From 0dcdd21abacbd04e540220e04b1507244e4ddc1d Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Thu, 27 Aug 2020 12:39:54 -0400
Subject: [PATCH 01/19] WIP: Try to make tkagg blitting threadsafe

current status: doesn't crash but also doesn't draw
when called from a thread
---
 lib/matplotlib/backends/_backend_tk.py   | 32 ++++++++++++
 lib/matplotlib/backends/backend_tkagg.py |  7 +--
 src/_tkagg.cpp                           | 63 ++++++++++++++++++++++++
 3 files changed, 96 insertions(+), 6 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index 69b57fc15d2a..42c92aca36b6 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -123,6 +123,7 @@ def __init__(self, figure, master=None, resize_callback=None):
             width=w, height=h, borderwidth=0, highlightthickness=0)
         self._tkphoto = tk.PhotoImage(
             master=self._tkcanvas, width=w, height=h)
+        self._findphoto_name = self._tkcanvas.register(self._findphoto)
         self._tkcanvas.create_image(w//2, h//2, image=self._tkphoto)
         self._resize_callback = resize_callback
         self._tkcanvas.bind("<Configure>", self.resize)
@@ -178,6 +179,37 @@ def resize(self, event):
             int(width / 2), int(height / 2), image=self._tkphoto)
         self.resize_event()
 
+    def _findphoto(self):
+        return _tkagg.findphoto(self._tkphoto.tk.interpaddr(), str(self._tkphoto))
+
+    def blit(self, bbox=None):
+        """
+        Blit *aggimage* to *photoimage*.
+
+        *offsets* is a tuple describing how to fill the ``offset`` field of the
+        ``Tk_PhotoImageBlock`` struct: it should be (0, 1, 2, 3) for RGBA8888 data,
+        (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.
+        """
+        data = np.asarray(self.renderer._renderer)
+        height, width = data.shape[:2]
+        dataptr = (height, width, data.ctypes.data)
+        if bbox is not None:
+            (x1, y1), (x2, y2) = bbox.__array__()
+            x1 = max(math.floor(x1), 0)
+            x2 = min(math.ceil(x2), width)
+            y1 = max(math.floor(y1), 0)
+            y2 = min(math.ceil(y2), height)
+            bboxptr = (x1, x2, y1, y2)
+        else:
+            self._tkphoto.blank()
+            bboxptr = (0, width, 0, height)
+        # photoptr = self._findphoto()  # Thread unsafe
+        photoptr = self._master.tk.call(self._findphoto_name)  # Thread safe
+        _tkagg.photoputblock(photoptr, dataptr, (0, 1, 2, 3), bboxptr)  # ???
+
     def draw_idle(self):
         # docstring inherited
         if not self._idle:
diff --git a/lib/matplotlib/backends/backend_tkagg.py b/lib/matplotlib/backends/backend_tkagg.py
index 484d5a40edbe..da01a5c4c58e 100644
--- a/lib/matplotlib/backends/backend_tkagg.py
+++ b/lib/matplotlib/backends/backend_tkagg.py
@@ -1,4 +1,3 @@
-from . import _backend_tk
 from .backend_agg import FigureCanvasAgg
 from ._backend_tk import (
     _BackendTk, FigureCanvasTk, FigureManagerTk, NavigationToolbar2Tk)
@@ -7,11 +6,7 @@
 class FigureCanvasTkAgg(FigureCanvasAgg, FigureCanvasTk):
     def draw(self):
         super().draw()
-        _backend_tk.blit(self._tkphoto, self.renderer._renderer, (0, 1, 2, 3))
-
-    def blit(self, bbox=None):
-        _backend_tk.blit(
-            self._tkphoto, self.renderer._renderer, (0, 1, 2, 3), bbox=bbox)
+        self.blit()
 
 
 @_BackendTk.export
diff --git a/src/_tkagg.cpp b/src/_tkagg.cpp
index d74800d5241d..3cfc68bcf38f 100644
--- a/src/_tkagg.cpp
+++ b/src/_tkagg.cpp
@@ -89,8 +89,71 @@ static PyObject *mpl_tk_blit(PyObject *self, PyObject *args)
     }
 }
 
+static PyObject *mpl_tk_findphoto(PyObject *self, PyObject *args)
+{
+    Tcl_Interp *interp;
+    char const *photo_name;
+    Tk_PhotoHandle photo;
+    if (!PyArg_ParseTuple(args, "O&s:findphoto",convert_voidptr, &interp, &photo_name)) {
+        goto exit;
+    }
+    if (!(photo = TK_FIND_PHOTO(interp, photo_name))) {
+        PyErr_SetString(PyExc_ValueError, "Failed to extract Tk_PhotoHandle");
+        goto exit;
+    }
+exit:
+    if (PyErr_Occurred()) {
+        return NULL;
+    } else {
+        return PyLong_FromVoidPtr((void *)photo);
+    }
+}
+
+static PyObject *mpl_tk_photoputblock(PyObject *self, PyObject *args)
+{
+    int height, width;
+    unsigned char *data_ptr;
+    int o0, o1, o2, o3;
+    int x1, x2, y1, y2;
+    Tk_PhotoHandle photo;
+    Tk_PhotoImageBlock block;
+    if (!PyArg_ParseTuple(args, "O&(iiO&)(iiii)(iiii):photoputblock",
+                          convert_voidptr, &photo,
+                          &height, &width, convert_voidptr, &data_ptr,
+                          &o0, &o1, &o2, &o3,
+                          &x1, &x2, &y1, &y2)) {
+        goto exit;
+    }
+    if (0 > y1 || y1 > y2 || y2 > height || 0 > x1 || x1 > x2 || x2 > width) {
+        PyErr_SetString(PyExc_ValueError, "Attempting to draw out of bounds");
+        goto exit;
+    }
+
+    Py_BEGIN_ALLOW_THREADS
+    block.pixelPtr = data_ptr + 4 * ((height - y2) * width + x1);
+    block.width = x2 - x1;
+    block.height = y2 - y1;
+    block.pitch = 4 * width;
+    block.pixelSize = 4;
+    block.offset[0] = o0;
+    block.offset[1] = o1;
+    block.offset[2] = o2;
+    block.offset[3] = o3;
+    TK_PHOTO_PUT_BLOCK_NO_COMPOSITE(
+        photo, &block, x1, height - y2, x2 - x1, y2 - y1);
+    Py_END_ALLOW_THREADS
+exit:
+    if (PyErr_Occurred()) {
+        return NULL;
+    } else {
+        Py_RETURN_NONE;
+    }
+}
+
 static PyMethodDef functions[] = {
     { "blit", (PyCFunction)mpl_tk_blit, METH_VARARGS },
+    { "findphoto", (PyCFunction)mpl_tk_findphoto, METH_VARARGS },
+    { "photoputblock", (PyCFunction)mpl_tk_photoputblock, METH_VARARGS },
     { NULL, NULL } /* sentinel */
 };
 

