fix: break plot<->item/canvas reference cycles to free GDI by refcount

QwtPlotItem and QwtPlotCanvas held a strong back-reference to their parent
QwtPlot, forming Python reference cycles that only the cyclic garbage
collector could reclaim. This delayed the release of native GDI handles and
could exhaust the ~10000 GDI-object limit on Windows under heavy plot
create/destroy churn. Make these back-references weak so a plot is freed by
reference counting alone, like the C++ Qwt parent/child model.

Author: PierreRaybaut

qwt/plot.py

@@ -20,6 +20,7 @@
 """
 
 import math
+import weakref
 
 import numpy as np
 from qtpy.QtCore import QEvent, QObject, QRectF, QSize, Qt, Signal
@@ -1815,16 +1816,23 @@ def attach(self, plot):
 
             :py:meth:`detach()`
         """
-        if plot is self.__data.plot:
+        if plot is self.plot():
             return
 
-        if self.__data.plot:
-            self.__data.plot.attachItem(self, False)
+        current = self.plot()
+        if current is not None:
+            current.attachItem(self, False)
 
-        self.__data.plot = plot
+        # Store the parent plot as a WEAK reference: an item must not keep its
+        # plot (and the plot's GDI-backed child widgets) alive. A strong
+        # back-reference here forms a ``plot <-> item`` reference cycle that can
+        # only be reclaimed by the cyclic garbage collector, which delays the
+        # release of native GDI handles and can lead to GDI-handle exhaustion
+        # on Windows when many plots are created and destroyed.
+        self.__data.plot = None if plot is None else weakref.ref(plot)
 
-        if self.__data.plot:
-            self.__data.plot.attachItem(self, True)
+        if plot is not None:
+            plot.attachItem(self, True)
 
     def detach(self):
         """
@@ -1854,7 +1862,8 @@ def plot(self):
         """
         :return: attached plot
         """
-        return self.__data.plot
+        ref = self.__data.plot
+        return None if ref is None else ref()
 
     def z(self):
         """
@@ -1881,11 +1890,12 @@ def setZ(self, z):
             :py:meth:`z()`, :py:meth:`QwtPlotDict.itemList()`
         """
         if self.__data.z != z:
-            if self.__data.plot:
-                self.__data.plot.attachItem(self, False)
+            plot = self.plot()
+            if plot is not None:
+                plot.attachItem(self, False)
             self.__data.z = z
-            if self.__data.plot:
-                self.__data.plot.attachItem(self, True)
+            if plot is not None:
+                plot.attachItem(self, True)
             self.itemChanged()
 
     def setTitle(self, title):
@@ -2091,8 +2101,9 @@ def itemChanged(self):
 
             :py:meth:`QwtPlot.legendChanged()`, :py:meth:`QwtPlot.autoRefresh()`
         """
-        if self.__data.plot:
-            self.__data.plot.autoRefresh()
+        plot = self.plot()
+        if plot is not None:
+            plot.autoRefresh()
 
     def legendChanged(self):
         """
@@ -2102,8 +2113,9 @@ def legendChanged(self):
 
             :py:meth:`QwtPlot.updateLegend()`, :py:meth:`itemChanged()`
         """
-        if self.testItemAttribute(QwtPlotItem.Legend) and self.__data.plot:
-            self.__data.plot.updateLegend(self)
+        plot = self.plot()
+        if self.testItemAttribute(QwtPlotItem.Legend) and plot is not None:
+            plot.updateLegend(self)
 
     def setAxes(self, xAxis, yAxis):
         """

qwt/plot_canvas.py

@@ -13,6 +13,7 @@
    :members:
 """
 
+import weakref
 from collections.abc import Sequence
 
 from qtpy.QtCore import QEvent, QObject, QPoint, QPointF, QRect, QRectF, QSize, Qt
@@ -442,7 +443,12 @@ class QwtPlotCanvas(QFrame):
 
     def __init__(self, plot=None):
         super(QwtPlotCanvas, self).__init__(plot)
-        self.__plot = plot
+        # Store the parent plot as a WEAK reference to avoid a
+        # ``plot <-> canvas`` reference cycle (the plot already owns the canvas
+        # through Qt's parent/child relationship). A strong back-reference here
+        # would keep the plot alive until the cyclic garbage collector runs,
+        # delaying the release of native GDI handles on Windows.
+        self.__plot = None if plot is None else weakref.ref(plot)
         self.setFrameStyle(QFrame.Panel | QFrame.Sunken)
         self.setLineWidth(2)
         self.__data = QwtPlotCanvas_PrivateData()
@@ -456,7 +462,8 @@ def plot(self):
         """
         :return: Parent plot widget
         """
-        return self.__plot
+        ref = self.__plot
+        return None if ref is None else ref()
 
     def setPaintAttribute(self, attribute, on=True):
         """
@@ -695,7 +702,9 @@ def drawCanvas(self, painter, withBackground):
             else:
                 #                print('**DEBUG: QwtPlotCanvas.drawCanvas')
                 painter.setClipRect(self.contentsRect(), Qt.IntersectClip)
-        self.plot().drawCanvas(painter)
+        plot = self.plot()
+        if plot is not None:
+            plot.drawCanvas(painter)
         painter.restore()
         if withBackground and hackStyledBackground:
             #  Now paint the border on top