select will timeout even if timeout is set to NULL

[versaloon]

Please include the following in your bug report:

Version of emscripten/emsdk:
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.8 (3ff7eff)
clang version 15.0.0 (https://github.com/llvm/llvm-project 80ec0ebfdc5692a58e0832125f2c6a991df9d63f)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: C:\Users\simon\Desktop\software\emsdk\upstream\bin

Failing command line in full:
compiling and linking are good.
emcc -pthread -sPTHREAD_POOL_SIZE=2 test.c
node --experimental-wasm-threads --experimental-wasm-bulk-memory .\a.out.js

Source code

#include <unistd.h>
#include <pthread.h>
#include <sys/select.h> 

#include <stdio.h>

int pipefd[2];

static void * __select_thread(void *arg)
{
    fd_set rfds;
    FD_ZERO(&rfds);
    FD_SET(pipefd[0], &rfds);

    int ret;
    char ch;
    while (1) {
        ret = select(pipefd[0] + 1, &rfds, NULL, NULL, NULL);
        printf("select returned %d\n", ret);
        if (0 == ret) {
            FD_SET(pipefd[0], &rfds);
        } else {
            read(pipefd[0], &ch, 1);
        }
    }

    return NULL;
}

int main(void)
{
    pipe(pipefd);

    char ch = 0;
    write(pipefd[1], &ch, 1);

    pthread_t thread;
    pthread_create(&thread, NULL, __select_thread, NULL);
    pthread_join(thread, NULL);

    printf("exit\n");
    return 0;
}

Desired output
On Ubuntu, the output is:
simonqian@simonqian-VirtualBox:$ gcc test.c -lpthread
simonqian@simonqian-VirtualBox:$ ./a.out
select returned 1

On nodejs, the output is:
PS Z:> emcc -pthread -sPTHREAD_POOL_SIZE=2 test.c
PS Z:> node --experimental-wasm-threads --experimental-wasm-bulk-memory .\a.out.js
select returned 1
select returned 0
select returned 0
select returned 0
select returned 0
select returned 0
......

[sbc100]

I think this is due to the fact that we can't bloc the main thread when running the browser and so our select implementation is basically just falls back to poll:

emscripten/src/library_syscall.js

Lines 591 to 595 in e1d97f5

	__syscall__newselect: function(nfds, readfds, writefds, exceptfds, timeout) {
	// readfds are supported,
	// writefds checks socket open status
	// exceptfds not supported
	// timeout is always 0 - fully async

Perhaps we should at least assert/warn when we return even though a timeout was requested?

Or perhaps we can actually block in some environments? If you would like to contribute here any patches would be welcome.

[ktock]

I've posted a PR related to this issue #25523.
@sbc100 Could you please take a look at that PR?