From 2ad422a78c3794ac5139364c0ea3d6a8d6de6030 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Thu, 24 Sep 2020 16:51:36 -0400
Subject: [PATCH 02/19] make tkagg blitting threadsafe

current status: Prone to races and hard-to-diagnose timeouts

(see https://github.com/python/cpython/pull/21299)
---
 lib/matplotlib/backends/_backend_tk.py | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index 42c92aca36b6..fe5a8c6c28b9 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -124,6 +124,8 @@ def __init__(self, figure, master=None, resize_callback=None):
         self._tkphoto = tk.PhotoImage(
             master=self._tkcanvas, width=w, height=h)
         self._findphoto_name = self._tkcanvas.register(self._findphoto)
+        self._photoputblock_name = self._tkcanvas.register(self._photoputblock)
+        self._tkpointers = None
         self._tkcanvas.create_image(w//2, h//2, image=self._tkphoto)
         self._resize_callback = resize_callback
         self._tkcanvas.bind("<Configure>", self.resize)
@@ -182,6 +184,10 @@ def resize(self, event):
     def _findphoto(self):
         return _tkagg.findphoto(self._tkphoto.tk.interpaddr(), str(self._tkphoto))
 
+    def _photoputblock(self):
+        photoptr, dataptr, bboxptr = self._tkpointers
+        _tkagg.photoputblock(photoptr, dataptr, (0, 1, 2, 3), bboxptr)
+
     def blit(self, bbox=None):
         """
         Blit *aggimage* to *photoimage*.
@@ -206,9 +212,9 @@ def blit(self, bbox=None):
         else:
             self._tkphoto.blank()
             bboxptr = (0, width, 0, height)
-        # photoptr = self._findphoto()  # Thread unsafe
-        photoptr = self._master.tk.call(self._findphoto_name)  # Thread safe
-        _tkagg.photoputblock(photoptr, dataptr, (0, 1, 2, 3), bboxptr)  # ???
+        photoptr = self._master.tk.call(self._findphoto_name)
+        self._tkpointers = photoptr, dataptr, bboxptr
+        self._master.tk.call(self._photoputblock_name)
 
     def draw_idle(self):
         # docstring inherited

From a4a920edf87f3a1399949f198934e1a50f4af6a8 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Fri, 25 Sep 2020 10:13:43 -0400
Subject: [PATCH 03/19] revert to unified blit code and prevent some races

current status: Relies on tkinter arcana, untested
---
 lib/matplotlib/backends/_backend_tk.py     | 71 ++++++++++------------
 lib/matplotlib/backends/backend_tkagg.py   |  6 +-
 lib/matplotlib/backends/backend_tkcairo.py |  2 +-
 src/_tkagg.cpp                             | 63 -------------------
 4 files changed, 37 insertions(+), 105 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index fe5a8c6c28b9..4d3e862b9861 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -44,10 +44,25 @@ def _restore_foreground_window_at_end():
             _c_internal_utils.Win32_SetForegroundWindow(foreground)
 
 
-def blit(photoimage, aggimage, offsets, bbox=None):
+_blit_args = {}
+_blit_tcl_name = None
+
+
+def _blit(argsid):
+    """
+    Thin wrapper to pass arguments to blit via tkapp.call
+    """
+    args = _blit_args.pop(argsid)
+    _tkagg.blit(*args)
+
+
+def blit(tk_instance, photoimage, aggimage, offsets, bbox=None):
     """
     Blit *aggimage* to *photoimage*.
 
+    A *tk_instance* such as a canvas is required execute the blit.
+    Tcl events must be dispatched to trigger a blit from a non-Tcl thread.
+
     *offsets* is a tuple describing how to fill the ``offset`` field of the
     ``Tk_PhotoImageBlock`` struct: it should be (0, 1, 2, 3) for RGBA8888 data,
     (2, 1, 0, 3) for little-endian ARBG32 (i.e. GBRA8888) data and (1, 2, 3, 0)
@@ -68,8 +83,22 @@ def blit(photoimage, aggimage, offsets, bbox=None):
     else:
         photoimage.blank()
         bboxptr = (0, width, 0, height)
-    _tkagg.blit(
+
+    args = (
         photoimage.tk.interpaddr(), str(photoimage), dataptr, offsets, bboxptr)
+    argsid = str(id)
+    _blit_args[argsid] = args
+
+    global _blit_tcl_name
+    # tkapp.call coerces all arguments to strings
+    try:
+        tk_instance.tk.call(_blit_tcl_name, argsid)
+    except tk.TclError:
+        # need to register with the tk root or constantly re-register
+        # each time tk_instance is destroyed
+        root = tk_instance._root()
+        _blit_tcl_name = root.register(_blit)
+        tk_instance.tk.call(_blit_tcl_name, argsid)
 
