Fix -4/-6 crash caused by broken filter statement

In python3 filter returns a generator instead of a list. When -4 or
-6 is used or if the host system has either of those address
families disabled len() gets called onto this filter expression
which results into a crash. This commit makes a filter expression
out of this statement and also adds a test for ipv4-only downloading.

servefile

@@ -849,9 +849,9 @@ class ServeFile():
 
 			# filter out ips we are not listening on
 			if not self.listenIPv6:
-				ips = filter(lambda ip: ":" not in ip, ips)
+				ips = [ip for ip in ips if '.' in ip]
 			if not self.listenIPv4:
-				ips = filter(lambda ip: "." not in ip, ips)
+				ips = [ip for ip in ips if ':' in ip]
 
 			return ips
 		return None

tests/test_servefile.py

@@ -2,6 +2,7 @@ import io
 import os
 import pytest
 import requests
+import socket
 import subprocess
 import tarfile
 import time
@@ -9,6 +10,12 @@ import urllib3
 
 # crudly written to learn more about pytest and to have a base for refactoring
 
+try:
+    ConnectionRefusedError
+    connrefused_exc = ConnectionRefusedError
+except NameError:
+    connrefused_exc = socket.error
+
 
 @pytest.fixture
 def run_servefile():
@@ -118,6 +125,18 @@ def test_specify_port(run_servefile, datadir):
     check_download(data, fname='testfile', port=8081)
 
 
+def test_ipv4_only(run_servefile, datadir):
+    data = "NOOT NOOT"
+    p = datadir({'testfile': data}) / 'testfile'
+    run_servefile([str(p), '-4'])
+
+    check_download(data, fname='testfile', host='127.0.0.1')
+
+    sock = socket.socket(socket.AF_INET6)
+    with pytest.raises(connrefused_exc):
+        sock.connect(("::1", 8080))
+
+
 def test_big_download(run_servefile, datadir):
     # test with about 10 mb of data
     data = "x" * (10 * 1024 ** 2)