The IPv6 detection in asyncio.base_events.create_server only detect if IPv6 is available instead of checking if the interface can actually support it.

I noticed that by using Python in a Docker container (example code to reproduce in attachment):


docker run -it --rm -v /tmp/test_ipv6.py:/src/test_ipv6.py python:3.6 python /src/test_ipv6.py


Will result in:


Traceback (most recent call last):
  File "/usr/local/lib/python3.6/asyncio/base_events.py", line 1043, in create_server
    sock.bind(sa)
OSError: [Errno 99] Cannot assign requested address

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/src/test_ipv6.py", line 11, in <module>
    server = loop.run_until_complete(server_creation)
  File "/usr/local/lib/python3.6/asyncio/base_events.py", line 466, in run_until_complete
    return future.result()
  File "/usr/local/lib/python3.6/asyncio/base_events.py", line 1047, in create_server
    % (sa, err.strerror.lower()))
OSError: [Errno 99] error while attempting to bind on address ('::1', 27015, 0, 0): cannot assign requested address


By default Docker containers have only IPv4 enabled:


1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
38: eth0@if39: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default 
    link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.2/16 scope global eth0
       valid_lft forever preferred_lft forever


I believe this detection mechanism should rely on the interface requested. I found this on the web for Python 2 that manage to get the info per interface: https://pastebin.com/VEnhF1Ht but it's using an external library.

However if you change the hostname to 127.0.0.1 it works normally.

Better than trying to detect IPv6 compatibility beforehand would probably to recognize the error and simply ignore it.

Note: errno 99 is EADDRNOTAVAIL.

Something like this could work (untested):

diff --git a/Lib/asyncio/base_events.py b/Lib/asyncio/base_events.py
index 33b8f48..413161a 100644
--- a/Lib/asyncio/base_events.py
+++ b/Lib/asyncio/base_events.py
@@ -1038,6 +1038,11 @@ class BaseEventLoop(events.AbstractEventLoop):
                     try:
                         sock.bind(sa)
                     except OSError as err:
+                        if err.errno == errno.EADDRNOTAVAIL:
+                            # See bpo-30945
+                            sockets.pop()
+                            sock.close()
+                            continue
                         raise OSError(err.errno, 'error while attempting '
                                       'to bind on address %r: %s'
                                       % (sa, err.strerror.lower())) from None

> Better than trying to detect IPv6 compatibility beforehand would probably to recognize the error and simply ignore it.


+1

Cécile, could you try the following patch?  I have no easy way to test here.

Sure! It seems to work, the process returns an exit code of 0 and I see no traceback but the message is still displayed in the terminal.

(Also I did something weird because your patch applies on branch master and I ran it with Python 3.6... I suppose it shouldn't be a problem)


[0] [11:54:13] /tmp > d run -it --rm -v /tmp:/tmp:ro -v ~/repos/cpython/Lib/asyncio:/usr/local/lib/python3.6/asyncio:ro python:3.6 python /tmp/test_ipv6.py
error while attempting to bind on address ('::1', 27015, 0, 0): cannot assign requested address
[0] [11:54:19] /tmp > d run -it --rm -v /tmp:/tmp:ro python:3.6 python /tmp/test_ipv6.py
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/asyncio/base_events.py", line 1043, in create_server
    sock.bind(sa)
OSError: [Errno 99] Cannot assign requested address

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/tmp/test_ipv6.py", line 11, in <module>
    server = loop.run_until_complete(server_creation)
  File "/usr/local/lib/python3.6/asyncio/base_events.py", line 466, in run_until_complete
    return future.result()
  File "/usr/local/lib/python3.6/asyncio/base_events.py", line 1047, in create_server
    % (sa, err.strerror.lower()))
OSError: [Errno 99] error while attempting to bind on address ('::1', 27015, 0, 0): cannot assign requested address
[1] [11:54:52] /tmp >

Cécile, thank you.  The reason the message is still displayed is that I turned the error into a warning (in other words, it is expected).

