watch: use EOFpipe to reduce dwaitpid wakeups

It's a bit inefficient to use a pipe, here.  However, using
dwaitpid() on a process that's not expected to exit soon is
also inefficient as it causes excessive wakeups as most of
our inbox-writing code expects synchronous waitpid().

This only affects -watch instances configured for NNTP and IMAP
clients.

3 files changed, 46 insertions, 4 deletions

diff --git a/MANIFEST b/MANIFEST
index f090175e..0b3835d8 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -114,6 +114,7 @@ lib/PublicInbox/DSPoll.pm
 lib/PublicInbox/Daemon.pm
 lib/PublicInbox/DirIdle.pm
 lib/PublicInbox/DummyInbox.pm
+lib/PublicInbox/EOFpipe.pm
 lib/PublicInbox/Emergency.pm
 lib/PublicInbox/Eml.pm
 lib/PublicInbox/EmlContentFoo.pm
diff --git a/lib/PublicInbox/EOFpipe.pm b/lib/PublicInbox/EOFpipe.pm
new file mode 100644
index 00000000..489caf82
--- /dev/null
+++ b/lib/PublicInbox/EOFpipe.pm
@@ -0,0 +1,24 @@
+# Copyright (C) 2020 all contributors <meta@public-inbox.org>
+# License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
+
+package PublicInbox::EOFpipe;
+use strict;
+use parent qw(PublicInbox::DS);
+use PublicInbox::Syscall qw(EPOLLIN EPOLLONESHOT);
+
+sub new {
+        my (undef, $rd, $cb, $arg) = @_;
+        my $self = bless {  cb => $cb, arg => $arg }, __PACKAGE__;
+        # 1031: F_SETPIPE_SZ, 4096: page size
+        fcntl($rd, 1031, 4096) if $^O eq 'linux';
+        $self->SUPER::new($rd, EPOLLIN|EPOLLONESHOT);
+}
+
+sub event_step {
+        my ($self) = @_;
+        if ($self->do_read(my $buf, 1) == 0) { # auto-closed
+                $self->{cb}->($self->{arg});
+        }
+}
+
+1;
diff --git a/lib/PublicInbox/Watch.pm b/lib/PublicInbox/Watch.pm
index db8d0396..17786377 100644
--- a/lib/PublicInbox/Watch.pm
+++ b/lib/PublicInbox/Watch.pm
@@ -14,7 +14,8 @@ use PublicInbox::Sigfd;
 use PublicInbox::DS qw(now);
 use PublicInbox::MID qw(mids);
 use PublicInbox::ContentHash qw(content_hash);
-use POSIX qw(_exit);
+use PublicInbox::EOFpipe;
+use POSIX qw(_exit WNOHANG);
 
 sub compile_watchheaders ($) {
         my ($ibx) = @_;
@@ -611,17 +612,30 @@ sub imap_idle_reap { # PublicInbox::DS::dwaitpid callback
                                 \&imap_idle_requeue, [ $self, $url_intvl ]);
 }
 
+sub reap { # callback for EOFpipe
+        my ($pid, $cb, $self) = @{$_[0]};
+        my $ret = waitpid($pid, 0);
+        if ($ret == $pid) {
+                $cb->($self, $pid); # poll_fetch_reap || imap_idle_reap
+        } else {
+                warn "W: waitpid($pid) => ", $ret // "($!)", "\n";
+        }
+}
+
 sub imap_idle_fork ($$) {
         my ($self, $url_intvl) = @_;
         my ($url, $intvl) = @$url_intvl;
+        pipe(my ($r, $w)) or die "pipe: $!";
         defined(my $pid = fork) or die "fork: $!";
         if ($pid == 0) {
+                close $r;
                 watch_atfork_child($self);
                 watch_imap_idle_1($self, $url, $intvl);
+                close $w;
                 _exit(0);
         }
         $self->{idle_pids}->{$pid} = $url_intvl;
-        PublicInbox::DS::dwaitpid($pid, \&imap_idle_reap, $self);
+        PublicInbox::EOFpipe->new($r, \&reap, [$pid, \&imap_idle_reap, $self]);
 }
 
 sub event_step {
@@ -689,24 +703,27 @@ sub watch_nntp_fetch_all ($$) {
 sub poll_fetch_fork ($) { # DS::add_timer callback
         my ($self, $intvl, $urls) = @{$_[0]};
         return if $self->{quit};
+        pipe(my ($r, $w)) or die "pipe: $!";
         my $oldset = watch_atfork_parent($self);
         my $pid = fork;
         if (defined($pid) && $pid == 0) {
+                close $r;
                 watch_atfork_child($self);
                 if ($urls->[0] =~ m!\Aimaps?://!i) {
                         watch_imap_fetch_all($self, $urls);
                 } else {
                         watch_nntp_fetch_all($self, $urls);
                 }
+                close $w;
                 _exit(0);
         }
         PublicInbox::Sigfd::sig_setmask($oldset);
         die "fork: $!"  unless defined $pid;
         $self->{poll_pids}->{$pid} = [ $intvl, $urls ];
-        PublicInbox::DS::dwaitpid($pid, \&poll_fetch_reap, $self);
+        PublicInbox::EOFpipe->new($r, \&reap, [$pid, \&poll_fetch_reap, $self]);
 }
 
-sub poll_fetch_reap { # PublicInbox::DS::dwaitpid callback
+sub poll_fetch_reap {
         my ($self, $pid) = @_;
         my $intvl_urls = delete $self->{poll_pids}->{$pid} or
                 die "BUG: PID=$pid (unknown) reaped: \$?=$?\n";