fix media server port allocation

the old code was allowing the main thread to read .port before it had
been updated, and was binding to sockets that were already in use on
Windows. instead, we use a system-assigned free port and block the main
thread until it's been allocated

aqt/main.py

@@ -1195,4 +1195,4 @@ Please ensure a profile is open and Anki is not busy, then try again."""),
         self.mediaServer.start()
 
     def baseHTML(self):
-        return '<base href="http://localhost:%d/">' % self.mediaServer.port
+        return '<base href="http://localhost:%d/">' % self.mediaServer.getPort()

aqt/mediasrv.py

@@ -32,21 +32,18 @@ class ThreadedHTTPServer(socketserver.ThreadingMixIn, http.server.HTTPServer):
 
 class MediaServer(threading.Thread):
 
+    _port = None
+    _ready = threading.Event()
+
     def run(self):
-        self.port = 10000
-        self.server = None
-        while not self.server:
-            try:
-                self.server = ThreadedHTTPServer(
-                        ("localhost", self.port), RequestHandler)
-            except OSError as e:
-                if e.errno == errno.EADDRINUSE:
-                    self.port += 1
-                    continue
-                raise
-            break
+        self.server = ThreadedHTTPServer(("localhost", 0), RequestHandler)
+        self._ready.set()
         self.server.serve_forever()
 
+    def getPort(self):
+        self._ready.wait()
+        return self.server.server_port
+
     def shutdown(self):
         self.server.shutdown()
 

aqt/webview.py

@@ -197,7 +197,7 @@ body { zoom: %f; %s }
 
     def webBundlePath(self, path):
         from aqt import mw
-        return "http://localhost:%d/_anki/%s" % (mw.mediaServer.port, path)
+        return "http://localhost:%d/_anki/%s" % (mw.mediaServer.getPort(), path)
 
     def bundledScript(self, fname):
         return '<script src="%s"></script>' % self.webBundlePath(fname)