unix, windows: add uv_loop_alive() function

[sam-github]

Useful to know when the the event loop is empty, this can't be done with
uv_run() without possibly blocking, or running some events (which might
empty the event loop as a side-effect).

[Nodejs-Jenkins]

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[saghul]

You can achieve the same with uv_run by using UV_RUN_NOWAIT mode. It returns 0 when the loop is finished and != 0 when there is still work to be done and uv_run should be called again.

if (!uv_run(loop, UV_RUN_NOWAIT)) {
    // loop is not alive
    ...
}

[sam-github]

Using NOWAIT was my first attempt, it looked promising, but won't work.

Problem is that it tells you that the loop is no longer alive after call, but not whether a handler had to be run to get down to zero.

Look at the linked pull request. If you try to rewrite using RUN_NOWAIT, you get this:

while(1) {
uv_run(ALL);
emit('keepalive');
// if nothing was scheduled, exit
r = uv_run(ONCE);
if(!r) {
// hm, maybe one thing was scheduled, then run, and now we should re-emit the keep alive
// or... maybe nothing was scheduled, and we should exit
// which is it? no way to know

[saghul]

Oh, I see.

[bnoordhuis]

const uv_loop_t*?

[sam-github]

Requires uv__loop_alive() to take const, as well. Is that OK?

[bnoordhuis]

Yep, that's fine. It should have been const in the first place, really.

[bnoordhuis]

LGTM but a test would be nice.

[sam-github]

I was hoping to avoid the test (because its a renamed version of an existing function), but I will write one. Speaking of which, I could just change the name of the existing private uv__loop_alive() and make its argument const, rather than having uv_loop_alive() wrap the private. Would you prefer that?

[bnoordhuis]

I was hoping to avoid the test (because its a renamed version of an existing function), but I will write one.

Rule of thumb is that all API additions should be covered by at least one test.

Speaking of which, I could just change the name of the existing private uv__loop_alive() and make its argument const, rather than having uv_loop_alive() wrap the private. Would you prefer that?

No. That makes it possible for outsiders to (inadvertently or otherwise) overload the uv_loop_alive symbol, plus it means each call goes through the PLT. It probably won't have a critical performance impact but it means I have to remember to set LD_BIND_NOW when I profile something.

(EDIT: Libuv is not very consistent in that regard; I tend to clean up such call sites when I encounter them.)

[sam-github]

/to @bnoordhuis, has test that both handles and requests count as alive

[saghul]

Looks like this was forgotten. Can you rebase on top of master and force push? You shouldn't have any problem, but since we use autotools now you need to modify Makefile.am to add the test file, build.mk no longer exists.

Thanks!

[saghul]

Thanks @sam-github! ✨