com/spesmilo/electrum/issues/7750

Each task we schedule on `TaskThread` can provide an optional `cancel` method.
When stopping `TaskThread`, we call this `cancel` method on all tasks in the queue.
If the currently running task does not implement `cancel`, `TaskThread.stop` will block
until that task finishes.

Note that there is a significant change in behaviour here:
`ElectrumWindow.run_coroutine_from_thread` and `ElectrumWindow.pay_lightning_invoice`
previously serialised the execution of their coroutines via wallet.thread.
This is no longer the case: they will now schedule coroutines immediately.
So for example, the GUI now allows trying to pay multiple LN invoices "concurrently".

electrum/gui/qt/bip39_recovery_dialog.py

@@ -2,6 +2,9 @@
 # Distributed under the MIT software license, see the accompanying
 # file LICENCE or http://www.opensource.org/licenses/mit-license.php
 
+import asyncio
+import concurrent.futures
+
 from PyQt5.QtCore import Qt
 from PyQt5.QtWidgets import QWidget, QVBoxLayout, QGridLayout, QLabel, QListWidget, QListWidgetItem
 
@@ -29,15 +32,27 @@ class Bip39RecoveryDialog(WindowModalDialog):
         self.content = QVBoxLayout()
         self.content.addWidget(QLabel(_('Scanning common paths for existing accounts...')))
         vbox.addLayout(self.content)
+
+        self.thread = TaskThread(self)
+        self.thread.finished.connect(self.deleteLater) # see #3956
+        network = Network.get_instance()
+        coro = account_discovery(network, self.get_account_xpub)
+        fut = asyncio.run_coroutine_threadsafe(coro, network.asyncio_loop)
+        self.thread.add(
+            fut.result,
+            on_success=self.on_recovery_success,
+            on_error=self.on_recovery_error,
+            cancel=fut.cancel,
+        )
+
         self.ok_button = OkButton(self)
         self.ok_button.clicked.connect(self.on_ok_button_click)
         self.ok_button.setEnabled(False)
-        vbox.addLayout(Buttons(CancelButton(self), self.ok_button))
+        cancel_button = CancelButton(self)
+        cancel_button.clicked.connect(fut.cancel)
+        vbox.addLayout(Buttons(cancel_button, self.ok_button))
         self.finished.connect(self.on_finished)
         self.show()
-        self.thread = TaskThread(self)
-        self.thread.finished.connect(self.deleteLater) # see #3956
-        self.thread.add(self.recovery, self.on_recovery_success, None, self.on_recovery_error)
 
     def on_finished(self):
         self.thread.stop()
@@ -47,11 +62,6 @@ class Bip39RecoveryDialog(WindowModalDialog):
         account = item.data(self.ROLE_ACCOUNT)
         self.on_account_select(account)
 
-    def recovery(self):
-        network = Network.get_instance()
-        coroutine = account_discovery(network, self.get_account_xpub)
-        return network.run_from_another_thread(coroutine)
-
     def on_recovery_success(self, accounts):
         self.clear_content()
         if len(accounts) == 0:
@@ -67,6 +77,9 @@ class Bip39RecoveryDialog(WindowModalDialog):
         self.content.addWidget(self.list)
 
     def on_recovery_error(self, exc_info):
+        e = exc_info[1]
+        if isinstance(e, concurrent.futures.CancelledError):
+            return
         self.clear_content()
         self.content.addWidget(QLabel(_('Error: Account discovery failed.')))
         _logger.error(f"recovery error", exc_info=exc_info)

electrum/gui/qt/main_window.py

@@ -37,6 +37,7 @@ from functools import partial
 import queue
 import asyncio
 from typing import Optional, TYPE_CHECKING, Sequence, List, Union
+import concurrent.futures
 
 from PyQt5.QtGui import QPixmap, QKeySequence, QIcon, QCursor, QFont
 from PyQt5.QtCore import Qt, QRect, QStringListModel, QSize, pyqtSignal, QPoint
@@ -318,16 +319,18 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
             self._update_check_thread.start()
 
     def run_coroutine_from_thread(self, coro, on_result=None):
+        fut = asyncio.run_coroutine_threadsafe(coro, self.network.asyncio_loop)
         def task():
             try:
-                f = asyncio.run_coroutine_threadsafe(coro, self.network.asyncio_loop)
+                r = fut.result()
-                r = f.result()
                 if on_result:
                     on_result(r)
+            except concurrent.futures.CancelledError:
+                self.logger.info(f"wallet.thread coro got cancelled: {coro}")
             except Exception as e:
                 self.logger.exception("exception in coro scheduled via window.wallet")
-                self.show_error_signal.emit(str(e))
+                self.show_error_signal.emit(repr(e))
-        self.wallet.thread.add(task)
+        self.wallet.thread.add(task, cancel=fut.cancel)
 
     def on_fx_history(self):
         self.history_model.refresh('fx_history')
@@ -400,7 +403,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
 
