library: staff signed in to Koha open Add books without a second login

Quick Add asks Koha (C4::Auth::check_cookie_auth via a secret-guarded svc script) whether the
browser's session may edit the catalogue; falls back to its own login otherwise. Cookie-authenticated
writes need an X-Quickadd header; the direct port never accepts cookies.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013wi3ZSot4XbUViwRFgLpRf

library/koha/quickadd-session.pl

@@ -0,0 +1,44 @@
+#!/usr/bin/perl
+# Installed as .../intranet/cgi-bin/svc/quickadd-session (URL /cgi-bin/koha/svc/quickadd-session).
+#
+# Lets the Add books page (Quick Add) ask: "is this browser signed in to the staff site as
+# someone allowed to edit the catalogue?" so staff do not have to log in a second time.
+#
+# It wraps Koha's own C4::Auth::check_cookie_auth (documented for exactly this: services that
+# are invoked by pages authenticated in the usual way) and only answers callers that present a
+# shared secret, so it cannot be used to probe sessions from outside. The browser's address is
+# passed in explicitly, so Koha's "session is tied to the IP that logged in" rule keeps working.
+use Modern::Perl;
+use CGI qw( -utf8 );
+use JSON qw( encode_json );
+use Digest::SHA qw( sha256_hex );
+use C4::Auth qw( check_cookie_auth );
+
+my $q = CGI->new;
+
+# Takes the CGI object as an argument: under Plack this file runs wrapped in a sub, where a named
+# sub would otherwise see a stale copy of a file-level variable.
+sub reply {
+    my ( $q, $status, $data ) = @_;
+    print $q->header( -type => 'application/json', -status => $status, -charset => 'utf-8', -cache_control => 'no-store' ),
+        encode_json($data);
+    exit;
+}
+
+# 1. The caller must know the shared secret (a file only the Koha user and root can read).
+my $secret = '';
+if ( open my $fh, '<', '/etc/koha/quickadd-sso.secret' ) { $secret = <$fh> // ''; $secret =~ s/\s+$//; close $fh }
+my $given = $ENV{HTTP_X_QUICKADD_SECRET} // '';
+reply( $q, '403 Forbidden', { status => 'forbidden' } )
+    unless length($secret) >= 16 && sha256_hex($given) eq sha256_hex($secret);
+
+# 2. Plausible-looking inputs only.
+my $sid = $q->param('sid') // '';
+my $ip  = $q->param('ip')  // '';
+reply( $q, '200 OK', { status => 'failed' } )
+    unless $sid =~ /\A[A-Za-z0-9]{16,64}\z/ && $ip =~ /\A[0-9A-Fa-f:.]{3,45}\z/;
+
+# 3. Ask Koha. Staff who may edit the catalogue (superlibrarians always may) are let in.
+my ( $status, $session ) = check_cookie_auth( $sid, { editcatalogue => 'edit_catalogue' }, { remote_addr => $ip } );
+reply( $q, '200 OK', { status => 'ok', user => scalar $session->param('id') } ) if $status eq 'ok' && $session;
+reply( $q, '200 OK', { status => $status || 'failed' } );