 
 class TimerTk(TimerBase):
@@ -123,9 +152,6 @@ def __init__(self, figure, master=None, resize_callback=None):
             width=w, height=h, borderwidth=0, highlightthickness=0)
         self._tkphoto = tk.PhotoImage(
             master=self._tkcanvas, width=w, height=h)
-        self._findphoto_name = self._tkcanvas.register(self._findphoto)
-        self._photoputblock_name = self._tkcanvas.register(self._photoputblock)
-        self._tkpointers = None
         self._tkcanvas.create_image(w//2, h//2, image=self._tkphoto)
         self._resize_callback = resize_callback
         self._tkcanvas.bind("<Configure>", self.resize)
@@ -181,41 +207,6 @@ def resize(self, event):
             int(width / 2), int(height / 2), image=self._tkphoto)
         self.resize_event()
 
-    def _findphoto(self):
-        return _tkagg.findphoto(self._tkphoto.tk.interpaddr(), str(self._tkphoto))
-
-    def _photoputblock(self):
-        photoptr, dataptr, bboxptr = self._tkpointers
-        _tkagg.photoputblock(photoptr, dataptr, (0, 1, 2, 3), bboxptr)
-
-    def blit(self, bbox=None):
-        """
-        Blit *aggimage* to *photoimage*.
-
-        *offsets* is a tuple describing how to fill the ``offset`` field of the
-        ``Tk_PhotoImageBlock`` struct: it should be (0, 1, 2, 3) for RGBA8888 data,
-        (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.
-        """
-        data = np.asarray(self.renderer._renderer)
-        height, width = data.shape[:2]
-        dataptr = (height, width, data.ctypes.data)
-        if bbox is not None:
-            (x1, y1), (x2, y2) = bbox.__array__()
-            x1 = max(math.floor(x1), 0)
-            x2 = min(math.ceil(x2), width)
-            y1 = max(math.floor(y1), 0)
-            y2 = min(math.ceil(y2), height)
-            bboxptr = (x1, x2, y1, y2)
-        else:
-            self._tkphoto.blank()
-            bboxptr = (0, width, 0, height)
-        photoptr = self._master.tk.call(self._findphoto_name)
-        self._tkpointers = photoptr, dataptr, bboxptr
-        self._master.tk.call(self._photoputblock_name)
-
     def draw_idle(self):
         # docstring inherited
         if not self._idle:
diff --git a/lib/matplotlib/backends/backend_tkagg.py b/lib/matplotlib/backends/backend_tkagg.py
index da01a5c4c58e..c652262ebd88 100644
--- a/lib/matplotlib/backends/backend_tkagg.py
+++ b/lib/matplotlib/backends/backend_tkagg.py
@@ -1,6 +1,6 @@
 from .backend_agg import FigureCanvasAgg
 from ._backend_tk import (
-    _BackendTk, FigureCanvasTk, FigureManagerTk, NavigationToolbar2Tk)
+    _BackendTk, FigureCanvasTk, FigureManagerTk, NavigationToolbar2Tk, blit)
 
 
 class FigureCanvasTkAgg(FigureCanvasAgg, FigureCanvasTk):
@@ -8,6 +8,10 @@ def draw(self):
         super().draw()
         self.blit()
 
+    def blit(self, bbox=None):
+        blit(self._tkcanvas, self._tkphoto,
+             self.renderer._renderer, (0, 1, 2, 3), bbox=bbox)
+
 
 @_BackendTk.export
 class _BackendTkAgg(_BackendTk):
diff --git a/lib/matplotlib/backends/backend_tkcairo.py b/lib/matplotlib/backends/backend_tkcairo.py
index a81fd0d92bb8..e7c8dd02480c 100644
--- a/lib/matplotlib/backends/backend_tkcairo.py
+++ b/lib/matplotlib/backends/backend_tkcairo.py
@@ -21,7 +21,7 @@ def draw(self):
         self.figure.draw(self._renderer)
         buf = np.reshape(surface.get_data(), (height, width, 4))
         _backend_tk.blit(
-            self._tkphoto, buf,
+            self._tkcanvas, self._tkphoto, buf,
             (2, 1, 0, 3) if sys.byteorder == "little" else (1, 2, 3, 0))
 
 
diff --git a/src/_tkagg.cpp b/src/_tkagg.cpp
index 3cfc68bcf38f..d74800d5241d 100644
--- a/src/_tkagg.cpp
+++ b/src/_tkagg.cpp
@@ -89,71 +89,8 @@ static PyObject *mpl_tk_blit(PyObject *self, PyObject *args)
     }
 }
 
