Re-fix exception caching in dviread.

anntzer · anntzer · commit 88fbc2666dc0 · 2024-10-02T00:06:43.000+02:00
The original solution using with_traceback didn't actually work because
with_traceback doesn't return a new exception instance but rather
modifies the original one in place, and the traceback will then be
attached to it by the raise statement.

Instead, we actually need to build a new instance, so reuse the
_ExceptionProxy machinery from font_manager (slightly expanded).
diff --git a/lib/matplotlib/cbook.py b/lib/matplotlib/cbook.py
@@ -32,6 +32,29 @@
 from matplotlib import _api, _c_internal_utils
 
 
+class _ExceptionInfo:
+    """
+    A class to carry exception information around.
+
+    This is used to store and later raise exceptions. It's an alternative to
+    directly storing Exception instances that circumvents traceback-related
+    issues: caching tracebacks can keep user's objects in local namespaces
+    alive indefinitely, which can lead to very surprising memory issues for
+    users and result in incorrect tracebacks.
+    """
+
+    def __init__(self, cls, *args):
+        self._cls = cls
+        self._args = args
+
+    @classmethod
+    def from_exception(cls, exc):
+        return cls(type(exc), *exc.args)
+
+    def to_exception(self):
+        return self._cls(*self._args)
+
+
 def _get_running_interactive_framework():
     """
     Return the interactive framework whose event loop is currently running, if
diff --git a/lib/matplotlib/dviread.py b/lib/matplotlib/dviread.py
@@ -340,7 +340,7 @@ def _read(self):
             byte = self.file.read(1)[0]
             self._dtable[byte](self, byte)
             if self._missing_font:
-                raise self._missing_font
+                raise self._missing_font.to_exception()
             name = self._dtable[byte].__name__
             if name == "_push":
                 down_stack.append(down_stack[-1])
@@ -368,14 +368,14 @@ def _read_arg(self, nbytes, signed=False):
     @_dispatch(min=0, max=127, state=_dvistate.inpage)
     def _set_char_immediate(self, char):
         self._put_char_real(char)
-        if isinstance(self.fonts[self.f], FileNotFoundError):
+        if isinstance(self.fonts[self.f], cbook._ExceptionInfo):
             return
         self.h += self.fonts[self.f]._width_of(char)
 
     @_dispatch(min=128, max=131, state=_dvistate.inpage, args=('olen1',))
     def _set_char(self, char):
         self._put_char_real(char)
-        if isinstance(self.fonts[self.f], FileNotFoundError):
+        if isinstance(self.fonts[self.f], cbook._ExceptionInfo):
             return
         self.h += self.fonts[self.f]._width_of(char)
 
@@ -390,7 +390,7 @@ def _put_char(self, char):
 
     def _put_char_real(self, char):
         font = self.fonts[self.f]
-        if isinstance(font, FileNotFoundError):
+        if isinstance(font, cbook._ExceptionInfo):
             self._missing_font = font
         elif font._vf is None:
             self.text.append(Text(self.h, self.v, font, char,
@@ -504,7 +504,7 @@ def _fnt_def_real(self, k, c, s, d, a, l):
             # and throw that error in Dvi._read.  For Vf, _finalize_packet
             # checks whether a missing glyph has been used, and in that case
             # skips the glyph definition.
-            self.fonts[k] = exc.with_traceback(None)
+            self.fonts[k] = cbook._ExceptionInfo.from_exception(exc)
             return
         if c != 0 and tfm.checksum != 0 and c != tfm.checksum:
             raise ValueError(f'tfm checksum mismatch: {n}')
diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
@@ -28,7 +28,6 @@
 from __future__ import annotations
 
 from base64 import b64encode
-from collections import namedtuple
 import copy
 import dataclasses
 from functools import lru_cache
@@ -133,8 +132,6 @@
     'sans',
 }
 
-_ExceptionProxy = namedtuple('_ExceptionProxy', ['klass', 'message'])
-
 # OS Font paths
 try:
     _HOME = Path.home()
@@ -1355,8 +1352,8 @@ 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, _ExceptionProxy):
-            raise ret.klass(ret.message)
+        if isinstance(ret, cbook._ExceptionInfo):
+            raise ret.to_exception()
         return ret
 
     def get_font_names(self):
@@ -1509,7 +1506,7 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
                 # This return instead of raise is intentional, as we wish to
                 # cache that it was not found, which will not occur if it was
                 # actually raised.
-                return _ExceptionProxy(
+                return cbook._ExceptionInfo(
                     ValueError,
                     f"Failed to find font {prop}, and fallback to the default font was "
                     f"disabled"
@@ -1535,7 +1532,7 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
                 # This return instead of raise is intentional, as we wish to
                 # cache that it was not found, which will not occur if it was
                 # actually raised.
-                return _ExceptionProxy(ValueError, "No valid font could be found")
+                return cbook._ExceptionInfo(ValueError, "No valid font could be found")
 
         return _cached_realpath(result)
 
