Jim Crist-Harif <[email protected]> added the comment:
> Is tornado the only example or you are aware of other libraries with such
> behavior?
A quick survey of other language network stacks didn't turn anything up, *But*
I also didn't find any implementations (other than asyncio & tornado) that bind
multiple sockets with a single api call (as `create_server` does).
I think part of the issue here is that dual IPV6 & IPV4 support is
intentionally disabled in asyncio (and tornado), so two sockets are needed (one
to support each interface). Other TCP implementations (e.g. both go and rust)
don't disable this, so one listener == one socket. This makes comparing API
designs across stacks harder - with e.g. Go it's straightforward to listen on a
random port on IPV4 & IPV6 with a single TCPListener, since both can be handled
by a single socket. Since this is disabled (by default) in asyncio we end up
using 2 sockets and run into the issue described above.
Also note that this issue will trigger for any address that resolves to
multiple interfaces (not just `host=""`). For example, on osx `localhost` will
resolve to `::1` and `127.0.0.1` by default, meaning that the following fairly
straightforward asyncio code has a bug in it:
```python
# Start a server on localhost with a random port
server = await loop.create_server(
EchoServerProtocol,
host="localhost",
port=0
)
# Retrieve and log the port
port = server.sockets[0].getsockname()[1]
print(f"listening at tcp://localhost:{port}")
```
As written, this looks correct enough, but on systems where localhost resolves
to multiple interfaces this will accidentally listen on multiple ports (instead
of one). This can be fixed with some additional logic external to asyncio, but
it makes for a much less straightforward asyncio example.
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<https://bugs.python.org/issue45693>
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