socketpair improvements

[cfriedt]

4 commits:

• improvements for socketpair sample app (it was broken for a while)
• rename socketpair_example.c to main.c
• minor improvements for socketpair.c
• additional <fcntl.h> corrections in the networking subsystem

Fixes #54347
Fixes #52360

[cfriedt]

I would really prefer to get this sample running in harness: console rather than harness: net so that we can run it on any host and have twister automatically check the output for SUCCESS using the regex property in sample.yaml. It's completely unnecessary for socketpair to depend on networking other than relying on some of the existing socket code.

Pushing that off to a later date though.

[rlubos]

@cfriedt net/lib/sockets/sockets_tls.c also needs an update.

[SeppoTakalo]

Is there a reason why we apply this workaround:

#ifdef CONFIG_ARCH_POSIX
#include <fcntl.h>
#else
#include <zephyr/posix/fcntl.h>
#endif

to all files, but not in the <zephyr/posix/fcntl.h> header itself?

[cfriedt]

Is there a reason why we apply this workaround:

#ifdef CONFIG_ARCH_POSIX
#include <fcntl.h>
#else
#include <zephyr/posix/fcntl.h>
#endif

to all files, but not in the <zephyr/posix/fcntl.h> header itself?

Yes, the eventual goal would be to include <zephyr/posix/fcntl.h> everywhere instead of <fcntl.h>[1]. However, getting there would require being able to test Zephyr's POSIX API on native_posix and we're not quite there yet. The foreseeable next plans for the posix api are to remove workarounds and to bring in slightly more mature posix headers. I would not recommend mixing them up again. It's actually somewhat helpful to identify where we conditionally include <fcntl.h> for native_posix as well rather than just continuing to sweep it under the carpet.

Specifically w.r.t. the network subsystem though, due to CONFIG_ARCH_POSIX and CONFIG_POSIX_API being mutually exclusive for the last few years, it has precipitated quite a bit of spaghetti between lib/os/, subsys/net, and lib/posix.

At the moment there is a cyclic dependency between net and posix that needs to be broken by a mutual one.

I.e. Currently, we have:

┌───────────┐       ┌───────────┐
│           ├──────►│           │
│   POSIX   │       │   NET     │
│           │◄──────┤           │
└───────────┘       └───────────┘

But we need

          ┌───────────┐
          │           │
          │    ??     │
          │           │
          └▲────────▲─┘
           │        │
           │        │
┌──────────┴┐       ├───────────┐
│           │       │           │
│   POSIX   ├───────►   NET     │
│           │       │           │
└───────────┘       └───────────┘

I'm thinking the correct name for ?? would be something like zfdio

Diagrams made with https://asciiflow.com/

[1] That assumes we don not want CONFIG_POSIX_API=y to be the default. If it were the default, that allow us to use all posix headers non-prefixed again everywhere (e.g. <fcntl.h>, <unistd.h>)

[cfriedt]

@cfriedt net/lib/sockets/sockets_tls.c also needs an update.

@rlubos - fixed!

[jukkar]

LGTM