rate limit: domain ceiling for all IPs, trusted app override

Invert level-2 semantics so domain limits apply to every client IP and
application limits with trusted IPs raise the ceiling above the domain
(still capped by level 1). Panel shows L1, validates maxima, and documents
the model on Settings.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/milter/ratelimit.go

@@ -8,9 +8,11 @@ import (
 )
 
 // overLimit reports whether the message currently being received should be
-// refused under a level-2 differentiated limit (guide § Rate limiting). It
-// checks the domain-level and application-level limits in turn; either being
-// exceeded is enough to refuse.
+// refused under a level-2 differentiated limit (guide § Rate limiting).
+//
+// Trusted application IPs (app limit active and client IP listed) use only the
+// app ceiling and skip the domain check. Everyone else is under the domain
+// ceiling when one is configured; otherwise only level 1 applies.
 //
 // It is deliberately fail-open: any store error, or the absence of a usable
 // limit, is treated as "not over limit" so a malfunction of the level-2
@@ -26,46 +28,48 @@ func (s *session) overLimit() bool {
 	if s.clientIP == "" {
 		return false // no client IP to key on; level-2 does not apply
 	}
-	checks := []struct{ scope, ref string }{
-		{store.RateLimitScopeDomain, domainOf(s.from)},
-		{store.RateLimitScopeApp, s.login},
-	}
-	var taken []*reservation
-	for _, c := range checks {
-		if c.ref == "" {
-			continue
-		}
-		rl, ok, err := s.rec.RateLimit(c.scope, c.ref)
+
+	if s.login != "" {
+		rl, ok, err := s.rec.RateLimit(store.RateLimitScopeApp, s.login)
 		if err != nil {
-			log.Printf("journal-milter: rate-limit lookup %s %q: %v (fail-open)", c.scope, c.ref, err)
-			continue
+			log.Printf("journal-milter: rate-limit lookup application %q: %v (fail-open)", s.login, err)
+		} else if ok && rl.Active() && rl.AllowsIP(s.clientIP) {
+			return s.enforceLimit(store.RateLimitScopeApp, s.login, rl)
 		}
-		// No limit configured, an inert draft, or a client IP outside the
-		// registered set: the differentiated limit does not apply here.
-		if !ok || !rl.Active() || !rl.AllowsIP(s.clientIP) {
-			continue
-		}
-		since := time.Now().Add(-time.Duration(rl.WindowSeconds) * time.Second)
-		n, err := s.rec.CountMessages(c.scope, c.ref, since)
-		if err != nil {
-			log.Printf("journal-milter: rate-limit count %s %q: %v (fail-open)", c.scope, c.ref, err)
-			continue
-		}
-		key := c.scope + "|" + c.ref
-		n += s.flight.count(key, since)
-		if n >= int64(rl.MaxMessages) {
-			log.Printf("journal-milter: %s %q over limit: %d/%d in %ds from %s — refusing 4xx",
-				c.scope, c.ref, n, rl.MaxMessages, rl.WindowSeconds, s.clientIP)
-			// The message is refused, so the slots claimed for the limits
-			// checked before this one must not stay claimed.
-			for _, r := range taken {
-				s.flight.release(r)
-			}
-			return true
-		}
-		taken = append(taken, s.flight.reserve(key))
 	}
-	s.reserved = append(s.reserved, taken...)
+
+	domain := domainOf(s.from)
+	if domain == "" {
+		return false
+	}
+	rl, ok, err := s.rec.RateLimit(store.RateLimitScopeDomain, domain)
+	if err != nil {
+		log.Printf("journal-milter: rate-limit lookup domain %q: %v (fail-open)", domain, err)
+		return false
+	}
+	if !ok || !rl.Active() {
+		return false
+	}
+	return s.enforceLimit(store.RateLimitScopeDomain, domain, rl)
+}
+
+// enforceLimit counts recent messages for scope/ref and refuses when at or
+// above the ceiling. On admit it reserves an in-flight slot on the session.
+func (s *session) enforceLimit(scope, ref string, rl store.RateLimit) bool {
+	since := time.Now().Add(-time.Duration(rl.WindowSeconds) * time.Second)
+	n, err := s.rec.CountMessages(scope, ref, since)
+	if err != nil {
+		log.Printf("journal-milter: rate-limit count %s %q: %v (fail-open)", scope, ref, err)
+		return false
+	}
+	key := scope + "|" + ref
+	n += s.flight.count(key, since)
+	if n >= int64(rl.MaxMessages) {
+		log.Printf("journal-milter: %s %q over limit: %d/%d in %ds from %s — refusing 4xx",
+			scope, ref, n, rl.MaxMessages, rl.WindowSeconds, s.clientIP)
+		return true
+	}
+	s.reserved = append(s.reserved, s.flight.reserve(key))
 	return false
 }