ResultSet::TaskRetries: add get_seconds_to_next_retry

Get the number of seconds before the next retriable task is ready.

This number is specifically intended to be used as a timeout, where
`undef` means never time out.

src/lib/Hydra/Schema/ResultSet/TaskRetries.pm

@@ -0,0 +1,41 @@
+package Hydra::Schema::ResultSet::TaskRetries;
+
+use strict;
+use warnings;
+use utf8;
+use base 'DBIx::Class::ResultSet';
+use List::Util qw(max);
+
+=head2 get_seconds_to_next_retry
+
+Query the database to identify how soon the next retryable task is due
+for being attempted again.
+
+If there are no tasks to be reattempted it returns undef.
+
+If a task's scheduled retry has passed, it returns 0.
+
+Otherwise, returns the number of seconds from now to look for work.
+
+=cut
+sub get_seconds_to_next_retry {
+    my ($self) = @_;
+
+    my $next_retry = $self->search(
+        {}, # any task
+        {
+            order_by => {
+                -asc => 'retry_at'
+            },
+            rows => 1,
+        }
+    )->get_column('retry_at')->first;
+
+    if (defined($next_retry)) {
+        return max(0, $next_retry - time());
+    } else {
+        return undef;
+    }
+}
+
+1;