net: download_using_mirrors() to download like fdroidclient does

tests/net.TestCase

@@ -121,6 +121,28 @@ class NetTest(unittest.TestCase):
             net.download_file('http://localhost:%d/f.txt' % server.port)
         server.stop()
 
+    def test_download_using_mirrors_retries(self):
+        server = RetryServer()
+        f = net.download_using_mirrors(
+            [
+                'https://fake.com/f.txt',  # 404 or 301 Redirect
+                'https://httpbin.org/status/403',
+                'https://httpbin.org/status/500',
+                'http://localhost:1/f.txt',  # ConnectionError
+                'http://localhost:%d/' % server.port,
+            ],
+        )
+        # strip the HTTP headers and compare the reply
+        self.assertEqual(server.reply.split(b'\n\n')[1], Path(f).read_bytes())
+        server.stop()
+
+    def test_download_using_mirrors_retries_not_forever(self):
+        """The retry logic should eventually exit with an error."""
+        server = RetryServer(failures=5)
+        with self.assertRaises(requests.exceptions.ConnectionError):
+            net.download_using_mirrors(['http://localhost:%d/' % server.port])
+        server.stop()
+
 
 if __name__ == "__main__":
     os.chdir(os.path.dirname(__file__))