Move locking of fontlist.json *into* json_dump.

anntzer · anntzer · commit 69ccb736982d · 2020-03-31T09:44:24.000+02:00
This makes it easier for end users to call json_dump (which is public
API) safely, given that _lock_path is not public API.
diff --git a/doc/api/next_api_changes/behaviour.rst b/doc/api/next_api_changes/behaviour.rst
@@ -147,3 +147,6 @@ The parameter ``s`` to `.Axes.annotate` and  `.pyplot.annotate` is renamed to
 The old parameter name remains supported, but
 support for it will be dropped in a future Matplotlib release.
 
+`.font_manager.json_dump` now locks the font manager dump file
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... to prevent multiple processes from writing to it at the same time.
diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
@@ -912,8 +912,11 @@ def json_dump(data, filename):
     File paths that are children of the Matplotlib data path (typically, fonts
     shipped with Matplotlib) are stored relative to that data path (to remain
     valid across virtualenvs).
+
+    This function temporarily locks the output file to prevent multiple
+    processes from overwriting one another's output.
     """
-    with open(filename, 'w') as fh:
+    with cbook._lock_path(filename), open(filename, 'w') as fh:
         try:
             json.dump(data, fh, cls=_JSONEncoder, indent=2)
         except OSError as e:
@@ -1333,8 +1336,7 @@ def _rebuild():
     global fontManager
     _log.info("Generating new fontManager, this may take some time...")
     fontManager = FontManager()
-    with cbook._lock_path(_fmcache):
-        json_dump(fontManager, _fmcache)
+    json_dump(fontManager, _fmcache)
 
 
 try:
