Policy scripts: the operator's own rules for routes, in Rhai

A new crate, bgp-policy, runs functions of a script on routes: on their
way in, after the general checks of a peer's import and before the
checks that can change (RPKI, next hop reachability); and on their way
out to a peer. A function is given the route and the peer, may refuse
the route, and may change its communities, large communities,
LOCAL_PREF and MED, and on the way out prepend our AS:

    fn from_peer(route, peer) {
        if route.has_community("65535:666") { return false; }
        route.add_community(LEARNED_FROM[peer.role]);
        route.local_pref = PREFERENCE[peer.role];
        true
    }

Configuration: policy { path | source, import_function, export_function }
and, for a peer, import { function } and export { function }. The script
is compiled at startup; `bgp --check` reads it and says which function
is whose. A function that fails on a route has refused it. Scripts have
no files, network or clock, and are stopped after 200,000 steps.

Route selection now goes by a LOCAL_PREF on a route from an external
peer: the decoder never lets a peer's own through, so one that is there
was set by policy.

What it costs was measured before it was built on (benches/policy.rs):
a call that tests a few communities and changes two takes 1 to 3
microseconds, some two hundred times what Rust would. So a function is
asked once for the routes of an UPDATE, which share their attributes,
unless it asks for the prefix. One peer sending 1.23 M routes over a
session: 0.37 s without a function, 0.80 s with one that marks and
ranks every route, 3.4 s if it also looks at the prefix.

Export applies to what is announced, which today is what the daemon
originates; learned routes are not passed on yet.

Tested by unit tests of the crate, sessions end to end, a fuzz target on
the example script (testdata/policy.rhai), and a lab phase in which a
script changes which of FRR's and BIRD's route is chosen and what each
of them is told.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/web/static/statusz.html

@@ -123,6 +123,10 @@ from <select id="lookup-peer"><option value="">any peer</option></select>
   const esc = s => String(s).replace(/[&<>"']/g, c => ({'&':'&amp;','<':'&lt;','>':'&gt;','"':'&quot;',"'":'&#39;'}[c]));
   const state = { status: null, peers: [], local: [], received: [], receivedPage: {}, refused: [], rpki: null, kernel: null };
   const DASH = '<span class="dim">-</span>';
+  // The functions of the policy script a peer's routes go through.
+  const policy = p => (p.import_function || p.export_function)
+    ? ` <span class="dim" title="Functions of the policy script: for routes from the peer, and for routes to it">${[p.import_function && 'in: ' + esc(p.import_function), p.export_function && 'out: ' + esc(p.export_function)].filter(Boolean).join(', ')}</span>`
+    : '';
 
   // ---- helpers -----------------------------------------------------------
   const fmtDur = s => {
@@ -182,13 +186,13 @@ from <select id="lookup-peer"><option value="">any peer</option></select>
       const up = p.state === 'Established';
       const detail = p.state === 'Backoff' ? `${esc(p.last_error || '')}; retry in ${until(p.retry_at_unix)}` : '';
       return `<tr>
-        <td>${p.name ? esc(p.name) : '<span class="dim">(inbound)</span>'}</td><td class="mono">${esc(p.peer_addr)}</td><td>${p.peer_asn}${p.role ? ` <span class="dim" title="${p.role_confirmed ? 'The peer says so too (RFC 9234)' : 'By our configuration; the peer has not said what it is'}">${esc(p.role)}${p.role_confirmed ? ' &check;' : ''}</span>` : ''}</td>
+        <td>${p.name ? esc(p.name) : '<span class="dim">(inbound)</span>'}${policy(p)}</td><td class="mono">${esc(p.peer_addr)}</td><td>${p.peer_asn}${p.role ? ` <span class="dim" title="${p.role_confirmed ? 'The peer says so too (RFC 9234)' : 'By our configuration; the peer has not said what it is'}">${esc(p.role)}${p.role_confirmed ? ' &check;' : ''}</span>` : ''}</td>
         <td><span class="${STATE_CLASS[p.state] || ''}">${esc(p.state)}</span></td><td>${up ? (p.direction === 'outbound' ? 'us' : 'peer') : DASH}</td>
         <td class="mono">${up ? esc(p.local_addr) : DASH}</td><td>${up ? fmtTime(p.established_at_unix) : DASH}</td>
         <td>${up ? age(p.established_at_unix) : DASH}</td><td>${up ? (p.four_octet_asn ? 'yes' : 'no') : DASH}</td>
         <td>${up ? esc((p.address_families || []).join(', ')) + addPath(p) + (p.extended_next_hop ? ' &middot; IPv4 with IPv6 next hops' : '') : DASH}</td>
         <td>${up ? p.routes : DASH}</td>
-        <td>${up ? (p.rejected_routes ? `<a class="bad" href="#refused">${p.rejected_routes}</a>` : 0) + (p.held_routes ? ` <span class="dim" title="Refused as RPKI or ASPA invalid and kept aside, to be taken if that changes">(${p.held_routes} held)</span>` : '') : DASH}</td><td>${detail}</td></tr>`;
+        <td>${up ? (p.rejected_routes ? `<a class="bad" href="#refused">${p.rejected_routes}</a>` : 0) + (p.held_routes ? ` <span class="dim" title="Refused as RPKI or ASPA invalid, or for a next hop that cannot be reached, and kept aside, to be taken if that changes">(${p.held_routes} held)</span>` : '') : DASH}</td><td>${detail}</td></tr>`;
     }).join(''));
     renderPeerChoices();
   }