matplotlib · timhoffm · Nov 2, 2020 · Aug 27, 2020 · Sep 24, 2020 · Sep 25, 2020
diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
@@ -44,6 +44,28 @@ def _restore_foreground_window_at_end():
             _c_internal_utils.Win32_SetForegroundWindow(foreground)
 
 
+_blit_args = {}
+# Initialize to a non-empty string that is not a Tcl command
+_blit_tcl_name = "29345091836409813"
+
+
+def _blit(argsid):
+    """
+    Thin wrapper to blit via tkapp.call
+
+    *argsid* is a unique string identifier to fetch the correct arguments from
+    the _blit_args dict, since args cannot be passed directly
+
+    photoimage blanking must occur in the same event and thread as blitting
+    to avoid flickering.
+    """
+    photoimage, dataptr, offsets, bboxptr, blank = _blit_args.pop(argsid)
+    if blank:
+        photoimage.blank()
+    _tkagg.blit(
+        photoimage.tk.interpaddr(), str(photoimage), dataptr, offsets, bboxptr)
+
+
 def blit(photoimage, aggimage, offsets, bbox=None):
     """
     Blit *aggimage* to *photoimage*.
@@ -53,7 +75,10 @@ def blit(photoimage, aggimage, offsets, bbox=None):
     (2, 1, 0, 3) for little-endian ARBG32 (i.e. GBRA8888) data and (1, 2, 3, 0)
     for big-endian ARGB32 (i.e. ARGB8888) data.
 
-    If *bbox* is passed, it defines the region that gets blitted.
+    If *bbox* is passed, it defines the region that gets blitted. That region
+    will NOT be blanked before blitting.
+
+    Tcl events must be dispatched to trigger a blit from a non-Tcl thread.
     """
     data = np.asarray(aggimage)
     height, width = data.shape[:2]
@@ -65,11 +90,32 @@ def blit(photoimage, aggimage, offsets, bbox=None):
         y1 = max(math.floor(y1), 0)
         y2 = min(math.ceil(y2), height)
         bboxptr = (x1, x2, y1, y2)
+        blank = False
     else:
-        photoimage.blank()
         bboxptr = (0, width, 0, height)
-    _tkagg.blit(
-        photoimage.tk.interpaddr(), str(photoimage), dataptr, offsets, bboxptr)
+        blank = True
+
+    # NOTE: _tkagg.blit is thread unsafe and will crash the process if called
+    # from a thread (GH#13293). Instead of blanking and blitting here,
+    # use tkapp.call to post a cross-thread event if this function is called
+    # from a non-Tcl thread.
+
+    # tkapp.call coerces all arguments to strings, so to avoid string parsing
+    # within _blit, pack up the arguments into a global data structure.
+    args = photoimage, dataptr, offsets, bboxptr, blank
+    # Need a unique key to avoid thread races.
+    # Again, make the key a string to avoid string parsing in _blit.
+    argsid = repr(id(args))
+    _blit_args[argsid] = args
+
+    global _blit_tcl_name
+    try:
+        photoimage.tk.call(_blit_tcl_name, argsid)
+    except tk.TclError:
+        # register _blit with code copied from tkinter.Misc._register
+        _blit_tcl_name = repr(id(_blit)) + _blit.__name__
+        photoimage.tk.createcommand(_blit_tcl_name, _blit)
+        photoimage.tk.call(_blit_tcl_name, argsid)
 
 
 class TimerTk(TimerBase):
@@ -402,10 +448,18 @@ def destroy(self, *args):
         if self.canvas._idle_callback:
             self.canvas._tkcanvas.after_cancel(self.canvas._idle_callback)
 
-        self.window.destroy()
+        # NOTE: events need to be flushed before issuing destroy (GH #9956),
+        # however, self.window.update() can break user code. This is the
+        # safest way to achieve a complete draining of the event queue,
+        # but it may require users to update() on their own to execute the
+        # completion in obscure corner cases.
+        def delayed_destroy():
+            self.window.destroy()
+
+            if self._owns_mainloop and not Gcf.get_num_fig_managers():
+                self.window.quit()
 
