FIX: do not cache exceptions

This leads to caching the tracebacks which can keep user's objects in local
namespaces alive indefinitely.  This can lead to very surprising memory issues
for users and will result in incorrect tracebacks.

Responsive to matplotlib#25406

lib/matplotlib/font_manager.py

@@ -26,6 +26,7 @@
 #   - 'light' is an invalid weight value, remove it.
 
 from base64 import b64encode
+from collections import namedtuple
 import copy
 import dataclasses
 from functools import lru_cache
@@ -128,6 +129,7 @@
     'sans',
 }
 
+_ExceptionProxy = namedtuple('_ExceptionProxy', ['klass', 'message'])
 
 # OS Font paths
 try:
@@ -1288,8 +1290,8 @@ def findfont(self, prop, fontext='ttf', directory=None,
         ret = self._findfont_cached(
             prop, fontext, directory, fallback_to_default, rebuild_if_missing,
             rc_params)
-        if isinstance(ret, Exception):
-            raise ret
+        if isinstance(ret, _ExceptionProxy):
+            raise ret.klass(ret.message)
         return ret
 
     def get_font_names(self):
@@ -1440,10 +1442,12 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
                                      fallback_to_default=False)
             else:
                 # This return instead of raise is intentional, as we wish to
-                # cache the resulting exception, which will not occur if it was
+                # cache that it was not found, which will not occur if it was
                 # actually raised.
-                return ValueError(f"Failed to find font {prop}, and fallback "
-                                  f"to the default font was disabled")
+                return _ExceptionProxy(
+                    ValueError,
+                    f"Failed to find font {prop}, and fallback to the default font was disabled"
+                )
         else:
             _log.debug('findfont: Matching %s to %s (%r) with score of %f.',
                        prop, best_font.name, best_font.fname, best_score)
@@ -1463,9 +1467,9 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
                     prop, fontext, directory, rebuild_if_missing=False)
             else:
                 # This return instead of raise is intentional, as we wish to
-                # cache the resulting exception, which will not occur if it was
+                # cache that it was not found, which will not occur if it was
                 # actually raised.
-                return ValueError("No valid font could be found")
+                return _ExceptionProxy(ValueError, "No valid font could be found")
 
         return _cached_realpath(result)
 

lib/matplotlib/tests/test_font_manager.py

@@ -1,4 +1,5 @@
 from io import BytesIO, StringIO
+import gc
 import multiprocessing
 import os
 from pathlib import Path
@@ -16,7 +17,7 @@
     json_dump, json_load, get_font, is_opentype_cff_font,
     MSUserFontDirectories, _get_fontconfig_fonts, ft2font,
     ttfFontProperty, cbook)
-from matplotlib import pyplot as plt, rc_context
+from matplotlib import pyplot as plt, rc_context, figure as mfigure
 
 has_fclist = shutil.which('fc-list') is not None
 
@@ -324,3 +325,25 @@ def test_get_font_names():
     assert set(available_fonts) == set(mpl_font_names)
     assert len(available_fonts) == len(mpl_font_names)
     assert available_fonts == mpl_font_names
+
+
+def test_donot_cache_tracebacks():
+
+    class SomeObject:
+        pass
+
+    def inner():
+        x = SomeObject()
+        fig = mfigure.Figure()
+        ax = fig.subplots()
+        fig.text(.5, .5, 'aardvark', family='doesnotexist')
+        with BytesIO() as out:
+            with warnings.catch_warnings():
+                warnings.filterwarnings('ignore')
+                fig.savefig(out, format='png')
+
+    inner()
+
+    for obj in gc.get_objects():
+        if isinstance(obj, SomeObject):
+            pytest.fail("object from inner stack still alive")