[New-bugs-announce] [issue32221] Converting ipv6 address to string representation using getnameinfo() is wrong.

[Марк Коренберг]

New submission from Марк Коренберг <socketpair at gmail.com>:

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')
```

----------
components: Library (Lib)
messages: 307651
nosy: socketpair
priority: normal
severity: normal
status: open
title: Converting ipv6 address to string representation using getnameinfo() is wrong.
type: performance
versions: Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32221>
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