filter/rubylang: reuse altid entry from inbox object

This allows users to DRY up their config a bit and avoid
specifying altid twice when reusing the NNTP-centric msgmap
for [ruby-*:\d+] serial numbers.

My current work-in-progress ~/.public-inbox/config entry
for the ruby-core list is:

------8<-------
[publicinbox "ruby-core"]
	address = ruby-core@ruby-lang.org
	url = //public-inbox.org/ruby-core
	mainrepo = /path/to/ruby-core.git
	newsgroup = inbox.comp.lang.ruby.core
	watchheader = List-Id:<ruby-core.ruby-lang.org>
	altid = serial:ruby-core:file=msgmap.sqlite3
	watch = maildir:/path/to/Maildir/.INBOX.ruby
	filter = PublicInbox::Filter::RubyLang

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/PublicInbox/Filter/RubyLang.pm b/lib/PublicInbox/Filter/RubyLang.pm
index ec4bc320..5240e8df 100644
--- a/lib/PublicInbox/Filter/RubyLang.pm
+++ b/lib/PublicInbox/Filter/RubyLang.pm
@@ -15,10 +15,13 @@ sub new {
         my ($class, %opts) = @_;
         my $altid = delete $opts{-altid};
         my $self = $class->SUPER::new(%opts);
+        my $ibx = $self->{-inbox};
         # altid = serial:ruby-core:file=msgmap.sqlite3
+        if (!$altid && $ibx && $ibx->{altid}) {
+                $altid ||= $ibx->{altid}->[0];
+        }
         if ($altid) {
                 require PublicInbox::MID; # mid_clean
-                my $ibx = $self->{-inbox};
                 require PublicInbox::AltId;
                 $self->{-altid} = PublicInbox::AltId->new($ibx, $altid, 1);
         }