Seeing this as well when running the cpython test suite in docker:

```
$ ./python -m test.test_asyncio

...

[18 similar traces omitted]
======================================================================
ERROR: test_sock_sendfile_zero_size (test.test_asyncio.test_unix_events.SelectorEventLoopUnixSockSendfileTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/code/Lib/test/test_asyncio/test_unix_events.py", line 559, in test_sock_sendfile_zero_size
    sock, proto = self.prepare()
  File "/code/Lib/test/test_asyncio/test_unix_events.py", line 483, in prepare
    lambda: proto, support.HOST, port))
  File "/code/Lib/test/test_asyncio/test_unix_events.py", line 476, in run_loop
    return self.loop.run_until_complete(coro)
  File "/code/Lib/asyncio/base_events.py", line 566, in run_until_complete
    return future.result()
  File "/code/Lib/asyncio/base_events.py", line 1346, in create_server
    % (sa, err.strerror.lower())) from None
OSError: [Errno 99] error while attempting to bind on address ('::1', 39527, 0, 0): cannot assign requested address

----------------------------------------------------------------------

```


I'm going to try and write a patch to skip these tests (there's already a helper)

Actually, my issue seems to be something more strange.

The host being passed in is `localhost` which resolves to:

```
>>> pprint.pprint(socket.getaddrinfo('localhost', 80))
[(<AddressFamily.AF_INET: 2>,
  <SocketKind.SOCK_STREAM: 1>,
  6,
  '',
  ('127.0.0.1', 80)),
 (<AddressFamily.AF_INET: 2>,
  <SocketKind.SOCK_DGRAM: 2>,
  17,
  '',
  ('127.0.0.1', 80)),
 (<AddressFamily.AF_INET: 2>,
  <SocketKind.SOCK_RAW: 3>,
  0,
  '',
  ('127.0.0.1', 80)),
 (<AddressFamily.AF_INET6: 10>,
  <SocketKind.SOCK_STREAM: 1>,
  6,
  '',
  ('::1', 80, 0, 0)),
 (<AddressFamily.AF_INET6: 10>,
  <SocketKind.SOCK_DGRAM: 2>,
  17,
  '',
  ('::1', 80, 0, 0)),
 (<AddressFamily.AF_INET6: 10>,
  <SocketKind.SOCK_RAW: 3>,
  0,
  '',
  ('::1', 80, 0, 0))]
```

asyncio is picking ipv6 because of this code:

https://github.com/python/cpython/blob/a445feb72902e4a3c5ae712f0c289309e1580d52/Lib/asyncio/base_events.py#L1334-L1340

despite my host not actually having an ipv6 network hooked up.

Applying this patch makes the tests pass for me, but I don't think the patch is appropriate (just hides the bug):

```
$ git diff
diff --git a/Lib/test/test_asyncio/test_unix_events.py b/Lib/test/test_asyncio/test_unix_events.py
index 5bd76d3..ff6c4e1 100644
--- a/Lib/test/test_asyncio/test_unix_events.py
+++ b/Lib/test/test_asyncio/test_unix_events.py
@@ -480,7 +480,7 @@ class SelectorEventLoopUnixSockSendfileTests(test_utils.TestCase):
         proto = self.MyProto(self.loop)
         port = support.find_unused_port()
         server = self.run_loop(self.loop.create_server(
-            lambda: proto, support.HOST, port))
+            lambda: proto, support.HOSTv4, port))
         self.run_loop(self.loop.sock_connect(sock, (support.HOST, port)))
 
         def cleanup():
```

I see that the patch hasn't been applied to master on GitHub. Is there anything else expected from me on this ticket?

Does anybody wants to make a PR to fix this?

> asyncio is picking ipv6 because of this code:

> https://github.com/python/cpython/blob/a445feb72902e4a3c5ae712f0c289309e1580d52/Lib/asyncio/base_events.py#L1334-L1340

> despite my host not actually having an ipv6 network hooked up.

Which should be fine; create_server enumerates all addresses and tries to connect to each one. IPV6_V6ONLY is only applied to one socket for one IPv6 address.  I think Antoine's patch is OK.