-static PyObject *mpl_tk_findphoto(PyObject *self, PyObject *args)
-{
-    Tcl_Interp *interp;
-    char const *photo_name;
-    Tk_PhotoHandle photo;
-    if (!PyArg_ParseTuple(args, "O&s:findphoto",convert_voidptr, &interp, &photo_name)) {
-        goto exit;
-    }
-    if (!(photo = TK_FIND_PHOTO(interp, photo_name))) {
-        PyErr_SetString(PyExc_ValueError, "Failed to extract Tk_PhotoHandle");
-        goto exit;
-    }
-exit:
-    if (PyErr_Occurred()) {
-        return NULL;
-    } else {
-        return PyLong_FromVoidPtr((void *)photo);
-    }
-}
-
-static PyObject *mpl_tk_photoputblock(PyObject *self, PyObject *args)
-{
-    int height, width;
-    unsigned char *data_ptr;
-    int o0, o1, o2, o3;
-    int x1, x2, y1, y2;
-    Tk_PhotoHandle photo;
-    Tk_PhotoImageBlock block;
-    if (!PyArg_ParseTuple(args, "O&(iiO&)(iiii)(iiii):photoputblock",
-                          convert_voidptr, &photo,
-                          &height, &width, convert_voidptr, &data_ptr,
-                          &o0, &o1, &o2, &o3,
-                          &x1, &x2, &y1, &y2)) {
-        goto exit;
-    }
-    if (0 > y1 || y1 > y2 || y2 > height || 0 > x1 || x1 > x2 || x2 > width) {
-        PyErr_SetString(PyExc_ValueError, "Attempting to draw out of bounds");
-        goto exit;
-    }
-
-    Py_BEGIN_ALLOW_THREADS
-    block.pixelPtr = data_ptr + 4 * ((height - y2) * width + x1);
-    block.width = x2 - x1;
-    block.height = y2 - y1;
-    block.pitch = 4 * width;
-    block.pixelSize = 4;
-    block.offset[0] = o0;
-    block.offset[1] = o1;
-    block.offset[2] = o2;
-    block.offset[3] = o3;
-    TK_PHOTO_PUT_BLOCK_NO_COMPOSITE(
-        photo, &block, x1, height - y2, x2 - x1, y2 - y1);
-    Py_END_ALLOW_THREADS
-exit:
-    if (PyErr_Occurred()) {
-        return NULL;
-    } else {
-        Py_RETURN_NONE;
-    }
-}
-
 static PyMethodDef functions[] = {
     { "blit", (PyCFunction)mpl_tk_blit, METH_VARARGS },
-    { "findphoto", (PyCFunction)mpl_tk_findphoto, METH_VARARGS },
-    { "photoputblock", (PyCFunction)mpl_tk_photoputblock, METH_VARARGS },
     { NULL, NULL } /* sentinel */
 };
 

From 321b7a0990297a8785e6f1e4cafc167a9e5a0746 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Fri, 25 Sep 2020 10:39:01 -0400
Subject: [PATCH 04/19] register _blit with Tcl without an extra widget

current status: Relies on tkinter arcana, untested
---
 lib/matplotlib/backends/_backend_tk.py     | 16 ++++++++--------
 lib/matplotlib/backends/backend_tkagg.py   |  3 +--
 lib/matplotlib/backends/backend_tkcairo.py |  2 +-
 3 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index 4d3e862b9861..fc36904a37c8 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -45,7 +45,8 @@ def _restore_foreground_window_at_end():
 
 
 _blit_args = {}
-_blit_tcl_name = None
+# Initialized to non-empty string that is not a current Tcl command
+_blit_tcl_name = "29345091836409813"
 
 
 def _blit(argsid):
@@ -56,7 +57,7 @@ def _blit(argsid):
     _tkagg.blit(*args)
 
 
-def blit(tk_instance, photoimage, aggimage, offsets, bbox=None):
+def blit(photoimage, aggimage, offsets, bbox=None):
     """
     Blit *aggimage* to *photoimage*.
 
@@ -92,13 +93,12 @@ def blit(tk_instance, photoimage, aggimage, offsets, bbox=None):
     global _blit_tcl_name
     # tkapp.call coerces all arguments to strings
     try:
-        tk_instance.tk.call(_blit_tcl_name, argsid)
+        photoimage.tk.call(_blit_tcl_name, argsid)
     except tk.TclError:
-        # need to register with the tk root or constantly re-register
-        # each time tk_instance is destroyed
-        root = tk_instance._root()
-        _blit_tcl_name = root.register(_blit)
-        tk_instance.tk.call(_blit_tcl_name, argsid)
+        # register _blit by copying code 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):
diff --git a/lib/matplotlib/backends/backend_tkagg.py b/lib/matplotlib/backends/backend_tkagg.py
index c652262ebd88..8768a729c6d0 100644
--- a/lib/matplotlib/backends/backend_tkagg.py
+++ b/lib/matplotlib/backends/backend_tkagg.py
@@ -9,8 +9,7 @@ def draw(self):
         self.blit()
 
     def blit(self, bbox=None):
-        blit(self._tkcanvas, 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/backends/backend_tkcairo.py b/lib/matplotlib/backends/backend_tkcairo.py
index e7c8dd02480c..a81fd0d92bb8 100644
--- a/lib/matplotlib/backends/backend_tkcairo.py
+++ b/lib/matplotlib/backends/backend_tkcairo.py
@@ -21,7 +21,7 @@ def draw(self):
         self.figure.draw(self._renderer)
         buf = np.reshape(surface.get_data(), (height, width, 4))
         _backend_tk.blit(
-            self._tkcanvas, self._tkphoto, buf,
+            self._tkphoto, buf,
             (2, 1, 0, 3) if sys.byteorder == "little" else (1, 2, 3, 0))
 
 

From 24d25456ccb37bf2be22f63e6d0ac65eeeea48ed Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Fri, 25 Sep 2020 10:40:34 -0400
Subject: [PATCH 05/19] docstring

---
 lib/matplotlib/backends/_backend_tk.py | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index fc36904a37c8..5d9a62bd5004 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -61,9 +61,6 @@ def blit(photoimage, aggimage, offsets, bbox=None):
     """
     Blit *aggimage* to *photoimage*.
 
