unvendorable dnslib (#887);

stolen/dnslib/ can be deleted and replaced with system lib

NOTE: unvendoring dnslib will make it impossible to communicate with
  devices which have a particular avahi bug; see 6c1cf68bca

README.md

@@ -2999,6 +2999,7 @@ force use of system modules  instead of the vendored versions:
 | env-var              | what it does |
 | -------------------- | ------------ |
 | `PRTY_SYS_ALL`       | all of the below |
+| `PRTY_SYS_DNSLIB`    | replace [stolen/dnslib](./copyparty/stolen/dnslib) with [upstream](https://pypi.org/project/dnslib/) |
 | `PRTY_SYS_IFADDR`    | replace [stolen/ifaddr](./copyparty/stolen/ifaddr) with [upstream](https://pypi.org/project/ifaddr/) |
 | `PRTY_SYS_QRCG`      | replace [stolen/qrcodegen.py](./copyparty/stolen/qrcodegen.py) with [upstream](https://github.com/nayuki/QR-Code-generator/blob/master/python/qrcodegen.py) |
 

copyparty/mdns.py

@@ -2,6 +2,7 @@
 from __future__ import print_function, unicode_literals
 
 import errno
+import os
 import random
 import select
 import socket
@@ -12,31 +13,64 @@ from ipaddress import IPv4Network, IPv6Network
 from .__init__ import TYPE_CHECKING
 from .__init__ import unicode as U
 from .multicast import MC_Sck, MCast
-from .stolen.dnslib import (
-    AAAA,
-)
-from .stolen.dnslib import CLASS as DC
-from .stolen.dnslib import (
-    NSEC,
-    PTR,
-    QTYPE,
-    RR,
-    SRV,
-    TXT,
-    A,
-    DNSHeader,
-    DNSQuestion,
-    DNSRecord,
-    set_avahi_379,
-)
 from .util import IP6_LL, CachedSet, Daemon, Netdev, list_ips, min_ex
 
+try:
+    if os.getenv("PRTY_SYS_ALL") or os.getenv("PRTY_SYS_DNSLIB"):
+        raise ImportError()
+    from .stolen.dnslib import (
+        AAAA,
+    )
+    from .stolen.dnslib import CLASS as DC
+    from .stolen.dnslib import (
+        NSEC,
+        PTR,
+        QTYPE,
+        RR,
+        SRV,
+        TXT,
+        A,
+        DNSHeader,
+        DNSQuestion,
+        DNSRecord,
+        set_avahi_379,
+    )
+
+    DNS_VND = True
+except ImportError:
+    DNS_VND = False
+    from dnslib import (
+        AAAA,
+    )
+    from dnslib import CLASS as DC
+    from dnslib import (
+        NSEC,
+        PTR,
+        QTYPE,
+        RR,
+        SRV,
+        TXT,
+        A,
+        Bimap,
+        DNSHeader,
+        DNSQuestion,
+        DNSRecord,
+    )
+
+    DC.forward[0x8001] = "F_IN"
+    DC.reverse["F_IN"] = 0x8001
+
 if TYPE_CHECKING:
     from .svchub import SvcHub
 
 if True:  # pylint: disable=using-constant-test
     from typing import Any, Optional, Union
 
+if os.getenv("PRTY_MODSPEC"):
+    from inspect import getsourcefile
+
+    print("PRTY_MODSPEC: dnslib:", getsourcefile(A))
+
 
 MDNS4 = "224.0.0.251"
 MDNS6 = "ff02::fb"
@@ -75,7 +109,7 @@ class MDNS(MCast):
         self.ngen = ngen
         self.ttl = 300
 
-        if not self.args.zm_nwa_1:
+        if not self.args.zm_nwa_1 and DNS_VND:
             set_avahi_379()
 
         zs = self.args.zm_fqdn or (self.args.name + ".local")