Explicit port 0 replaced with default port

[gmacon]

Please confirm the following

• I agree to follow the aio-libs Code of Conduct

• I have checked the current issues for duplicates.

• I understand this is open source software provided for free and that I might not receive a timely response.
• I am positive I am NOT reporting a (potential) security
  vulnerability, to the best of my knowledge. (These must be shared by
  submitting this report form instead, if
  any hesitation exists.)

• I am willing to submit a pull request with reporoducers as xfailing test cases or even entire fix. (Assign this issue to me.)

Describe the bug

If a URL has an explicit port of 0 and a scheme with a default port (such as http), then the port and explicit_port attributes disagree on what the port is.

To Reproduce

import yarl
u = yarl.URL("http://a.co:0")
assert u.port == u.explicit_port

Expected behavior

If explicit_port is not None, then port should be the same as explicit_port.

Logs/tracebacks

Python 3.10.12 (main, Sep 11 2024, 15:47:36) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import yarl
>>> u = yarl.URL("http://a.co:0")
>>> assert u.port == u.explicit_port
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError
>>> u.port
80
>>> u.explicit_port
0

Python Version

$ python --version
Python 3.10.12

multidict Version

$ python -m pip show multidict
Name: multidict
Version: 6.1.0
Location: $PWD/.venv/lib/python3.10/site-packages
Requires: typing-extensions
Required-by: yarl

propcache Version

$ python -m pip show propcache
Name: propcache
Version: 0.2.0
Location: $PWD/.venv/lib/python3.10/site-packages
Requires:
Required-by: yarl

yarl Version

$ python -m pip show yarl
Name: yarl
Version: 1.17.2.dev0
Location: $PWD/.venv/lib/python3.10/site-packages
Editable project location: $PWD
Requires: idna, multidict, propcache
Required-by:
$ git describe
v1.17.1-1-gdfc41db

OS

Ubuntu 22.04

Additional context

I discovered this because Hypothesis can generate such a URL from its hypothesis.provisional.urls strategy. I think yarl's implementation of port (as self.explicit_port or self._default_port) is wrong because the URL grammar permits the port to be 0 (the production is just port = *DIGIT), but I can't think of a situation where a URL with port 0 would be useful.

[webknjaz]

Thanks for the report! Outside of URLs, port 0 may have different semantics assigned. For example, it can be a so-called "ephemeral port" that is passed into the socket bind syscalls and then the kernel assigns an unoccupied port that nobody claimed, at which point it turns into a concrete port with a non-zero number.

I can imagine that someone may want to represent a pre-binding URL for logging purposes. Or for when an interactive app doesn't bind to a port until a user clicks the button.

I can also imagine 0 having different semantics for non-HTTP URLs (yarl is agnostic, after all).

So I'd say — send us an xfailing test and perhaps some Hypothesis tests, for starters.

No rush, though — we're leading an aiohttp sprint day on Saturday and this might be a good issue for the participants to look into.

[webknjaz]

It doesn't look like anybody picked this up...

[asvetlov]

I can imagine the only usage for port 0: make an url that listens on any available port.
Honestly I never met this notation in real life before this issue.