Warn if a temporary config/cache dir must be created.

anntzer · anntzer · commit 694d69e00989 · 2020-04-01T14:59:12.000+02:00
Paying the cost of regen'ing the font cache on every import seems a bit
silly when one can just set MPLCONFIGDIR to a persistent directory.
Moreover the tmpdir approach is brittle against forking; this is fixable
but somewhat complex to do.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -431,16 +431,6 @@ def get_home():
         return None
 
 
-def _create_tmp_config_or_cache_dir():
-    """
-    If the config or cache directory cannot be created, create a temporary one.
-    """
-    configdir = os.environ['MPLCONFIGDIR'] = (
-        tempfile.mkdtemp(prefix='matplotlib-'))
-    atexit.register(shutil.rmtree, configdir)
-    return configdir
-
-
 def _get_xdg_config_dir():
     """
     Return the XDG configuration directory, according to the `XDG
@@ -474,7 +464,19 @@ def _get_config_or_cache_dir(xdg_base):
     else:
         if os.access(str(configdir), os.W_OK) and configdir.is_dir():
             return str(configdir)
-    return _create_tmp_config_or_cache_dir()
+    # If the config or cache directory cannot be created or is not a writable
+    # directory, create a temporary one.
+    tmpdir = os.environ["MPLCONFIGDIR"] = \
+        tempfile.mkdtemp(prefix="matplotlib-")
+    atexit.register(shutil.rmtree, tmpdir)
+    _log.warning(
+        "Matplotlib created a temporary config/cache directory at %s because "
+        "the default path (%s) is not a writable directory; it is highly "
+        "recommended to set the MPLCONFIGDIR environment variable to a "
+        "writable directory, in particular to speed up the import of "
+        "Matplotlib and to better support multiprocessing.",
+        configdir, tmpdir)
+    return tmpdir
 
 
 @_logged_cached('CONFIGDIR=%s')
diff --git a/lib/matplotlib/tests/test_matplotlib.py b/lib/matplotlib/tests/test_matplotlib.py
@@ -1,5 +1,26 @@
+import os
+import subprocess
+import sys
+
+import pytest
+
 import matplotlib
-import matplotlib.rcsetup
+
+
+@pytest.mark.skipif(
+    os.name == "nt", reason="chmod() doesn't work as is on Windows")
+def test_tmpconfigdir_warning(tmpdir):
+    """Test that a warning is emitted if a temporary configdir must be used."""
+    mode = os.stat(tmpdir).st_mode
+    try:
+        os.chmod(tmpdir, 0)
+        proc = subprocess.run(
+            [sys.executable, "-c", "import matplotlib"],
+            env={**os.environ, "MPLCONFIGDIR": str(tmpdir)},
+            stderr=subprocess.PIPE, universal_newlines=True, check=True)
+        assert "set the MPLCONFIGDIR" in proc.stderr
+    finally:
+        os.chmod(tmpdir, mode)
 
 
 def test_use_doc_standard_backends():
