Clean up NetworkManager after downloads finished.

Fixes #490.

qutebrowser/browser/downloads.py

@@ -797,14 +797,19 @@ class DownloadManager(QAbstractListModel):
         Return:
             A boolean.
         """
+        assert nam.adopted_downloads == 0
         for download in self.downloads:
-            if download.reply is not None and download.reply.manager() is nam:
-                return True
-            if (download.done and (not download.successful) and
-                    download.retry_info.manager is nam):
-                # user could request retry after tab is closed.
-                return True
-        return False
+            running_download = (download.reply is not None and
+                                download.reply.manager() is nam)
+            # user could request retry after tab is closed.
+            failed_download = (download.done and (not download.successful) and
+                               download.retry_info.manager is nam)
+            if running_download or failed_download:
+                log.downloads.debug("Found running/failed downloads, "
+                                    "adopting the NAM.")
+                nam.adopted_downloads += 1
+                download.destroyed.connect(nam.on_adopted_download_destroyed)
+        return nam.adopted_downloads
 
     def can_clear(self):
         """Check if there are finished downloads to clear."""

qutebrowser/browser/network/networkmanager.py

@@ -43,6 +43,11 @@ class NetworkManager(QNetworkAccessManager):
     """Our own QNetworkAccessManager.
 
     Attributes:
+        adopted_downloads: If downloads are running with this QNAM but the
+                           associated tab gets closed already, the NAM gets
+                           reparented to the DownloadManager. This counts the
+                           still running downloads, so the QNAM can clean
+                           itself up when this reaches zero again.
         _requests: Pending requests.
         _scheme_handlers: A dictionary (scheme -> handler) of supported custom
                           schemes.
@@ -62,6 +67,7 @@ class NetworkManager(QNetworkAccessManager):
             # http://www.riverbankcomputing.com/pipermail/pyqt/2014-November/035045.html
             super().__init__(parent)
         log.init.debug("NetworkManager init done")
+        self.adopted_downloads = 0
         self._win_id = win_id
         self._tab_id = tab_id
         self._requests = []
@@ -207,6 +213,19 @@ class NetworkManager(QNetworkAccessManager):
             # switched from private mode to normal mode
             self._set_cookiejar()
 
+    @pyqtSlot()
+    def on_adopted_download_destroyed(self):
+        """Check if we can clean up if an adopted download was destroyed.
+
+        See the description for adopted_downloads for details.
+        """
+        self.adopted_downloads -= 1
+        log.downloads.debug("Adopted download destroyed, {} left.".format(
+            self.adopted_downloads))
+        assert self.adopted_downloads >= 0
+        if self.adopted_downloads == 0:
+            self.deleteLater()
+
     # WORKAROUND for:
     # http://www.riverbankcomputing.com/pipermail/pyqt/2014-September/034806.html
     #