Support retrying downloads with Qt 5.10

Fixes #2787

doc/changelog.asciidoc

@@ -25,6 +25,7 @@ Added
 - A new `qute://bindings` page, opened by `:bind`, shows all keybindings.
 - `:session-load` has a new `--delete` flag which deletes the
   session after loading it.
+- QtWebEngine: Retrying downloads is now supported with Qt 5.10 or newer.
 
 Changed
 ~~~~~~~

qutebrowser/browser/webengine/webenginedownloads.py

@@ -96,9 +96,15 @@ class DownloadItem(downloads.AbstractDownloadItem):
         self._qt_item.cancel()
 
     def retry(self):
-        # https://bugreports.qt.io/browse/QTBUG-56840
-        raise downloads.UnsupportedOperationError(
-            "Retrying downloads is unsupported with QtWebEngine")
+        state = self._qt_item.state()
+        assert state == QWebEngineDownloadItem.DownloadInterrupted, state
+
+        try:
+            self._qt_item.resume()
+        except AttributeError:
+            raise downloads.UnsupportedOperationError(
+                "Retrying downloads is unsupported with QtWebEngine on "
+                "Qt/PyQt < 5.10")
 
     def _get_open_filename(self):
         return self._filename

tests/end2end/features/downloads.feature

@@ -193,13 +193,23 @@ Feature: Downloading things from a website.
             does-not-exist
             does-not-exist
 
-    @qtwebkit_skip
-    Scenario: Retrying a failed download with QtWebEngine
+    @qtwebkit_skip @qt<5.10
+    Scenario: Retrying a failed download with QtWebEngine (Qt < 5.10)
         When I open data/downloads/issue2298.html
         And I run :click-element id download
         And I wait for "Download error: *" in the log
         And I run :download-retry
-        Then the error "Retrying downloads is unsupported with QtWebEngine" should be shown
+        Then the error "Retrying downloads is unsupported *" should be shown
+
+    @qtwebkit_skip @qt>=5.10
+    Scenario: Retrying a failed download with QtWebEngine (Qt >= 5.10)
+        When I open data/downloads/issue2298.html
+        And I run :click-element id download
+        And I wait for "Download error: *" in the log
+        And I run :download-retry
+        # For some reason it doesn't actually try again here, but let's hope it
+        # works e.g. on a connection loss, which we can't test automatically.
+        Then "Retrying downloads is unsupported *" should not be logged
 
     Scenario: Retrying with count
         When I run :download http://localhost:(port)/data/downloads/download.bin