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Date: Wed, 1 Mar 2023 23:12:05 +0000
From: Eric Wong <bofh@yhbt.net>
To: unicorn-public@yhbt.net
Subject: Re: [PATCH] epollexclusive: avoid Ruby object allocation for buffer
Message-ID: <20230301231205.M669849@dcvr>
References: <20230215075813.1732969-1-bofh@yhbt.net>
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In-Reply-To: <20230215075813.1732969-1-bofh@yhbt.net>
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Eric Wong <bofh@yhbt.net> wrote:
> Leaving a dangling Ruby object around is suboptimal since it
> adds to GC pressure.
> 
> `__attribute__((__cleanup__(foo)))' is an old gcc extension to
> provide automatic cleanup functionality by running a pre-declared
> function at the end of scope.

Unfortunately, this can't be usable due to exceptions.
Even if we avoid throwing exceptions ourselves; Thread#raise can
be called by a Rack app which spawns a background thread

>  	n = (long)rb_thread_call_without_gvl(do_wait, &epw, RUBY_UBF_IO, NULL);

Thread#raise can hit rb_thread_call_wihtout_gvl

> -	if (n < 0) {
> -		if (errno != EINTR) rb_sys_fail("epoll_wait");
> -		n = 0;
> -	}
> +	if (n < 0 && errno != EINTR) rb_sys_fail("epoll_wait");
>  	/* Linux delivers events in order received */
>  	for (i = 0; i < n; i++) {
>  		struct epoll_event *ev = &epw.events[i];
> @@ -109,7 +113,6 @@ get_readers(VALUE epio, VALUE ready, VALUE readers, VALUE timeout_msec)
>  		if (RTEST(obj))
>  			rb_ary_push(ready, obj);
>  	}

While the `for' loop can be moved ahead of the rb_sys_fail call
and limited in scope, Thread#raise can't be predicted.

I suppose rb_ensure works, but that's a PITA.