Abstract over postgres' LISTEN/NOTIFY

This lets us test the event loop if we wanted, and lets us
test the listening behavior in isolation.

src/lib/Hydra/PostgresListener.pm

@@ -0,0 +1,105 @@
+package Hydra::PostgresListener;
+
+use strict;
+use warnings;
+use IO::Select;
+
+=head1 Hydra::PostgresListener
+
+An abstraction around using Postgres' LISTEN / NOTIFY in an event loop.
+
+=cut
+
+=head2 new
+
+Arguments:
+
+=over 1
+
+=item C<$dbh>
+L<DBI::db> The database connection.
+
+=back
+
+=cut
+
+sub new {
+    my ($self, $dbh) = @_;
+    my $sel = IO::Select->new($dbh->func("getfd"));
+
+    return bless {
+        "dbh" => $dbh,
+        "sel" => $sel,
+    }, $self;
+}
+
+=head2 subscribe
+
+Subscribe to the named channel for messages
+
+Arguments:
+
+=over 1
+
+=item C<$channel>
+
+The channel name.
+
+=back
+
+=cut
+
+sub subscribe {
+    my ($self, $channel) = @_;
+    $channel = $self->{'dbh'}->quote_identifier($channel);
+    $self->{'dbh'}->do("listen $channel");
+}
+
+=head2 block_for_messages
+
+Wait for messages to arrive within the specified timeout.
+
+Arguments:
+
+=over 1
+
+=item C<$timeout>
+The maximum number of seconds to wait for messages.
+
+Optional: if unspecified, block forever.
+
+=back
+
+Returns: a sub, call the sub repeatedly to get a message. The sub
+will return undef when there are no pending messages.
+
+Example:
+
+  my $events = $listener->block_for_messages();
+  while (my $message = $events->()) {
+    ...
+  }
+
+=cut
+
+sub block_for_messages {
+    my ($self, $timeout) = @_;
+
+    $self->{'sel'}->can_read($timeout);
+
+    return sub {
+        my $notify = $self->{'dbh'}->func("pg_notifies");
+        if (defined($notify)) {
+            my ($channelName, $pid, $payload) = @$notify;
+            return {
+                channel => $channelName,
+                pid => $pid,
+                payload => $payload,
+            }
+        } else {
+            return undef
+        }
+    }
+}
+
+1;

src/script/hydra-notify

@@ -3,9 +3,9 @@
 use strict;
 use utf8;
 use Hydra::Plugin;
+use Hydra::PostgresListener;
 use Hydra::Helper::Nix;
 use Hydra::Helper::AddBuilds;
-use IO::Select;
 use Getopt::Long;
 
 STDERR->autoflush(1);
@@ -26,9 +26,10 @@ my @plugins = Hydra::Plugin->instantiate(db => $db, config => $config);
 
 my $dbh = $db->storage->dbh;
 
-$dbh->do("listen build_started");
-$dbh->do("listen build_finished");
-$dbh->do("listen step_finished");
+my $listener = Hydra::PostgresListener->new($dbh);
+$listener->subscribe("build_started");
+$listener->subscribe("build_finished");
+$listener->subscribe("step_finished");
 
 sub buildStarted {
     my ($buildId) = @_;
@@ -112,15 +113,13 @@ for my $build ($db->resultset('Builds')->search(
 
 
 # Process incoming notifications.
-my $fd = $dbh->func("getfd");
-my $sel = IO::Select->new($fd);
-
 while (!$queued_only) {
-    $sel->can_read;
+    my $messages = $listener->block_for_messages();
+    while (my $message = $messages->()) {
 
-    while (my $notify = $dbh->func("pg_notifies")) {
-
-        my ($channelName, $pid, $payload) = @$notify;
+        my $channelName = $message->{"channel"};
+        my $pid = $message->{"pid"};
+        my $payload = $message->{"payload"};
         #print STDERR "got '$channelName' from $pid: $payload\n";
 
         my @payload = split /\t/, $payload;

t/PostgresListener.t

@@ -0,0 +1,79 @@
+use strict;
+use Setup;
+
+my %ctx = test_init();
+
+require Hydra::Model::DB;
+
+use Hydra::PostgresListener;
+use Test2::V0;
+
+my $db = Hydra::Model::DB->new;
+my $dbh = $db->storage->dbh;
+
+my $listener = Hydra::PostgresListener->new($dbh);
+
+$listener->subscribe("foo");
+$listener->subscribe("bar");
+
+is(undef, $listener->block_for_messages(0)->(), "There is no message");
+is(undef, $listener->block_for_messages(0)->(), "There is no message");
+is(undef, $listener->block_for_messages(0)->(), "There is no message");
+
+$dbh->do("notify foo, ?", undef, "hi");
+my $event = $listener->block_for_messages(0)->();
+is($event->{'channel'}, "foo", "The channel matches");
+isnt($event->{'pid'}, undef, "The pid is set");
+is($event->{'payload'}, "hi", "The payload matches");
+
+is(undef, $listener->block_for_messages(0)->(), "There is no message");
+
+
+like(
+    dies {
+        local $SIG{ALRM} = sub { die "timeout" };
+        alarm 1;
+        $listener->block_for_messages->();
+        alarm 0;
+    },
+    qr/timeout/,
+    "An unspecified block should block forever"
+);
+
+like(
+    dies {
+        local $SIG{ALRM} = sub { die "timeout" };
+        alarm 1;
+        $listener->block_for_messages(2)->();
+        alarm 0;
+    },
+    qr/timeout/,
+    "A 2-second block goes longer than 1 second"
+);
+
+ok(
+    lives {
+        local $SIG{ALRM} = sub { die "timeout" };
+        alarm 2;
+        is(undef, $listener->block_for_messages(1)->(), "A one second block returns undef data after timeout");
+        alarm 0;
+    },
+    "A 1-second block expires within 2 seconds"
+);
+
+subtest "with wacky channel names" => sub {
+    my $channel = "foo! very weird channel names...; select * from t where 1 = 1";
+    my $escapedChannel = $dbh->quote_identifier($channel);
+
+    $listener->subscribe($channel);
+
+    is(undef, $listener->block_for_messages(0)->(), "There is no message");
+
+    $dbh->do("notify $escapedChannel, ?", undef, "hi");
+    my $event = $listener->block_for_messages(0)->();
+    is($event->{'channel'}, $channel, "The channel matches");
+    isnt($event->{'pid'}, undef, "The pid is set");
+    is($event->{'payload'}, "hi", "The payload matches");
+};
+
+done_testing;