-    A *tk_instance* such as a canvas is required execute the blit.
-    Tcl events must be dispatched to trigger a blit from a non-Tcl thread.
-
     *offsets* is a tuple describing how to fill the ``offset`` field of the
     ``Tk_PhotoImageBlock`` struct: it should be (0, 1, 2, 3) for RGBA8888 data,
     (2, 1, 0, 3) for little-endian ARBG32 (i.e. GBRA8888) data and (1, 2, 3, 0)

From 7196d37bf44bfa7ce2ae0bf3af7bdd37bbef910d Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Fri, 25 Sep 2020 10:41:24 -0400
Subject: [PATCH 06/19] missing id argument

---
 lib/matplotlib/backends/_backend_tk.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index 5d9a62bd5004..4d076d251c95 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -84,7 +84,7 @@ def blit(photoimage, aggimage, offsets, bbox=None):
 
     args = (
         photoimage.tk.interpaddr(), str(photoimage), dataptr, offsets, bboxptr)
-    argsid = str(id)
+    argsid = repr(id(args))
     _blit_args[argsid] = args
 
     global _blit_tcl_name

From b63950e9aef2d580c0ffb1bcc2b5b9176e6371c3 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Fri, 25 Sep 2020 11:15:04 -0400
Subject: [PATCH 07/19] make blank occur in same event as blit

drawing from a thread produces an intolerable flicker unless blank and blit are called from the same event in the main thread
---
 lib/matplotlib/backends/_backend_tk.py | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index 4d076d251c95..9e9dd7ae72e3 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -53,7 +53,11 @@ def _blit(argsid):
     """
     Thin wrapper to pass arguments to blit via tkapp.call
     """
-    args = _blit_args.pop(argsid)
+    photoimage, dataptr, offsets, bboxptr, blank = _blit_args.pop(argsid)
+    args = (
+        photoimage.tk.interpaddr(), str(photoimage), dataptr, offsets, bboxptr)
+    if blank:
+        photoimage.blank()
     _tkagg.blit(*args)
 
 
@@ -78,12 +82,12 @@ 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)
+        blank = True
 
-    args = (
-        photoimage.tk.interpaddr(), str(photoimage), dataptr, offsets, bboxptr)
+    args = photoimage, dataptr, offsets, bboxptr, blank
     argsid = repr(id(args))
     _blit_args[argsid] = args
 

From 9180607cbe2dab31132023d80c6772a8b1955972 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Fri, 25 Sep 2020 16:04:00 -0400
Subject: [PATCH 08/19] test for thread safety

---
 lib/matplotlib/backends/_backend_tk.py        |  5 ++--
 .../tests/test_backends_interactive.py        | 23 +++++++++++++++++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index 9e9dd7ae72e3..26e5227bf032 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -54,11 +54,10 @@ def _blit(argsid):
     Thin wrapper to pass arguments to blit via tkapp.call
     """
     photoimage, dataptr, offsets, bboxptr, blank = _blit_args.pop(argsid)
-    args = (
-        photoimage.tk.interpaddr(), str(photoimage), dataptr, offsets, bboxptr)
     if blank:
         photoimage.blank()
-    _tkagg.blit(*args)
+    _tkagg.blit(
+        photoimage.tk.interpaddr(), str(photoimage), dataptr, offsets, bboxptr)
 
 
 def blit(photoimage, aggimage, offsets, bbox=None):
diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
index 000ff971bc97..aee0f37d3aaa 100644
--- a/lib/matplotlib/tests/test_backends_interactive.py
+++ b/lib/matplotlib/tests/test_backends_interactive.py
@@ -72,6 +72,7 @@ def _get_testable_interactive_backends():
 import io
 import json
 import sys
+import threading
 from unittest import TestCase
 
 import matplotlib as mpl
@@ -146,6 +147,28 @@ def check_alt_backend(alt_backend):
     # FIXME: This should be enabled everywhere once Qt5 is fixed on macOS to
     # not resize incorrectly.
     assert_equal(result.getvalue(), result_after.getvalue())
+    
+# Test artists 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(0.1)
+
+def thread_artist_work():
+    ax.plot([1,3,6])
+
+def thread_draw_work():
+    fig.canvas.draw()
+    fig.canvas.stop_event_loop()
+
+t = threading.Thread(target=thread_artist_work)
+t.start()
+# artists never wait for the event loop to run, so just join
+t.join()
+
+t = threading.Thread(target=thread_draw_work)
+t.start()
+fig.canvas.start_event_loop()
+t.join()
 """
 _test_timeout = 10  # Empirically, 1s is not enough on Travis.
 

From e692b625bd290b92eaf71ebf9304b6944ccc67a7 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Mon, 28 Sep 2020 06:38:55 -0400
Subject: [PATCH 09/19] improve thread safety test

---
 .../tests/test_backends_interactive.py        | 36 ++++++++++++++-----
 1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
index aee0f37d3aaa..644dead74543 100644
--- a/lib/matplotlib/tests/test_backends_interactive.py
+++ b/lib/matplotlib/tests/test_backends_interactive.py
@@ -67,7 +67,6 @@ 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
@@ -147,28 +146,49 @@ def check_alt_backend(alt_backend):
     # FIXME: This should be enabled everywhere once Qt5 is fixed on macOS to
     # not resize incorrectly.
     assert_equal(result.getvalue(), result_after.getvalue())
-    
-# Test artists and drawing does not crash from thread (no other guarantees)
+
+# Test artists and drawing from thread does not crash (no other guarantees)
 fig, ax = plt.subplots()
 # plt.pause needed vs plt.show(block=False) at least on toolbar2-tkagg
-plt.pause(0.1)
+plt.pause(0.5)
+
+exc_info = None
 
 def thread_artist_work():
-    ax.plot([1,3,6])
+    try:
+        ax.plot([1,3,6])
+    except Exception as e:
+        # Propagate error to main thread
+        global exc_info
+        exc_info = sys.exc_info()
 
