listen:detect backlog parameter as in std. library

[alexs-sh]

Define the default listen backlog close to the standard library.

This helps avoid hardcoded unsynchronized values and allows better control of default values depending on the target. This is the initial version of the change that addresses issue #1872

Selecting a “valid” default value can be tricky due to:

• It often serves only as a hint and may be rounded, trimmed, or ignored by the OS.
• It is sometimes provided as a "magic" value, for example, -1. This value is undocumented and not standard, but it is often used to represent the largest possible backlog size. This happens due to signed/unsigned conversion and rounding to the upper bound performed by the OS.
• Default values vary depending on the OS and its version. Common defaults include: -1, 128, 1024, and 4096

As the topic could affect many targets, reviews, comments, and reports are very welcome.

Thank you

[Darksonn]

Formatting is broken here.

[alexs-sh]

Thanks. I’ll double-check. I already ran cargo fmt before submitting the PR

[Thomasdezeeuw]

Can this be a constant (not function)?

Also it shouldn't need allow(dead_code)

[alexs-sh]

Thx.

1. Yes, it's possible to have constant. I started with this approach, but I felt that the function better combines logically related code into one piece. Anyway, I’ll prepare a version using a constant and send it for review.
2. Removing allow(dead_code) causes fails if net feature is not enabled.

 cargo build
   Compiling log v0.4.22
   Compiling mio v1.0.4 (/mnt/public-storage/projects/mio)
error: function `get_listen_backlog_size` is never used
   --> src/sys/mod.rs:101:21
    |
101 | pub(crate) const fn get_listen_backlog_size() -> i32 {
    |                     ^^^^^^^^^^^^^^^^^^^^^^^
    |
note: the lint level is defined here
   --> src/lib.rs:6:5
    |
6   |     dead_code
    |     ^^^^^^^^^

error: could not compile `mio` (lib) due to 1 previous error

[Thomasdezeeuw]

cfg_select! once stable would be handy here.

[alexs-sh]

Yeah, I agree. It’s pretty wordy now

[Thomasdezeeuw]

CI found one actual error:

  error: unused import: `std::convert::TryInto`
    --> src\sys\windows\tcp.rs:50:9
     |
  50 |     use std::convert::TryInto;

I think the rest can be ignored, a rebase should fix them.

[Thomasdezeeuw]

The 3 remaining failures seem legit, libc::SOMAXCONN is not defined for those platforms.

[alexs-sh]

The 3 remaining failures seem legit, libc::SOMAXCONN is not defined for those platforms.

Good to know. Should I also add some fallback or special values for those targets? Following the STD is good, but not if it’s breaking things that were previously buildable

[Thomasdezeeuw]

Good to know. Should I also add some fallback or special values for those targets? Following the STD is good, but not if it’s breaking things that were previously buildable

Ideally we can add SOMAXCONN to libc for those targets, but since that will likely take some time I would also be ok to defaulting 1024 (the old value) for those.

[Thomasdezeeuw]

Thanks @alexs-sh

[stepancheg]

Define the default listen backlog close to the standard library.

FWIW code you referenced is for unix socket.

For TCP socket default is 128.

https://github.com/rust-lang/rust/blob/76c5ed2847cdb26ef2822a3a165d710f6b772217/library/std/src/sys/net/connection/socket/mod.rs#L540-L552

The change seems good anyway.