matplotlib · tacaswell · Apr 28, 2022 · Dec 18, 2021 · Dec 18, 2021 · Apr 8, 2022
diff --git a/environment.yml b/environment.yml
@@ -47,6 +47,7 @@ dependencies:
   - nbformat!=5.0.0,!=5.0.1
   - pandas!=0.25.0
   - pikepdf
+  - psutil
   - pre-commit
   - pydocstyle>=5.1.0
   - pytest!=4.6.0,!=5.4.0

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
@@ -474,18 +474,22 @@ def destroy(self, *args):
             self.canvas._tkcanvas.after_cancel(self.canvas._event_loop_id)
 
         # 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.
+        # however, self.window.update() can break user code. An async callback
+        # is the safest way to achieve a complete draining of the event queue,
+        # but it leaks if no tk event loop is running. Therefore we explicitly
+        # check for an event loop and choose our best guess.
         def delayed_destroy():
             self.window.destroy()
 
             if self._owns_mainloop and not Gcf.get_num_fig_managers():
                 self.window.quit()
 
-        # "after idle after 0" avoids Tcl error/race (GH #19940)
-        self.window.after_idle(self.window.after, 0, delayed_destroy)
+        if cbook._get_running_interactive_framework() == "tk":
+            # "after idle after 0" avoids Tcl error/race (GH #19940)
+            self.window.after_idle(self.window.after, 0, delayed_destroy)
+        else:
+            self.window.update()
+            delayed_destroy()
 
     def get_window_title(self):
         return self.window.wm_title()

diff --git a/lib/matplotlib/tests/test_backend_tk.py b/lib/matplotlib/tests/test_backend_tk.py
@@ -50,8 +50,9 @@ def test_func():
             )
         except subprocess.TimeoutExpired:
             pytest.fail("Subprocess timed out")
-        except subprocess.CalledProcessError:
-            pytest.fail("Subprocess failed to test intended behavior")
+        except subprocess.CalledProcessError as e:
+            pytest.fail("Subprocess failed to test intended behavior\n"
+                        + str(e.stderr))
         else:
             # macOS may actually emit irrelevant errors about Accelerated
             # OpenGL vs. software OpenGL, so suppress them.
@@ -146,6 +147,7 @@ def target():
     thread.join()
 
 
+@pytest.mark.backend('TkAgg', skip_on_importerror=True)
 @pytest.mark.flaky(reruns=3)
 @_isolated_tk_test(success_count=0)
 def test_never_update():
@@ -159,14 +161,12 @@ def test_never_update():
 
     plt.draw()  # Test FigureCanvasTkAgg.
     fig.canvas.toolbar.configure_subplots()  # Test NavigationToolbar2Tk.
+    # Test FigureCanvasTk filter_destroy callback
+    fig.canvas.get_tk_widget().after(100, plt.close, fig)
 
     # Check for update() or update_idletasks() in the event queue, functionally
     # equivalent to tkinter.Misc.update.
-    # Must pause >= 1 ms to process tcl idle events plus extra time to avoid
-    # flaky tests on slow systems.
-    plt.pause(0.1)
-
-    plt.close(fig)  # Test FigureCanvasTk filter_destroy callback
+    plt.show(block=True)
 
     # Note that exceptions would be printed to stderr; _isolated_tk_test
     # checks them.

diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
@@ -195,8 +195,7 @@ def _test_thread_impl():
     future = ThreadPoolExecutor().submit(fig.canvas.draw)
     plt.pause(0.5)  # flush_events fails here on at least Tkagg (bpo-41176)
     future.result()  # Joins the thread; rethrows any exception.
-    plt.close()
-    fig.canvas.flush_events()  # pause doesn't process events after close
+    plt.close()  # backend is responsible for flushing any events here
 
 
 _thread_safe_backends = _get_testable_interactive_backends()
@@ -503,3 +502,46 @@ def test_blitting_events(env):
     # blitting is not properly implemented
     ndraws = proc.stdout.count("DrawEvent")
     assert 0 < ndraws < 5
+
+
+# The source of this function gets extracted and run in another process, so it
+# must be fully self-contained.
+def _test_figure_leak():
+    import sys
+
+    import psutil
+    from matplotlib import pyplot as plt
+    # Second argument is pause length, but if zero we should skip pausing
+    t = float(sys.argv[1])
+
+    # Warmup cycle, this reasonably allocates a lot
+    p = psutil.Process()
+    fig = plt.figure()
+    if t:
+        plt.pause(t)
+    plt.close(fig)
+    mem = p.memory_full_info().uss
+
+    for _ in range(5):
+        fig = plt.figure()
+        if t:
+            plt.pause(t)
+        plt.close(fig)
+    growth = p.memory_full_info().uss - mem
+
+    print(growth)
+
+
+@pytest.mark.parametrize("env", _get_testable_interactive_backends())
+@pytest.mark.parametrize("time", ["0.0", "0.1"])
+def test_figure_leak_20490(env, time):
+    pytest.importorskip("psutil", reason="psutil needed to run this test")
+
+    # We can't yet directly identify the leak
+    # so test with a memory growth threshold
+    acceptable_memory_leakage = 2_000_000
+
+    result = _run_helper(_test_figure_leak, time, timeout=_test_timeout, **env)
+
+    growth = int(result.stdout)
+    assert growth <= acceptable_memory_leakage
diff --git a/requirements/testing/all.txt b/requirements/testing/all.txt
@@ -2,6 +2,7 @@
 
 certifi
 coverage<6.3
+psutil
 pytest!=4.6.0,!=5.4.0
 pytest-cov
 pytest-rerunfailures