On X11 initialize fontconfig in the GUI thread as Qt also uses fontconfig internally and fontconfig is not thread safe

src/calibre/gui2/convert/mobi_output.py

@@ -11,7 +11,6 @@ from PyQt4.Qt import Qt
 from calibre.gui2.convert.mobi_output_ui import Ui_Form
 from calibre.gui2.convert import Widget
 from calibre.gui2.widgets import FontFamilyModel
-from calibre.utils.fonts import fontconfig
 
 font_family_model = None
 
@@ -28,6 +27,7 @@ class PluginWidget(Widget, Ui_Form):
                     'mobi_ignore_margins',
                 'dont_compress', 'no_inline_toc', 'masthead_font','personal_doc']
                 )
+        from calibre.utils.fonts import fontconfig
         self.db, self.book_id = db, book_id
 
         global font_family_model

src/calibre/gui2/widgets.py

@@ -19,7 +19,6 @@ from calibre.gui2 import NONE, error_dialog, pixmap_to_data, gprefs
 from calibre.constants import isosx
 from calibre.gui2.filename_pattern_ui import Ui_Form
 from calibre import fit_image
-from calibre.utils.fonts import fontconfig
 from calibre.ebooks import BOOK_EXTENSIONS
 from calibre.ebooks.metadata.meta import metadata_from_filename
 from calibre.utils.config import prefs, XMLConfig
@@ -283,6 +282,7 @@ class FontFamilyModel(QAbstractListModel):
 
     def __init__(self, *args):
         QAbstractListModel.__init__(self, *args)
+        from calibre.utils.fonts import fontconfig
         try:
             self.families = fontconfig.find_font_families()
         except:

src/calibre/utils/fonts/__init__.py

@@ -7,15 +7,19 @@ __copyright__ = '2009, Kovid Goyal <kovid@kovidgoyal.net>'
 __docformat__ = 'restructuredtext en'
 
 import os, sys
-from threading import Thread
 
-from calibre.constants import plugins, iswindows
+from calibre.constants import plugins, iswindows, islinux, isfreebsd
 
 _fc, _fc_err = plugins['fontconfig']
 
 if _fc is None:
     raise RuntimeError('Failed to load fontconfig with error:'+_fc_err)
 
+if islinux or isfreebsd:
+    Thread = object
+else:
+    from threading import Thread
+
 class FontConfig(Thread):
 
     def __init__(self):
@@ -45,6 +49,7 @@ class FontConfig(Thread):
             self.failed = True
 
     def wait(self):
+        if not (islinux or isfreebsd):
             self.join()
         if self.failed:
             raise RuntimeError('Failed to initialize fontconfig')
@@ -144,6 +149,12 @@ class FontConfig(Thread):
         return fonts if all else (fonts[0] if fonts else None)
 
 fontconfig = FontConfig()
+if islinux or isfreebsd:
+    # On X11 Qt also uses fontconfig, so initialization must happen in the
+    # main thread. In any case on X11 initializing fontconfig should be very
+    # fast
+    fontconfig.run()
+else:
     fontconfig.start()
 
 def test():