inboxidle: new class to detect inbox changes

This will be used to implement IMAP IDLE, first.

Eventually, it may be used to trigger other things:

* incremental internal updates for manifest.js.gz
* restart `git cat-file' processes on pack index unlink
* IMAP IDLE-like long-polling HTTP endpoint

And maybe more things we haven't thought of, yet.

It uses Linux::Inotify2 or IO::KQueue depending on what packages
are installed and what the kernel supports.  It falls back to
nanosecond-aware Time::HiRes::stat() (available with Perl 5.10.0+)
on systems lacking Linux::Inotify2 and IO::KQueue.

In the future, a pure Perl alternative to Linux::Inotify2 may be
supplied for users of architectures we already support signalfd
and epoll on.

v2 changes:
- avoid O_TRUNC on lock file
- change ctime on Linux systems w/o inotify
- fix naming of comments and fields

1 files changed, 39 insertions, 0 deletions

diff --git a/lib/PublicInbox/FakeInotify.pm b/lib/PublicInbox/FakeInotify.pm
new file mode 100644
index 00000000..bd610463
--- /dev/null
+++ b/lib/PublicInbox/FakeInotify.pm
@@ -0,0 +1,39 @@
+# Copyright (C) 2020 all contributors <meta@public-inbox.org>
+# License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
+
+# for systems lacking Linux::Inotify2 or IO::KQueue, just emulates
+# enough of Linux::Inotify2
+package PublicInbox::FakeInotify;
+use strict;
+use Time::HiRes qw(stat);
+my $IN_CLOSE = 0x08 | 0x10; # match Linux inotify
+
+sub new { bless { watch => {} }, __PACKAGE__ }
+
+# behaves like Linux::Inotify2->watch
+sub watch {
+        my ($self, $path, $mask, $cb) = @_;
+        my @st = stat($path) or return;
+        $self->{watch}->{"$path\0$mask"} = [ @st, $cb ];
+}
+
+# behaves like non-blocking Linux::Inotify2->poll
+sub poll {
+        my ($self) = @_;
+        my $watch = $self->{watch} or return;
+        for my $x (keys %$watch) {
+                my ($path, $mask) = split(/\0/, $x, 2);
+                my @now = stat($path) or next;
+                my $prv = $watch->{$x};
+                my $cb = $prv->[-1];
+                # 10: ctime, 7: size
+                if ($prv->[10] != $now[10]) {
+                        if (($mask & $IN_CLOSE) == $IN_CLOSE) {
+                                eval { $cb->() };
+                        }
+                }
+                @$prv = (@now, $cb);
+        }
+}
+
+1;