     def on_error(self, exc_info):
         e = exc_info[1]
-        if isinstance(e, UserCancelled):
+        if isinstance(e, (UserCancelled, concurrent.futures.CancelledError)):
             pass
         elif isinstance(e, UserFacingException):
             self.show_error(str(e))
@@ -1592,11 +1595,8 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         if not self.question(msg):
             return
         self.save_pending_invoice()
-        def task():
         coro = self.wallet.lnworker.pay_invoice(invoice, amount_msat=amount_msat, attempts=LN_NUM_PAYMENT_ATTEMPTS)
-            fut = asyncio.run_coroutine_threadsafe(coro, self.network.asyncio_loop)
+        self.run_coroutine_from_thread(coro)
-            return fut.result()
-        self.wallet.thread.add(task)
 
     def on_request_status(self, wallet, key, status):
         if wallet != self.wallet:

electrum/gui/qt/util.py

@@ -29,6 +29,7 @@ from PyQt5.QtWidgets import (QPushButton, QLabel, QMessageBox, QHBoxLayout,
 from electrum.i18n import _, languages
 from electrum.util import FileImportFailed, FileExportFailed, make_aiohttp_session, resource_path
 from electrum.invoices import PR_UNPAID, PR_PAID, PR_EXPIRED, PR_INFLIGHT, PR_UNKNOWN, PR_FAILED, PR_ROUTING, PR_UNCONFIRMED
+from electrum.logging import Logger
 
 if TYPE_CHECKING:
     from .main_window import ElectrumWindow
@@ -902,7 +903,7 @@ class PasswordLineEdit(QLineEdit):
         super().clear()
 
 
-class TaskThread(QThread):
+class TaskThread(QThread, Logger):
     '''Thread that runs background tasks.  Callbacks are guaranteed
     to happen in the context of its parent.'''
 
@@ -911,24 +912,35 @@ class TaskThread(QThread):
         cb_success: Optional[Callable]
         cb_done: Optional[Callable]
         cb_error: Optional[Callable]
+        cancel: Optional[Callable] = None
 
     doneSig = pyqtSignal(object, object, object)
 
     def __init__(self, parent, on_error=None):
-        super(TaskThread, self).__init__(parent)
+        QThread.__init__(self, parent)
+        Logger.__init__(self)
         self.on_error = on_error
         self.tasks = queue.Queue()
+        self._cur_task = None  # type: Optional[TaskThread.Task]
+        self._stopping = False
         self.doneSig.connect(self.on_done)
         self.start()
 
-    def add(self, task, on_success=None, on_done=None, on_error=None):
+    def add(self, task, on_success=None, on_done=None, on_error=None, *, cancel=None):
+        if self._stopping:
+            self.logger.warning(f"stopping or already stopped but tried to add new task.")
+            return
         on_error = on_error or self.on_error
-        self.tasks.put(TaskThread.Task(task, on_success, on_done, on_error))
+        task_ = TaskThread.Task(task, on_success, on_done, on_error, cancel=cancel)
+        self.tasks.put(task_)
 
     def run(self):
         while True:
+            if self._stopping:
+                break
             task = self.tasks.get()  # type: TaskThread.Task
-            if not task:
+            self._cur_task = task
+            if not task or self._stopping:
                 break
             try:
                 result = task.task()
@@ -944,7 +956,21 @@ class TaskThread(QThread):
             cb_result(result)
 
     def stop(self):
-        self.tasks.put(None)
+        self._stopping = True
+        # try to cancel currently running task now.
+        # if the task does not implement "cancel", we will have to wait until it finishes.
+        task = self._cur_task
+        if task and task.cancel:
+            task.cancel()
+        # cancel the remaining tasks in the queue
+        while True:
+            try:
+                task = self.tasks.get_nowait()
+            except queue.Empty:
+                break
+            if task and task.cancel:
+                task.cancel()
+        self.tasks.put(None)  # in case the thread is still waiting on the queue
         self.exit()
         self.wait()
 

electrum/lnworker.py

@@ -1106,7 +1106,9 @@ class LNWallet(LNWorker):
         self.save_payment_info(info)
         self.wallet.set_label(key, lnaddr.get_description())
 
+        self.logger.info(f"pay_invoice starting session for RHASH={payment_hash.hex()}")
         self.set_invoice_status(key, PR_INFLIGHT)
+        success = False
         try:
             await self.pay_to_node(
                 node_pubkey=invoice_pubkey,
@@ -1121,8 +1123,9 @@ class LNWallet(LNWorker):
             success = True
         except PaymentFailure as e:
             self.logger.info(f'payment failure: {e!r}')
-            success = False
             reason = str(e)
+        finally:
+            self.logger.info(f"pay_invoice ending session for RHASH={payment_hash.hex()}. {success=}")
         if success:
             self.set_invoice_status(key, PR_PAID)
             util.trigger_callback('payment_succeeded', self.wallet, key)