https://github.com/python/cpython/pull/4724


`recvfrom` from multicast socket is painfull slow. In fact, it returns sender address in form:

`('fe80::941f:f6ff:fe04:c560%qwe', 42133, 0, 78)`
which is superfluous, since interface-name part (`%qwe`) is not actually used. Actually, scopeid (`78`) signify interface/scope/zone_id. This tuple can be used for `.sendto()` either with this interface-name-part or without.

The problem is in the performance. For each `recvrfom()`, `getnameinfo()` internally converts interface index to interface name using three syscalls, i.e. `socket(), getsockopt()?, close()` , which slows down receiving (I have not measured result, but see additional syscalls in `strace`).

In order to convert from tuple to string-based full address one may use `getnameinfo()`:
As you can see, initial interface is ignored (but without my patch it is also validated uselessly):
```
In[1]: socket.getnameinfo(('fe80::941f:f6ff:fe04:c560%qwe', 42133, 0, 78), socket.NI_NUMERICHOST)
Out[1]: ('fe80::941f:f6ff:fe04:c560%qwe', '42133')
In[2]: socket.getnameinfo(('fe80::941f:f6ff:fe04:c560', 42133, 0, 78), socket.NI_NUMERICHOST)
Out[2]: ('fe80::941f:f6ff:fe04:c560%qwe', '42133')
```

Please look at my PR

> `recvfrom` from multicast socket is painfull slow.

What do you mean? Can you give results for a benchmark of your choice?  How much does your PR speed it up?

Original (not patched) python:

```
In [1]s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
In [2]: s.bind(('ff02::1de:c0db', 1234, 0, 2))
In [3]: timeit s.getsockname()
The slowest run took 12.06 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 3.8 µs per loop

In [5]: d = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
In [6]: d.bind(('::', 1235, 0, 0))
In [7]: timeit d.getsockname()
The slowest run took 23.18 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 532 ns per loop
```

On patched version, times for both cases should be the same exactly (i.e. 532 ns)

So it may affect applications, that work with UDP and .recvfrom()

Wow.  That's a heavy price to pay indeed.

Unfortunately I'm not competent on IPv6.  I'm cc-ing other core developers so that they may chime in.

After my patch:

$ python3.7 -m timeit --setup 'import socket; s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM); s.bind(("ff02::1de:c0db", 1234, 0, 2))' 's.getsockname()'
500000 loops, best of 5: 613 nsec per loop

$ python3.7 -m timeit --setup 'import socket; s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM); s.bind(("::", 1235, 0, 0))' 's.getsockname()'
500000 loops, best of 5: 420 nsec per loop

I thought the improvements were way bigger than 200nsec, no?

No, Improvement for typical case (not scoped IPv6 addresses) was not the target, but fortunatelly it also sped up.

The PR looks good, but I don't have the capacity right now to merge it. Ill need some time for research to double check if it's ok to drop scope id. Will try to find it in the next few days.

So, PR is ready. Please review.

> So, PR is ready. Please review.

I understand, however I still don't have time for it at the moment. If another core dev wants to take initiative, please by all means do.

I can/will take another look at this after January 15.

New changeset 47c0b1f7d4115e6f15e6776c1f91d28e7d96fe0c by Yury Selivanov (Коренберг Марк) in branch 'master':
bpo-32221: makeipaddr(): remove interface part + speedup (GH-4724)
https://github.com/python/cpython/commit/47c0b1f7d4115e6f15e6776c1f91d28e7d96fe0c

Merged.  Thanks, Mark!

Mark,

Two tests fail on MacOS X now (newly added tests).  I'm reverting the change for now.  If you figure this out, please re-submit a PR for a quick approval.


======================================================================
ERROR: test_getaddrinfo_ipv6_scopeid (test.test_socket.GeneralModuleTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/yury/dev/pydev/cpython/Lib/test/test_socket.py", line 1612, in test_getaddrinfo_ipv6_scopeid
    ifindex = socket.if_nametoindex(test_interface)
OSError: no interface with this name

======================================================================
ERROR: test_getnameinfo_ipv6_scopeid (test.test_socket.GeneralModuleTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/yury/dev/pydev/cpython/Lib/test/test_socket.py", line 1636, in test_getnameinfo_ipv6_scopeid
    nameinfo = socket.getnameinfo(sockaddr, socket.NI_NUMERICHOST | socket.NI_NUMERICSERV)
socket.gaierror: [Errno 4] Non-recoverable failure in name resolution

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

New changeset 0ceb717689b04c0540d78c1ba93c0572c66c0994 by Yury Selivanov in branch 'master':
Revert "bpo-32221: makeipaddr(): remove interface part + speedup (GH-4724)" (#5394)
https://github.com/python/cpython/commit/0ceb717689b04c0540d78c1ba93c0572c66c0994

Mark, I think this still land in beta-2.  Please try to figure out what's going on on Mac OS (I have no time to take care of this myself).

New changeset 7766b96ab80b04509bbac708ee5ecf3c1c5934fc by Yury Selivanov (Коренберг Марк) in branch 'master':
bpo-32221: makeipaddr(): remove interface part + speedup (GH-5449) (#5449)
https://github.com/python/cpython/commit/7766b96ab80b04509bbac708ee5ecf3c1c5934fc

New changeset 0442599961f966a3dc7f3fe6a3c0d5765fcf2082 by Yury Selivanov (Miss Islington (bot)) in branch '3.7':
bpo-32221: makeipaddr(): remove interface part + speedup (GH-5449) (GH-5449) (#5641)
https://github.com/python/cpython/commit/0442599961f966a3dc7f3fe6a3c0d5765fcf2082