Backport PR #14535: Invalidate FT2Font cache when fork()ing.

tacaswell · MeeseeksDev[bot] · commit b81f8331e625 · 2019-06-13T15:00:10.000Z
diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
@@ -1321,12 +1321,20 @@ def is_opentype_cff_font(filename):
         return False
 
 
-_get_font = lru_cache(64)(ft2font.FT2Font)
 _fmcache = os.path.join(
     mpl.get_cachedir(), 'fontlist-v{}.json'.format(FontManager.__version__))
 fontManager = None
 
 
+_get_font = lru_cache(64)(ft2font.FT2Font)
+# FT2Font objects cannot be used across fork()s because they reference the same
+# FT_Library object.  While invalidating *all* existing FT2Fonts after a fork
+# would be too complicated to be worth it, the main way FT2Fonts get reused is
+# via the cache of _get_font, which we can empty upon forking (in Py3.7+).
+if hasattr(os, "register_at_fork"):
+    os.register_at_fork(after_in_child=_get_font.cache_clear)
+
+
 def get_font(filename, hinting_factor=None):
     if hinting_factor is None:
         hinting_factor = rcParams['text.hinting_factor']
diff --git a/lib/matplotlib/tests/test_font_manager.py b/lib/matplotlib/tests/test_font_manager.py
@@ -1,4 +1,5 @@
 from io import BytesIO
+import multiprocessing
 import os
 from pathlib import Path
 import shutil
@@ -184,3 +185,17 @@ def test_user_fonts_win32():
     # Now, the font should be available
     fonts = findSystemFonts()
     assert any(font_test_file in font for font in fonts)
+
+
+def _model_handler(_):
+    fig, ax = plt.subplots()
+    fig.savefig(BytesIO(), format="pdf")
+    plt.close()
+
+
+@pytest.mark.skipif(not hasattr(os, "register_at_fork"),
+                    reason="Cannot register at_fork handlers")
+def test_fork():
+    _model_handler(0)  # Make sure the font cache is filled.
+    ctx = multiprocessing.get_context("fork")
+    ctx.Pool(processes=2).map(_model_handler, range(2))
