Actually, as far as I can tell, it never did anything other than cause
signals to be delivered that were promptly ignored. It appears to have
been added in eaf1f651cf86.

There are two things in ngx_master_process_cycle() with names related
to SIGIO. One is adding SIGIO to the set. That is removed in this
patch. The other is a variable named sigio, added in 8abb88374c6c.
This variable does not appear to have anything to do with SIGIO
despite the name.

I ran the tests with this patch and they all passed. Receiving signals
isn't free, so this patch may improve performance.
On Wed, Jun 27, 2018 at 5:57 AM Ruslan Ermilov <ru@nginx.com> wrote:
>
> On Mon, Jun 25, 2018 at 11:16:12AM -0700, Ian Gudger via nginx-devel wrote:
> > # HG changeset patch
> > # User Ian Gudger <igudger@google.com>
> > # Date 1529449008 25200
> > # Tue Jun 19 15:56:48 2018 -0700
> > # Node ID 9427538acbc50142afbe91a11a1d4f907a00d257
> > # Parent 118885f7a5774962f1145693d9c26a4c199ca6ea
> > Core: remove unused FIOASYNC.
> >
> > FIOASYNC, F_SETOWN and SIGIO seem to no longer serve any function.
>
> Can you decode your "seem to no longer server any function", please?
>
> > diff --git a/src/os/unix/ngx_process.c b/src/os/unix/ngx_process.c
> > --- a/src/os/unix/ngx_process.c
> > +++ b/src/os/unix/ngx_process.c
> > @@ -71,8 +71,6 @@ ngx_signal_t signals[] = {
> >
> > { SIGINT, "SIGINT", "", ngx_signal_handler },
> >
> > - { SIGIO, "SIGIO", "", ngx_signal_handler },
> > -
> > { SIGCHLD, "SIGCHLD", "", ngx_signal_handler },
> >
> > { SIGSYS, "SIGSYS, SIG_IGN", "", NULL },
> > @@ -87,7 +85,6 @@ ngx_pid_t
> > ngx_spawn_process(ngx_cycle_t *cycle, ngx_spawn_proc_pt proc, void *data,
> > char *name, ngx_int_t respawn)
> > {
> > - u_long on;
> > ngx_pid_t pid;
> > ngx_int_t s;
> >
> > @@ -142,21 +139,6 @@ ngx_spawn_process(ngx_cycle_t *cycle, ng
> > return NGX_INVALID_PID;
> > }
> >
> > - on = 1;
> > - if (ioctl(ngx_processes[s].channel[0], FIOASYNC, &on) == -1) {
> > - ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_errno,
> > - "ioctl(FIOASYNC) failed while spawning
> > \"%s\"", name);
> > - ngx_close_channel(ngx_processes[s].channel, cycle->log);
> > - return NGX_INVALID_PID;
> > - }
> > -
> > - if (fcntl(ngx_processes[s].channel[0], F_SETOWN, ngx_pid) == -1) {
> > - ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_errno,
> > - "fcntl(F_SETOWN) failed while spawning
> > \"%s\"", name);
> > - ngx_close_channel(ngx_processes[s].channel, cycle->log);
> > - return NGX_INVALID_PID;
> > - }
> > -
> > if (fcntl(ngx_processes[s].channel[0], F_SETFD, FD_CLOEXEC) == -1) {
> > ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_errno,
> > "fcntl(FD_CLOEXEC) failed while spawning \"%s\"",
> > @@ -394,10 +376,6 @@ ngx_signal_handler(int signo, siginfo_t
> > ngx_sigalrm = 1;
> > break;
> >
> > - case SIGIO:
> > - ngx_sigio = 1;
> > - break;
> > -
> > case SIGCHLD:
> > ngx_reap = 1;
> > break;
> > @@ -433,8 +411,6 @@ ngx_signal_handler(int signo, siginfo_t
> >
> > case ngx_signal_value(NGX_RECONFIGURE_SIGNAL):
> > case ngx_signal_value(NGX_CHANGEBIN_SIGNAL):
> > - case SIGIO:
> > - action = ", ignoring";
>
> Removing setting of an "action" variable looks like an error.
>
> > break;
> > }
> >
> > diff --git a/src/os/unix/ngx_process_cycle.c b/src/os/unix/ngx_process_cycle.c
> > --- a/src/os/unix/ngx_process_cycle.c
> > +++ b/src/os/unix/ngx_process_cycle.c
> > @@ -34,7 +34,6 @@ ngx_pid_t ngx_pid;
> > ngx_pid_t ngx_parent;
> >
> > sig_atomic_t ngx_reap;
> > -sig_atomic_t ngx_sigio;
> > sig_atomic_t ngx_sigalrm;
> > sig_atomic_t ngx_terminate;
> > sig_atomic_t ngx_quit;
> > @@ -88,7 +87,6 @@ ngx_master_process_cycle(ngx_cycle_t *cy
> > sigemptyset(&set);
> > sigaddset(&set, SIGCHLD);
> > sigaddset(&set, SIGALRM);
> > - sigaddset(&set, SIGIO);
> > sigaddset(&set, SIGINT);
> > sigaddset(&set, ngx_signal_value(NGX_RECONFIGURE_SIGNAL));
> > sigaddset(&set, ngx_signal_value(NGX_REOPEN_SIGNAL));
> > diff --git a/src/os/unix/ngx_process_cycle.h b/src/os/unix/ngx_process_cycle.h
> > --- a/src/os/unix/ngx_process_cycle.h
> > +++ b/src/os/unix/ngx_process_cycle.h
> > @@ -47,7 +47,6 @@ extern ngx_uint_t ngx_daemonized;
> > extern ngx_uint_t ngx_exiting;
> >
> > extern sig_atomic_t ngx_reap;
> > -extern sig_atomic_t ngx_sigio;
> > extern sig_atomic_t ngx_sigalrm;
> > extern sig_atomic_t ngx_quit;
> > extern sig_atomic_t ngx_debug_quit;
>
> There's also a SIGIO related code in ngx_master_process_cycle(),
> added in 8abb88374c6c.
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