 def thread_draw_work():
-    fig.canvas.draw()
-    fig.canvas.stop_event_loop()
+    try:
+        fig.canvas.draw()
+    except Exception as e:
+        # Propagate error to main thread
+        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 is not None:  # Raise thread error
+    raise exc_info[1].with_traceback(exc_info[2])
+
 t = threading.Thread(target=thread_draw_work)
+timer = fig.canvas.new_timer(1.)
+timer.add_callback(FigureCanvasBase.key_press_event, fig.canvas, "q")
+fig.canvas.mpl_connect("draw_event", lambda event: timer.start())
+fig.canvas.mpl_connect("close_event", print)
 t.start()
 fig.canvas.start_event_loop()
 t.join()
+
+if exc_info is not None:  # Raise thread error
+    raise exc_info[1].with_traceback(exc_info[2])
 """
 _test_timeout = 10  # Empirically, 1s is not enough on Travis.
 
@@ -193,7 +213,7 @@ def test_interactive_backend(backend, toolbar):
     if proc.returncode:
         pytest.fail("The subprocess returned with non-zero exit status "
                     f"{proc.returncode}.")
-    assert proc.stdout.count("CloseEvent") == 1
+    assert proc.stdout.count("CloseEvent") == 2
 
 
 @pytest.mark.skipif('TF_BUILD' in os.environ,

From e20118963a222b9d373f3a9e3f06835063b47f3a Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Mon, 28 Sep 2020 07:00:46 -0400
Subject: [PATCH 10/19] fix tcl errors by delaying the widget destruction

---
 lib/matplotlib/backends/_backend_tk.py | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index 26e5227bf032..2fc3be07bebb 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -431,10 +431,18 @@ 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()

From 8c979cc117fbc82955dd96ed14d7f8b7be51064b Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Mon, 28 Sep 2020 07:39:45 -0400
Subject: [PATCH 11/19] blit docs & comments

---
 lib/matplotlib/backends/_backend_tk.py | 27 +++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index 2fc3be07bebb..d97f294bc5b9 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -45,13 +45,19 @@ def _restore_foreground_window_at_end():
 
 
 _blit_args = {}
-# Initialized to non-empty string that is not a current Tcl command
+# Initialize to a non-empty string that is not a Tcl command
 _blit_tcl_name = "29345091836409813"
 
 
 def _blit(argsid):
     """
-    Thin wrapper to pass arguments to blit via tkapp.call
+    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:
@@ -69,7 +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]
@@ -86,16 +95,24 @@ def blit(photoimage, aggimage, offsets, bbox=None):
         bboxptr = (0, width, 0, height)
         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
-    # tkapp.call coerces all arguments to strings
     try:
         photoimage.tk.call(_blit_tcl_name, argsid)
     except tk.TclError:
-        # register _blit by copying code from tkinter.Misc._register
+        # 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)

From 346314808050ccda74966a545b1fd170b9e1f679 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Mon, 28 Sep 2020 11:41:29 -0400
Subject: [PATCH 12/19] appease CI

---
 .../tests/test_backends_interactive.py        | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
index 644dead74543..55b1096ec3a7 100644
--- a/lib/matplotlib/tests/test_backends_interactive.py
+++ b/lib/matplotlib/tests/test_backends_interactive.py
@@ -147,17 +147,17 @@ def check_alt_backend(alt_backend):
     # not resize incorrectly.
     assert_equal(result.getvalue(), result_after.getvalue())
 
-# Test artists and drawing from thread does not crash (no other guarantees)
+# Test artists 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)
+plt.pause(0.1)
 
 exc_info = None
 
 def thread_artist_work():
     try:
         ax.plot([1,3,6])
-    except Exception as e:
+    except:
         # Propagate error to main thread
         global exc_info
         exc_info = sys.exc_info()
@@ -165,7 +165,7 @@ def thread_artist_work():
 def thread_draw_work():
     try:
         fig.canvas.draw()
-    except Exception as e:
+    except:
         # Propagate error to main thread
         global exc_info
         exc_info = sys.exc_info()
@@ -175,19 +175,18 @@ def thread_draw_work():
 # artists never wait for the event loop to run, so just join
 t.join()
 
-if exc_info is not None:  # Raise thread error
+if exc_info:  # Raise thread error
     raise exc_info[1].with_traceback(exc_info[2])
 
 t = threading.Thread(target=thread_draw_work)
-timer = fig.canvas.new_timer(1.)
-timer.add_callback(FigureCanvasBase.key_press_event, fig.canvas, "q")
-fig.canvas.mpl_connect("draw_event", lambda event: timer.start())
 fig.canvas.mpl_connect("close_event", print)
 t.start()
-fig.canvas.start_event_loop()
+plt.pause(0.1)  # flush_events fails on at least Tkagg (bpo-41176)
 t.join()
+plt.close()
+fig.canvas.flush_events()  # pause doesn't process events after close
 
-if exc_info is not None:  # Raise thread error
+if exc_info:  # Raise thread error
     raise exc_info[1].with_traceback(exc_info[2])
 """
 _test_timeout = 10  # Empirically, 1s is not enough on Travis.

