Catch RemoteDisconnected case when starting downloading

A common error is to connect to the remote server successfully but raise a
`http_client.RemoteDisconnected` exception when trying to begin
downloading. Its caused by call `prox_rec_res.begin(...)` which calls
`http_client._read_status()`. In that case, we also add the target
`hostname:port` to the `bad_hostnames_ports` cache.

Modify 2 unit tests to clear the `bad_hostnames_ports` cache because
localhost is added from previous tests and this breaks them.

tests/test_warcprox.py

@@ -1986,6 +1986,10 @@ def test_socket_timeout_response(
 def test_empty_response(
         warcprox_, http_daemon, https_daemon, archiving_proxies,
         playback_proxies):
+    # localhost:server_port was added to the `bad_hostnames_ports` cache by
+    # previous tests and this causes subsequent tests to fail. We clear it.
+    warcprox_.proxy.bad_hostnames_ports.clear()
+
     url = 'http://localhost:%s/empty-response' % http_daemon.server_port
     response = requests.get(url, proxies=archiving_proxies, verify=False)
     assert response.status_code == 502
@@ -2001,6 +2005,10 @@ def test_payload_digest(warcprox_, http_daemon):
     Tests that digest is of RFC2616 "entity body"
     (transfer-decoded but not content-decoded)
     '''
+    # localhost:server_port was added to the `bad_hostnames_ports` cache by
+    # previous tests and this causes subsequent tests to fail. We clear it.
+    warcprox_.proxy.bad_hostnames_ports.clear()
+
     class HalfMockedMitm(warcprox.mitmproxy.MitmProxyHandler):
         def __init__(self, url):
             self.path = url

warcprox/mitmproxy.py

@@ -549,6 +549,18 @@ class MitmProxyHandler(http_server.BaseHTTPRequestHandler):
             if not is_connection_dropped(self._remote_server_conn):
                 self._conn_pool._put_conn(self._remote_server_conn)
         except Exception as e:
+            # A common error is to connect to the remote server successfully
+            # but raise a `RemoteDisconnected` exception when trying to begin
+            # downloading. Its caused by prox_rec_res.begin(...) which calls
+            # http_client._read_status(). In that case, the host is also bad
+            # and we must add it to `bad_hostnames_ports` cache.
+            if type(e) == http_client.RemoteDisconnected:
+                with self.server.bad_hostnames_ports_lock:
+                    host_port = self._hostname_port_cache_key()
+                    self.server.bad_hostnames_ports[host_port] = 1
+                    self.logger.info('bad_hostnames_ports cache size: %d',
+                                     len(self.server.bad_hostnames_ports))
+
             self._remote_server_conn.sock.shutdown(socket.SHUT_RDWR)
             self._remote_server_conn.sock.close()
             raise