FIX: make the cache in font_manager._get_font keyed by thread id

tacaswell · anntzer · tacaswell · commit ad7a7931e3f0 · 2021-03-05T17:29:02.000-05:00
This prevents segfaults when multiple threads try to manipulate the FT2Font object simultaneously. closes #19560 Co-authored-by: Antony Lee <anntzer.lee@gmail.com>
diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
@@ -33,8 +33,10 @@
 import subprocess
 import sys
 try:
+    import threading
     from threading import Timer
 except ImportError:
+    import dummy_threading as threading
     from dummy_threading import Timer
 
 import matplotlib as mpl
@@ -1394,7 +1396,12 @@ def is_opentype_cff_font(filename):
         return False
 
 
-_get_font = lru_cache(64)(ft2font.FT2Font)
+@lru_cache(64)
+def _get_font(filename, hinting_factor, *, _kerning_factor, thread_id):
+    return ft2font.FT2Font(
+        filename, hinting_factor, _kerning_factor=_kerning_factor)
+
+
 # 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
@@ -1409,8 +1416,10 @@ def get_font(filename, hinting_factor=None):
     filename = _cached_realpath(filename)
     if hinting_factor is None:
         hinting_factor = rcParams['text.hinting_factor']
+    # also key on the thread ID to prevent segfaults with multi-threading
     return _get_font(filename, hinting_factor,
-                     _kerning_factor=rcParams['text.kerning_factor'])
+                     _kerning_factor=rcParams['text.kerning_factor'],
+                     thread_id=threading.get_ident())
 
 
 def _load_fontmanager(*, try_read_cache=True):
diff --git a/lib/matplotlib/tests/test_font_manager.py b/lib/matplotlib/tests/test_font_manager.py
@@ -3,9 +3,11 @@
 import os
 from pathlib import Path
 import shutil
+import subprocess
 import sys
 import warnings
 
+
 import numpy as np
 import pytest
 
@@ -215,3 +217,41 @@ def test_missing_family(caplog):
         "findfont: Generic family 'sans' not found because none of the "
         "following families were found: this-font-does-not-exist",
     ]
+
+
+def _test_threading():
+    import threading
+    from matplotlib.ft2font import LOAD_NO_HINTING
+    import matplotlib.font_manager as fm
+
+    N = 10
+    b = threading.Barrier(N)
+
+    def bad_idea(n):
+        b.wait()
+        for j in range(100):
+            font = fm.get_font(fm.findfont("DejaVu Sans"))
+            font.set_text(str(n), 0.0, flags=LOAD_NO_HINTING)
+
+    threads = [
+        threading.Thread(target=bad_idea, name=f"bad_thread_{j}", args=(j,))
+        for j in range(N)
+    ]
+
+    for t in threads:
+        t.start()
+
+    for t in threads:
+        t.join()
+
+
+def test_fontcache_thread_safe():
+    import inspect
+
+    proc = subprocess.run(
+        [sys.executable, "-c",
+         inspect.getsource(_test_threading) + '\n_test_threading()']
+    )
+    if proc.returncode:
+        pytest.fail("The subprocess returned with non-zero exit status "
+                    f"{proc.returncode}.")
