Provide an OS independent way to set/get UV_THREADPOOL_SIZE

[joy4eg]

Currently, if you want to increase thread pool size, you need to write something like that:

static int xsetenv(const char *key, const char *value)
{
#ifdef _WIN32
	return (int)_putenv_s(key, value);
#else
	return setenv(key, value, 1);
#endif
}

void uv_set_thread_num(unsigned int n)
{
	char buf[0xFF];

	snprintf(buf, sizeof(buf), "%u", n);
	xsetenv("UV_THREADPOOL_SIZE", buf);
}

Which is not really convenient.
Instead, it would be really nice to have some low-level function that does this thing for you.
Something that you can call during early init procedure, like uv_replace_allocator().

[saghul]

Maybe uv_os_setenv ? https://docs.libuv.org/en/v1.x/misc.html#c.uv_os_setenv

[joy4eg]

Maybe uv_os_setenv ? https://docs.libuv.org/en/v1.x/misc.html#c.uv_os_setenv

Yeah, looks like I missed it :D

Anyway, having a native API would be still better rather than dealing with env.

[saghul]

A native API would not be very reliable really, because just setting the env var doesn't do anything if the threadpool has already been started anyway...

[joy4eg]

That's the point. uv_set_thread_num can return 0 if the thread pool is not running yet, and it was able to update the configuration.
If the thread pool is already running, we can return an error (<0).

[saghul]

SGTM. Ping @libuv/collaborators

[vtjnash]

SGRM. setenv is also thread-unsafe by design in posix, so it is good to have safe alternatives

[juanarbol]

I can investigate on this one.

[saghul]

Go for it!