From fd97233b0581c8062a704788a64539e151dbfdee Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Sat, 24 Oct 2020 09:15:32 -0400
Subject: [PATCH 13/19] Split thread tests from other interactive tests

ignore wx
---
 .../tests/test_backends_interactive.py        | 67 ++++++++++++++-----
 1 file changed, 52 insertions(+), 15 deletions(-)

diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
index 55b1096ec3a7..0d631799b1a0 100644
--- a/lib/matplotlib/tests/test_backends_interactive.py
+++ b/lib/matplotlib/tests/test_backends_interactive.py
@@ -146,8 +146,51 @@ def check_alt_backend(alt_backend):
     # FIXME: This should be enabled everywhere once Qt5 is fixed on macOS to
     # not resize incorrectly.
     assert_equal(result.getvalue(), result_after.getvalue())
+"""
+_test_timeout = 10  # Empirically, 1s is not enough on Travis.
+
+
+@pytest.mark.parametrize("backend", _get_testable_interactive_backends())
+@pytest.mark.parametrize("toolbar", ["toolbar2", "toolmanager"])
+@pytest.mark.flaky(reruns=3)
+def test_interactive_backend(backend, toolbar):
+    if backend == "macosx":
+        if toolbar == "toolmanager":
+            pytest.skip("toolmanager is not implemented for macosx.")
+        if toolbar == "toolbar2" and os.environ.get('TRAVIS'):
+            # See https://github.com/matplotlib/matplotlib/issues/18213
+            pytest.skip("toolbar2 for macosx is buggy on Travis.")
+
+    proc = subprocess.run(
+        [sys.executable, "-c", _test_script,
+         json.dumps({"toolbar": toolbar})],
+        env={**os.environ, "MPLBACKEND": backend, "SOURCE_DATE_EPOCH": "0"},
+        timeout=_test_timeout,
+        stdout=subprocess.PIPE, universal_newlines=True)
+    if proc.returncode:
+        pytest.fail("The subprocess returned with non-zero exit status "
+                    f"{proc.returncode}.")
+    assert proc.stdout.count("CloseEvent") == 1
+
+
+_thread_test_script = """\
+import json
+import sys
+import threading
+from unittest import TestCase
+
+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]))
+assert_equal = TestCase().assertEqual
+assert_raises = TestCase().assertRaises
 
-# Test artists and drawing does not crash from thread (no other guarantees)
+# 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(0.1)
@@ -159,6 +202,7 @@ def thread_artist_work():
         ax.plot([1,3,6])
     except:
         # Propagate error to main thread
+        import sys
         global exc_info
         exc_info = sys.exc_info()
 
@@ -167,6 +211,7 @@ def thread_draw_work():
         fig.canvas.draw()
     except:
         # Propagate error to main thread
+        import sys
         global exc_info
         exc_info = sys.exc_info()
 
@@ -181,7 +226,7 @@ def thread_draw_work():
 t = threading.Thread(target=thread_draw_work)
 fig.canvas.mpl_connect("close_event", print)
 t.start()
-plt.pause(0.1)  # flush_events fails on at least Tkagg (bpo-41176)
+plt.pause(0.1)  # 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
@@ -189,30 +234,22 @@ def thread_draw_work():
 if exc_info:  # Raise thread error
     raise exc_info[1].with_traceback(exc_info[2])
 """
-_test_timeout = 10  # Empirically, 1s is not enough on Travis.
 
 
 @pytest.mark.parametrize("backend", _get_testable_interactive_backends())
-@pytest.mark.parametrize("toolbar", ["toolbar2", "toolmanager"])
 @pytest.mark.flaky(reruns=3)
-def test_interactive_backend(backend, toolbar):
-    if backend == "macosx":
-        if toolbar == "toolmanager":
-            pytest.skip("toolmanager is not implemented for macosx.")
-        if toolbar == "toolbar2" and os.environ.get('TRAVIS'):
-            # See https://github.com/matplotlib/matplotlib/issues/18213
-            pytest.skip("toolbar2 for macosx is buggy on Travis.")
-
+def test_interactive_thread_safety(backend):
     proc = subprocess.run(
-        [sys.executable, "-c", _test_script,
-         json.dumps({"toolbar": toolbar})],
+        [sys.executable, "-c", _thread_test_script],
         env={**os.environ, "MPLBACKEND": backend, "SOURCE_DATE_EPOCH": "0"},
         timeout=_test_timeout,
         stdout=subprocess.PIPE, universal_newlines=True)
     if proc.returncode:
+        if backend == "wx":
+            pytest.xfail("Ignoring deprecated wx failure. Use wxagg.")
         pytest.fail("The subprocess returned with non-zero exit status "
                     f"{proc.returncode}.")
-    assert proc.stdout.count("CloseEvent") == 2
+    assert proc.stdout.count("CloseEvent") == 1
 
 
 @pytest.mark.skipif('TF_BUILD' in os.environ,

From bbb82ac438589f5e01b99bc8e70850b48e032272 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Sat, 24 Oct 2020 10:59:50 -0400
Subject: [PATCH 14/19] ignore -> xfail for wx and macosx

---
 .../tests/test_backends_interactive.py        | 24 ++++++++++++-------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
index 0d631799b1a0..0fc682102b4b 100644
--- a/lib/matplotlib/tests/test_backends_interactive.py
+++ b/lib/matplotlib/tests/test_backends_interactive.py
@@ -235,20 +235,28 @@ def thread_draw_work():
     raise exc_info[1].with_traceback(exc_info[2])
 """
 
-
-@pytest.mark.parametrize("backend", _get_testable_interactive_backends())
+_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,
+        timeout=_test_timeout, check=True,
         stdout=subprocess.PIPE, universal_newlines=True)
-    if proc.returncode:
-        if backend == "wx":
-            pytest.xfail("Ignoring deprecated wx failure. Use wxagg.")
-        pytest.fail("The subprocess returned with non-zero exit status "
-                    f"{proc.returncode}.")
     assert proc.stdout.count("CloseEvent") == 1
 
 

From 59d5c0635b75a49e72c37298041753bc12641cb9 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Sat, 24 Oct 2020 12:01:13 -0400
Subject: [PATCH 15/19] increase event loop time for CI, cleanup imports

