Reenable QtWebKit cache with Qt 5.9.

This was fixed here:
https://codereview.qt-project.org/#/c/190818/

See #2427

CHANGELOG.asciidoc

@@ -43,8 +43,8 @@ Changed
 - PAC on QtWebKit now supports SOCKS5 as type.
 - Comments in the config file are now before the individual
   options instead of being before sections.
-- The HTTP cache is disabled with QtWebKit on Qt 5.8 now as it leads to frequent
-  crashes due to a Qt bug.
+- The HTTP cache is disabled with QtWebKit on Qt 5.7.1 and 5.8 now as it leads
+  to frequent crashes due to a Qt bug.
 - stdin is now closed immediately for processes spawned from qutebrowser
 - When ui -> message-timeout is set to 0, messages are now never cleared.
 

qutebrowser/browser/webkit/cache.py

@@ -54,7 +54,8 @@ class DiskCache(QNetworkDiskCache):
         if size is None:
             size = 1024 * 1024 * 50  # default from QNetworkDiskCachePrivate
         # WORKAROUND for https://bugreports.qt.io/browse/QTBUG-59909
-        if qtutils.version_check('5.7.1'):  # pragma: no cover
+        if (qtutils.version_check('5.7.1') and  # pragma: no cover
+                not qtutils.version_check('5.9')):
             size = 0
         self.setMaximumCacheSize(size)
 

tests/unit/browser/webkit/test_cache.py

@@ -25,9 +25,9 @@ from qutebrowser.browser.webkit import cache
 from qutebrowser.utils import qtutils
 
 
-pytestmark = pytest.mark.skipif(qtutils.version_check('5.7.1'),
-                                reason="QNetworkDiskCache is broken on Qt >= "
-                                       "5.7.1")
+pytestmark = pytest.mark.skipif(
+    qtutils.version_check('5.7.1') and not qtutils.version_check('5.9'),
+    reason="QNetworkDiskCache is broken on Qt 5.7.1 and 5.8")
 
 
 def preload_cache(cache, url='http://www.example.com/', content=b'foobar'):