policy/v2: add Grant.HasVia and tighten the changelog entry

CHANGELOG.md

@@ -137,7 +137,7 @@ clients, and how to run the same setup without Nix.
 - Fix SSH access surviving the removal of its policy rules; clients kept their last SSH rules, and a pending SSH check could still be approved [#3517](https://github.com/juanfont/headscale/pull/3517)
 - Fix SSH check periods coming from the first check rule for a node pair instead of the rule for the login user [#3517](https://github.com/juanfont/headscale/pull/3517)
 - Policy changes resend a node's SSH policy only when it changed, sparing clients a full netmap rebuild [#3517](https://github.com/juanfont/headscale/pull/3517)
-- Map generation no longer resolves every policy rule's sources and destinations for each peer when the policy has no `via` grants, which made map responses slow and CPU-bound on large tailnets [#3512](https://github.com/juanfont/headscale/issues/3512)
+- Fix every grant being fully resolved as if it had `via` when the policy has no `via` grants, slowing map generation on large tailnets [#3538](https://github.com/juanfont/headscale/pull/3538)
 
 ## 0.29.5 (202x-xx-xx)
 

hscontrol/policy/v2/policy.go

@@ -1369,7 +1369,7 @@ func (pm *PolicyManager) ViaRoutesForPeer(viewer, peer types.NodeView) types.Via
 	}
 
 	// Only via grants can add to the result, and ACLs never carry via.
-	if !slices.ContainsFunc(pm.pol.Grants, grantHasVia) {
+	if !slices.ContainsFunc(pm.pol.Grants, Grant.HasVia) {
 		return result
 	}
 
@@ -1419,7 +1419,7 @@ func (pm *PolicyManager) ViaRoutesForPeer(viewer, peer types.NodeView) types.Via
 	}
 
 	for i, grant := range grants {
-		if len(grant.Via) == 0 {
+		if !grant.HasVia() {
 			continue
 		}
 
@@ -1502,7 +1502,7 @@ func (pm *PolicyManager) ViaRoutesForPeer(viewer, peer types.NodeView) types.Via
 		// otherwise grants for other viewer groups would incorrectly
 		// demote the peer.
 		for i, grant := range grants {
-			if len(grant.Via) == 0 {
+			if !grant.HasVia() {
 				continue
 			}
 
@@ -1552,7 +1552,7 @@ func (pm *PolicyManager) ViaRoutesForPeer(viewer, peer types.NodeView) types.Via
 		// [state.State.RoutesForPeer] can apply normal
 		// [policy.ReduceRoutes] + primary logic.
 		for i, grant := range grants {
-			if len(grant.Via) > 0 {
+			if grant.HasVia() {
 				continue
 			}
 
@@ -1590,10 +1590,6 @@ func (pm *PolicyManager) ViaRoutesForPeer(viewer, peer types.NodeView) types.Via
 	return result
 }
 
-func grantHasVia(grant Grant) bool {
-	return len(grant.Via) > 0
-}
-
 // grantReachesInternet reports whether a grant's destinations include
 // the internet. Neither the wildcard nor autogroup:internet resolves
 // to 0.0.0.0/0, so check the aliases themselves.

hscontrol/policy/v2/types.go

@@ -1850,6 +1850,11 @@ type Grant struct {
 	Via []Tag `json:"via,omitzero"`
 }
 
+// HasVia reports whether the grant routes through via-tagged nodes.
+func (g Grant) HasVia() bool {
+	return len(g.Via) > 0
+}
+
 // NodeAttrGrant attaches Tailscale node capabilities (and/or an IP-pool
 // preference) to every node selected by Targets. The Targets aliases are
 // resolved exactly like ACL/grant sources, so users, groups, tags, hosts,