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/

1 files changed, 12 insertions, 3 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 {