support jumps in limit downloads

like when unchecking 'limit downloads' or entering a value manually in limit downloads.
Fixes #340

src/gpodder/download.py

@@ -401,12 +401,15 @@ class DownloadQueueManager(object):
         """Spawn new worker threads if necessary
         """
         with self.worker_threads_access:
-            if not self.tasks.has_work():
-                return
-
-            if len(self.worker_threads) == 0 or \
-                    len(self.worker_threads) < self._config.max_downloads or \
-                    not self._config.max_downloads_enabled:
+            work_count = self.tasks.available_work_count()
+            if self._config.max_downloads_enabled:
+                # always allow at least 1 download
+                max_downloads = max(int(self._config.max_downloads), 1)
+                spawn_limit = max_downloads - len(self.worker_threads)
+            else:
+                spawn_limit = work_count
+            logger.info('%r tasks to do, can start at most %r threads', work_count, spawn_limit)
+            for i in range(0, min(work_count, spawn_limit)):
                 # We have to create a new thread here, there's work to do
                 logger.info('Starting new worker thread.')
 

src/gpodder/gtkui/download.py

@@ -142,18 +142,22 @@ class DownloadStatusModel(Gtk.ListStore):
         return False
 
     def has_work(self):
-        return any(task for task in
-                (row[DownloadStatusModel.C_TASK] for row in self)
-                if task.status == task.QUEUED)
+        return any(self._work_gen())
+
+    def available_work_count(self):
+        return len(list(self._work_gen()))
 
     def get_next(self):
         with self.set_downloading_access:
-            result = next(task for task in
-                    (row[DownloadStatusModel.C_TASK] for row in self)
-                    if task.status == task.QUEUED)
+            result = next(self._work_gen())
             self.set_downloading(result)
         return result
 
+    def _work_gen(self):
+        return (task for task in
+                    (row[DownloadStatusModel.C_TASK] for row in self)
+                    if task.status == task.QUEUED)
+
     def set_downloading(self, task):
         with self.set_downloading_access:
             if task.status is task.DOWNLOADING:

src/gpodder/gtkui/main.py

@@ -170,6 +170,7 @@ class gPodder(BuilderWidget, dbus.service.Object):
         # When the amount of maximum downloads changes, notify the queue manager
         changed_cb = lambda spinbutton: self.download_queue_manager.update_max_downloads()
         self.spinMaxDownloads.connect('value-changed', changed_cb)
+        self.cbMaxDownloads.connect('toggled', changed_cb)
 
         # Keep a reference to the last add podcast dialog instance
         self._add_podcast_dialog = None