More intelligent harfbuzz choosing. Fixes rendering segfaults.

TODO

@@ -44,7 +44,6 @@ Improvements / minor features
 
 - Print nicer error message when platform plugin is missing
   (on Arch: libxkbcommon-x11)
-- Force new harfbuzz engine with linux and 5.3.0
 - Add harfbuzz forcing to version info
 - Print some meaningful message (via qInstallMsgHandler) when platform plugins
   are missing, especially with xcb (on Arch: libxkbcommon-x11)

qutebrowser/utils/earlyinit.py

@@ -66,16 +66,38 @@ def fix_harfbuzz(args):
         args: The argparse namespace.
     """
     from qutebrowser.utils.log import init as logger
+    from PyQt5.QtCore import qVersion
     if 'PyQt5.QtWidgets' in sys.modules:
         logger.warning("Harfbuzz fix attempted but QtWidgets is already "
                        "imported!")
     if sys.platform.startswith('linux') and args.harfbuzz == 'auto':
-        os.environ['QT_HARFBUZZ'] = 'old'
-    elif args.harfbuzz == 'old':
-        os.environ['QT_HARFBUZZ'] = 'old'
-    elif args.harfbuzz == 'new':
-        os.environ['QT_HARFBUZZ'] = 'new'
-    # else: use system default harfbuzz
+        # Lets use the most stable variant.
+        #
+        # - On Qt 5.2 (and probably earlier) the new engine probably has more
+        #   crashes and is also experimental.
+        #
+        # - On Qt 5.3.0 there's a bug that affects a lot of websites:
+        #   https://bugreports.qt-project.org/browse/QTBUG-39278
+        #   So the new engine will be more stable.
+        #
+        # - On Qt 5.3.1 this bug hopefully will be fixed and the old engine
+        #   will be the more stable one again.
+        if qVersion() == '5.3.0':
+            logger.debug("Using new harfbuzz engine (auto)")
+            os.environ['QT_HARFBUZZ'] = 'new'
+        else:
+            logger.debug("Using old harfbuzz engine (auto)")
+            os.environ['QT_HARFBUZZ'] = 'old'
+    elif args.harfbuzz in ['old', 'new']:
+        # forced harfbuzz variant
+        # FIXME looking at the Qt code, 'new' isn't a valid value, but leaving
+        # it empty and using new yields different behaviour...
+        logger.debug("Using {} harfbuzz engine (forced)".format(args.harfbuzz))
+        os.environ['QT_HARFBUZZ'] = args.harfbuzz
+    else:
+        # use system default harfbuzz
+        logger.debug("Using system harfbuzz engine")
+        pass
 
 
 # At this point we can safely import Qt stuff, but we can't be sure it's