inboxidle: avoid needless syscalls on refresh

We don't have to replace a bunch of existing watches
with identical new ones.  On Linux with Linux::Inotify2
installed, this avoids a storm of inotify_add_watch(2)
and inotify_rm_watch(2) syscalls on SIGHUP with -imapd
and "-extindex --watch"

(cherry picked from commit 10bf54305da8422d9ece6b809996092c1c4b1786)

Note: this seems to fix missed wakeups with many watches,
so it's in the stable branch.

Link: https://public-inbox.org/meta/20201226201115.GA30142@dcvr/

2 files changed, 17 insertions, 24 deletions

diff --git a/lib/PublicInbox/InboxIdle.pm b/lib/PublicInbox/InboxIdle.pm
index 357bd216..60948bea 100644
--- a/lib/PublicInbox/InboxIdle.pm
+++ b/lib/PublicInbox/InboxIdle.pm
@@ -30,17 +30,26 @@ sub in2_arm ($$) { # PublicInbox::Config::each_inbox callback
         }
         my $inot = $self->{inot};
         my $cur = $self->{pathmap}->{$dir} //= [];
+        my $lock = "$dir/".($ibx->version >= 2 ? 'inbox.lock' : 'ssoma.lock');
 
         # transfer old subscriptions to the current inbox, cancel the old watch
-        if (my $old_ibx = $cur->[0]) {
+        my $old_ibx = $cur->[0];
+        $cur->[0] = $ibx;
+        if ($old_ibx) {
                 $ibx->{unlock_subs} and
                         die "BUG: $dir->{unlock_subs} should not exist";
                 $ibx->{unlock_subs} = $old_ibx->{unlock_subs};
+
+                # Linux::Inotify2::Watch::name matches if watches are the
+                # same, no point in replacing a watch of the same name
+                if ($cur->[1]->name eq $lock) {
+                        $self->{on_unlock}->{$lock} = $ibx;
+                        return;
+                }
+                # rare, name changed (v1 inbox converted to v2)
                 $cur->[1]->cancel; # Linux::Inotify2::Watch::cancel
         }
-        $cur->[0] = $ibx;
 
-        my $lock = "$dir/".($ibx->version >= 2 ? 'inbox.lock' : 'ssoma.lock');
         if (my $w = $cur->[1] = $inot->watch($lock, $IN_MODIFY)) {
                 $self->{on_unlock}->{$w->name} = $ibx;
         } else {
diff --git a/t/imapd.t b/t/imapd.t
index c1c52839..a464ad86 100644
--- a/t/imapd.t
+++ b/t/imapd.t
@@ -296,27 +296,11 @@ $pi_config->each_inbox(sub {
 
         # ensure IDLE persists across HUP, w/o extra watches or FDs
         $td->kill('HUP') or BAIL_OUT "failed to kill -imapd: $!";
-        SKIP: {
-                skip 'no inotify fdinfo (or support)', 2 if !@ino_info;
-                my (@tmp, %prev);
-                local $/ = "\n";
-                my $end = time + 5;
-                until (time > $end) {
-                        select undef, undef, undef, 0.01;
-                        open my $fh, '<', $ino_fdinfo or
-                                                BAIL_OUT "$ino_fdinfo: $!";
-                        %prev = map { $_ => 1 } @ino_info;
-                        @tmp = grep(/^inotify wd:/, <$fh>);
-                        if (scalar(@tmp) == scalar(@ino_info)) {
-                                delete @prev{@tmp};
-                                last if scalar(keys(%prev)) == @ino_info;
-                        }
-                }
-                is(scalar @tmp, scalar @ino_info,
-                        'old inotify watches replaced');
-                is(scalar keys %prev, scalar @ino_info,
-                        'no previous watches overlap');
-        };
+        for my $n (1..2) { # kick the event loop so we know HUP is done
+                my $m = $imap_client->new(%mic_opt);
+                ok($m->login && $m->IsAuthenticated && $m->logout,
+                        "connection $n works after HUP");
+        }
 
         open($fh, '<', 't/data/0001.patch') or BAIL_OUT("open: $!");
         run_script(['-mda', '--no-precheck'], $env, { 0 => $fh }) or