fix processEvents() reentrancy bug in progress manager window handling (#3030)

* fix progress manager close window race condition

* use monotonic clock for time deltas in progress manager

* restructure progress manager finish logic

qt/aqt/progress.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import time
+from concurrent.futures import Future
 from dataclasses import dataclass
 
 import aqt.forms
@@ -162,7 +163,7 @@ class ProgressManager:
         self._counter = min
         self._min = min
         self._max = max
-        self._firstTime = time.time()
+        self._firstTime = time.monotonic()
         self._show_timer = QTimer(self.mw)
         self._show_timer.setSingleShot(True)
         self._show_timer.start(immediate and 100 or 600)
@@ -205,7 +206,7 @@ class ProgressManager:
         maybeShow: bool = True,
         max: int | None = None,
     ) -> None:
-        # print self._min, self._counter, self._max, label, time.time() - self._lastTime
+        # print self._min, self._counter, self._max, label, time.monotonic() - self._lastTime
         if not self.mw.inMainThread():
             print("progress.update() called on wrong thread")
             return
@@ -223,25 +224,49 @@ class ProgressManager:
             self._win.form.progressBar.setValue(self._counter)
 
     def finish(self) -> None:
-        # we must latch the levels update and perform it after cleanup has occured or we expose ourselves to a race
-        # condition where a second progress could see levels == 0 and wrongly assume everything is in a clean state
-        next_levels = self._levels - 1
-        next_levels = max(0, next_levels)
-        if next_levels == 0:
-            if self._win:
-                self._closeWin()
-            if self._busy_cursor_timer:
-                self._busy_cursor_timer.stop()
-                self._busy_cursor_timer = None
-            self._restore_cursor()
-            if self._show_timer:
-                self._show_timer.stop()
-                self._show_timer = None
-        if self._backend_timer:
-            self._backend_timer.stop()
-            self._backend_timer.deleteLater()
-            self._backend_timer = None
-        self._levels = next_levels
+        def do_window_cleanup(future: Future | None = None):
+            # this method can be called in an async fashion from taskman where a future
+            # is passed or in synchronous manner from the main thread
+            if future is not None:
+                future.result()
+
+            next_levels = self._levels - 1
+            next_levels = max(0, next_levels)
+            if next_levels == 0:
+                if self._win:
+                    self._closeWin()
+                if self._busy_cursor_timer:
+                    self._busy_cursor_timer.stop()
+                    self._busy_cursor_timer = None
+                self._restore_cursor()
+                if self._show_timer:
+                    self._show_timer.stop()
+                    self._show_timer = None
+            if self._backend_timer:
+                self._backend_timer.stop()
+                self._backend_timer.deleteLater()
+                self._backend_timer = None
+            self._levels = next_levels
+
+        # if the window is not currently shown, we can do cleanup immediately, if it is
+        # currently shown then we need to give the window system a half-second to
+        # present the window before we close it again - fixes progress window getting
+        # stuck, especially on ubuntu 16.10+
+        elapsed_time = time.monotonic() - self._shown
+        time_to_wait = 0.5 - elapsed_time
+        # NOTE: we must not yield control if the window is not shown since we don't want
+        # to expose ourselves to the possibility of something showing the window in the
+        # meantime
+        if (not self._shown) or (time_to_wait <= 0):
+            do_window_cleanup()
+        else:
+            # NOTE: we can't use self.single_shot here since it won't call the callback
+            # until _levels = 0, but if we are in this branch then _levels > 0
+            self.mw.taskman.run_in_background(
+                lambda time_to_wait=time_to_wait: time.sleep(time_to_wait),
+                do_window_cleanup,
+                uses_collection=False,
+            )
 
     def clear(self) -> None:
         "Restore the interface after an error."
@@ -254,25 +279,15 @@ class ProgressManager:
             return
         if self._shown:
             return
-        delta = time.time() - self._firstTime
+        delta = time.monotonic() - self._firstTime
         if delta > 0.5:
             self._showWin()
 
     def _showWin(self) -> None:
-        self._shown = time.time()
+        self._shown = time.monotonic()
         self._win.show()
 
     def _closeWin(self) -> None:
-        if self._shown:
-            while True:
-                # give the window system a second to present
-                # window before we close it again - fixes
-                # progress window getting stuck, especially
-                # on ubuntu 16.10+
-                elap = time.time() - self._shown
-                if elap >= 0.5:
-                    break
-                self.app.processEvents(QEventLoop.ProcessEventsFlag.ExcludeUserInputEvents)  # type: ignore #possibly related to https://github.com/python/mypy/issues/6910
         # if the parent window has been deleted, the progress dialog may have
         # already been dropped; delete it if it hasn't been
         if not sip.isdeleted(self._win):