-        if self._owns_mainloop and not Gcf.get_num_fig_managers():
-            self.window.quit()
+        self.window.after_idle(delayed_destroy)
 
     def get_window_title(self):
         return self.window.wm_title()

diff --git a/lib/matplotlib/backends/backend_tkagg.py b/lib/matplotlib/backends/backend_tkagg.py
@@ -1,17 +1,15 @@
-from . import _backend_tk
 from .backend_agg import FigureCanvasAgg
 from ._backend_tk import (
-    _BackendTk, FigureCanvasTk, FigureManagerTk, NavigationToolbar2Tk)
+    _BackendTk, FigureCanvasTk, FigureManagerTk, NavigationToolbar2Tk, blit)
 
 
 class FigureCanvasTkAgg(FigureCanvasAgg, FigureCanvasTk):
     def draw(self):
         super().draw()
-        _backend_tk.blit(self._tkphoto, self.renderer._renderer, (0, 1, 2, 3))
+        self.blit()
 
     def blit(self, bbox=None):
-        _backend_tk.blit(
-            self._tkphoto, self.renderer._renderer, (0, 1, 2, 3), bbox=bbox)
+        blit(self._tkphoto, self.renderer._renderer, (0, 1, 2, 3), bbox=bbox)
 
 
 @_BackendTk.export

diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
@@ -67,11 +67,11 @@ def _get_testable_interactive_backends():
 # early.  Also, gtk3 redefines key_press_event with a different signature, so
 # we directly invoke it from the superclass instead.
 _test_script = """\
-import importlib
 import importlib.util
 import io
 import json
 import sys
+import threading
 from unittest import TestCase
 
 import matplotlib as mpl
@@ -173,6 +173,90 @@ def test_interactive_backend(backend, toolbar):
     assert proc.stdout.count("CloseEvent") == 1
 
 
+_thread_test_script = """\
+import json
+import sys
+import threading
+
+from matplotlib import pyplot as plt, rcParams
+rcParams.update({
+    "webagg.open_in_browser": False,
+    "webagg.port_retries": 1,
+})
+if len(sys.argv) >= 2:  # Second argument is json-encoded rcParams.
+    rcParams.update(json.loads(sys.argv[1]))
+
+# Test artist creation and drawing does not crash from thread
+# No other guarantees!
+fig, ax = plt.subplots()
+# plt.pause needed vs plt.show(block=False) at least on toolbar2-tkagg
+plt.pause(0.5)
+
+exc_info = None
+
+def thread_artist_work():
+    try:
+        ax.plot([1,3,6])
+    except:
+        # Propagate error to main thread
+        import sys
+        global exc_info
+        exc_info = sys.exc_info()
+
+def thread_draw_work():
+    try:
+        fig.canvas.draw()
+    except:
+        # Propagate error to main thread
+        import sys
+        global exc_info
+        exc_info = sys.exc_info()
+
+t = threading.Thread(target=thread_artist_work)
+t.start()
+# artists never wait for the event loop to run, so just join
+t.join()
+
+if exc_info:  # Raise thread error
+    raise exc_info[1].with_traceback(exc_info[2])
+
+t = threading.Thread(target=thread_draw_work)
+fig.canvas.mpl_connect("close_event", print)
+t.start()
+plt.pause(0.5)  # flush_events fails here on at least Tkagg (bpo-41176)
+t.join()
+plt.close()
+fig.canvas.flush_events()  # pause doesn't process events after close
+
+if exc_info:  # Raise thread error
+    raise exc_info[1].with_traceback(exc_info[2])
+"""
+
+_thread_safe_backends = _get_testable_interactive_backends()
+# Known unsafe backends. Remove the xfails if they start to pass!
+if "wx" in _thread_safe_backends:
+    _thread_safe_backends.remove("wx")
+    _thread_safe_backends.append(
+        pytest.param("wx", marks=pytest.mark.xfail(
+            raises=subprocess.CalledProcessError, strict=True)))
+if "macosx" in _thread_safe_backends:
+    _thread_safe_backends.remove("macosx")
+    _thread_safe_backends.append(
+        pytest.param("macosx", marks=pytest.mark.xfail(
+            raises=subprocess.TimeoutExpired, strict=True)))
+
+
+@pytest.mark.parametrize("backend", _thread_safe_backends)
+@pytest.mark.flaky(reruns=3)
+def test_interactive_thread_safety(backend):
+    proc = subprocess.run(
+        [sys.executable, "-c", _thread_test_script],
+        env={**os.environ, "MPLBACKEND": backend, "SOURCE_DATE_EPOCH": "0"},
+        timeout=_test_timeout, check=True,
+        stdout=subprocess.PIPE, universal_newlines=True)
+    assert proc.stdout.count("CloseEvent") == 1
+
+
 @pytest.mark.skipif('TF_BUILD' in os.environ,
                     reason="this test fails an azure for unknown reasons")
 @pytest.mark.skipif(os.name == "nt", reason="Cannot send SIGINT on Windows.")