---
 lib/matplotlib/tests/test_backends_interactive.py | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
index 0fc682102b4b..6ccdcd921d63 100644
--- a/lib/matplotlib/tests/test_backends_interactive.py
+++ b/lib/matplotlib/tests/test_backends_interactive.py
@@ -177,7 +177,6 @@ def test_interactive_backend(backend, toolbar):
 import json
 import sys
 import threading
-from unittest import TestCase
 
 from matplotlib import pyplot as plt, rcParams
 rcParams.update({
@@ -186,14 +185,12 @@ def test_interactive_backend(backend, toolbar):
 })
 if len(sys.argv) >= 2:  # Second argument is json-encoded rcParams.
     rcParams.update(json.loads(sys.argv[1]))
-assert_equal = TestCase().assertEqual
-assert_raises = TestCase().assertRaises
 
 # 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(0.1)
+plt.pause(0.5)
 
 exc_info = None
 
@@ -226,7 +223,7 @@ def thread_draw_work():
 t = threading.Thread(target=thread_draw_work)
 fig.canvas.mpl_connect("close_event", print)
 t.start()
-plt.pause(0.1)  # flush_events fails here on at least Tkagg (bpo-41176)
+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

From 9255fb6af0958d55b08a3d78636e46a9b430dc60 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Tue, 27 Oct 2020 08:44:32 -0400
Subject: [PATCH 16/19] respond to comments

---
 lib/matplotlib/backends/_backend_tk.py   | 5 ++---
 lib/matplotlib/backends/backend_tkagg.py | 6 ++++--
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index d97f294bc5b9..326df873214e 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -1,3 +1,4 @@
+import uuid
 from contextlib import contextmanager
 import logging
 import math
@@ -46,7 +47,7 @@ def _restore_foreground_window_at_end():
 
 _blit_args = {}
 # Initialize to a non-empty string that is not a Tcl command
-_blit_tcl_name = "29345091836409813"
+_blit_tcl_name = "mpl_blit_" + uuid.uuid4().hex
 
 
 def _blit(argsid):
@@ -108,12 +109,10 @@ def blit(photoimage, aggimage, offsets, bbox=None):
     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)
 
diff --git a/lib/matplotlib/backends/backend_tkagg.py b/lib/matplotlib/backends/backend_tkagg.py
index 8768a729c6d0..ca15bf818142 100644
--- a/lib/matplotlib/backends/backend_tkagg.py
+++ b/lib/matplotlib/backends/backend_tkagg.py
@@ -1,6 +1,7 @@
+from . import _backend_tk
 from .backend_agg import FigureCanvasAgg
 from ._backend_tk import (
-    _BackendTk, FigureCanvasTk, FigureManagerTk, NavigationToolbar2Tk, blit)
+    _BackendTk, FigureCanvasTk, FigureManagerTk, NavigationToolbar2Tk)
 
 
 class FigureCanvasTkAgg(FigureCanvasAgg, FigureCanvasTk):
@@ -9,7 +10,8 @@ def draw(self):
         self.blit()
 
     def blit(self, bbox=None):
-        blit(self._tkphoto, self.renderer._renderer, (0, 1, 2, 3), bbox=bbox)
+        _backend_tk.blit(
+            self._tkphoto, self.renderer._renderer, (0, 1, 2, 3), bbox=bbox)
 
 
 @_BackendTk.export

From d1984c5b1ca963710aa82d930e2b75e67ab5bcd1 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Tue, 27 Oct 2020 09:41:46 -0400
Subject: [PATCH 17/19] remove lingering threading import

---
 lib/matplotlib/tests/test_backends_interactive.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
index 6ccdcd921d63..ecf9c513bd1b 100644
--- a/lib/matplotlib/tests/test_backends_interactive.py
+++ b/lib/matplotlib/tests/test_backends_interactive.py
@@ -71,7 +71,6 @@ def _get_testable_interactive_backends():
 import io
 import json
 import sys
-import threading
 from unittest import TestCase
 
 import matplotlib as mpl

From 78a1e43b1016c3a4871e6b6f447d08fdba4f1589 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Tue, 27 Oct 2020 09:55:52 -0400
Subject: [PATCH 18/19] Apply suggestions from code review

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
---
 lib/matplotlib/backends/_backend_tk.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index 326df873214e..afe51e7c5c26 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -52,10 +52,10 @@ def _restore_foreground_window_at_end():
 
 def _blit(argsid):
     """
-    Thin wrapper to blit via tkapp.call
+    Thin wrapper to blit called 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
+    the ``_blit_args`` dict, since arguments cannot be passed directly.
 
     photoimage blanking must occur in the same event and thread as blitting
     to avoid flickering.
@@ -106,7 +106,7 @@ def blit(photoimage, aggimage, offsets, bbox=None):
     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))
+    argsid = str(id(args))
     _blit_args[argsid] = args
 
     try:

From 0b1660bfbdf742d795edb1e742c0ee66b40bdef0 Mon Sep 17 00:00:00 2001
From: richardsheridan <richard.sheridan@gmail.com>
Date: Tue, 27 Oct 2020 10:00:52 -0400
Subject: [PATCH 19/19] narrow TclError handling

---
 lib/matplotlib/backends/_backend_tk.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index afe51e7c5c26..a6389944f3cf 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -111,8 +111,9 @@ def blit(photoimage, aggimage, offsets, bbox=None):
 
     try:
         photoimage.tk.call(_blit_tcl_name, argsid)
-    except tk.TclError:
-        # register _blit with code copied from tkinter.Misc._register
+    except tk.TclError as e:
+        if "invalid command name" not in str(e):
+            raise
         photoimage.tk.createcommand(_blit_tcl_name, _blit)
         photoimage.tk.call(_blit_tcl_name, argsid)