Disallow PAC proxies with QtWebEngine

See #4577

doc/changelog.asciidoc

@@ -85,6 +85,8 @@ Fixed
 - The outdated header `X-Do-Not-Track` is no longer sent.
 - A javascript error on page load when using Qt 5.12 was fixed.
 - `window.print()` works with Qt 5.12 now.
+- PAC proxies were never correctly supported with QtWebEngine, but are now
+  explicitly disallowed.
 
 v1.5.2
 ------

qutebrowser/browser/network/pac.py

@@ -208,6 +208,8 @@ class PACResolver:
         Return:
             A list of QNetworkProxy objects in order of preference.
         """
+        qtutils.ensure_valid(query.url())
+
         if from_file:
             string_flags = QUrl.PrettyDecoded
         else:

qutebrowser/browser/network/proxy.py

@@ -20,10 +20,12 @@
 """Handling of proxies."""
 
 
+from PyQt5.QtCore import QUrl
 from PyQt5.QtNetwork import QNetworkProxy, QNetworkProxyFactory
 
 from qutebrowser.config import config, configtypes
-from qutebrowser.utils import objreg
+from qutebrowser.utils import objreg, message, usertypes, urlutils
+from qutebrowser.misc import objects
 from qutebrowser.browser.network import pac
 
 
@@ -33,6 +35,18 @@ def init():
     objreg.register('proxy-factory', proxy_factory)
     QNetworkProxyFactory.setApplicationProxyFactory(proxy_factory)
 
+    config.instance.changed.connect(_warn_for_pac)
+    _warn_for_pac()
+
+
+@config.change_filter('content.proxy', function=True)
+def _warn_for_pac():
+    """Show a warning if PAC is used with QtWebEngine."""
+    proxy = config.val.content.proxy
+    if (isinstance(proxy, pac.PACFetcher) and
+            objects.backend == usertypes.Backend.QtWebEngine):
+        message.error("PAC support isn't implemented for QtWebEngine yet!")
+
 
 def shutdown():
     QNetworkProxyFactory.setApplicationProxyFactory(None)
@@ -70,6 +84,10 @@ class ProxyFactory(QNetworkProxyFactory):
             # ref. http://doc.qt.io/qt-5/qnetworkproxyfactory.html#systemProxyForQuery
             proxies = QNetworkProxyFactory.systemProxyForQuery(query)
         elif isinstance(proxy, pac.PACFetcher):
+            if objects.backend == usertypes.Backend.QtWebEngine:
+                # Looks like query.url() is always invalid on QtWebEngine...
+                proxies = [urlutils.proxy_from_url(QUrl('direct://'))]
+            else:
                 proxies = proxy.resolve(query)
         else:
             proxies = [proxy]