zebra: fix SRv6 encap lost during recursive nexthop resolution

tests/topotests/bgp_srv6_recursive_nhop_encap/r1/frr.conf

@@ -0,0 +1,74 @@
+! SPDX-License-Identifier: ISC
+!
+! r1 — router under test
+!
+! Topology:
+!   r1 ---eth0-eth0--- r2
+!
+! r1 receives 2600:1000::101/128 from r2 via IPv6 VPN with SRv6 encap and
+! imports it into vrf test1.  A local static IPv4 route for 198.0.0.150/32
+! points at that IPv6 nexthop, triggering the recursive resolution code path
+! in nexthop_set_resolved() that the fix targets.
+!
+hostname r1
+!
+interface lo
+ ipv6 address 2001:db8:1:1::1/128
+!
+interface eth0
+ ipv6 address 2001:db8:12::1/64
+!
+segment-routing
+ srv6
+  locators
+   locator loc1
+    prefix 2001:db8:1:1::/64 block-len 40 node-len 24 func-bits 16
+   !
+  !
+ !
+!
+ip forwarding
+ipv6 forwarding
+!
+! Underlay reachability to r2's loopback (VPN BGP nexthop)
+ipv6 route 2001:db8:2:2::/64 2001:db8:12::2
+!
+! Underlay reachability to r2's SRv6 locator (needed for kernel to forward SRv6 packets)
+ipv6 route 2001:db8:3::/48 2001:db8:12::2
+!
+! The IPv4 route under test: NH is an IPv6 address only reachable via the
+! SRv6 VPN route imported into vrf test1.  No SRv6 SID is associated with
+! this static route — its nexthop->nh_srv6 is NULL — which is exactly the
+! condition that exposed the bug.
+ip route 198.0.0.150/32 2600:1000::101 vrf test1
+!
+line vty
+!
+router bgp 65001
+ bgp router-id 1.1.1.1
+ no bgp ebgp-requires-policy
+ no bgp default ipv4-unicast
+ neighbor 2001:db8:12::2 remote-as 65002
+ neighbor 2001:db8:12::2 timers 3 10
+ neighbor 2001:db8:12::2 timers connect 1
+ neighbor 2001:db8:12::2 capability extended-nexthop
+ !
+ segment-routing srv6
+  locator loc1
+ !
+ address-family ipv6 vpn
+  neighbor 2001:db8:12::2 activate
+ exit-address-family
+!
+router bgp 65001 vrf test1
+ bgp router-id 1.1.1.1
+ no bgp ebgp-requires-policy
+ !
+ address-family ipv6 unicast
+  ! Import the SRv6 VPN route for 2600:1000::101/128 so the recursive
+  ! nexthop of 198.0.0.150/32 can be resolved through it.
+  rd vpn export 65001:10
+  rt vpn both 0:20
+  import vpn
+ exit-address-family
+!

tests/topotests/bgp_srv6_recursive_nhop_encap/r1/ipv4_rib_test1.json

@@ -0,0 +1,19 @@
+{
+    "198.0.0.150/32": [
+        {
+            "prefix": "198.0.0.150/32",
+            "protocol": "static",
+            "installed": true,
+            "nexthops": [
+                {
+                    "fib": true,
+                    "active": true,
+                    "seg6": {
+                        "segs": "2001:db8:3:1::"
+                    },
+                    "srv6EncapBehavior": "H.Encaps"
+                }
+            ]
+        }
+    ]
+}

tests/topotests/bgp_srv6_recursive_nhop_encap/r1/ipv6_rib_test1.json

@@ -0,0 +1,19 @@
+{
+    "2600:1000::101/128": [
+        {
+            "prefix": "2600:1000::101/128",
+            "protocol": "bgp",
+            "installed": true,
+            "nexthops": [
+                {
+                    "fib": true,
+                    "active": true,
+                    "seg6": {
+                        "segs": "2001:db8:3:1::"
+                    },
+                    "srv6EncapBehavior": "H.Encaps"
+                }
+            ]
+        }
+    ]
+}

tests/topotests/bgp_srv6_recursive_nhop_encap/r2/frr.conf

@@ -0,0 +1,66 @@
+! SPDX-License-Identifier: ISC
+!
+! r2 — peer / origin router
+!
+! r2 exports 2600:1000::101/128 from vrf test1 as an IPv6 VPN route with an
+! SRv6 SID drawn from locator 2001:db8:3::/48.  r1 imports this route into
+! its own vrf test1 so the recursive resolution of 198.0.0.150/32 goes
+! through a nexthop that carries SRv6 encapsulation information.
+!
+hostname r2
+!
+interface lo
+ ipv6 address 2001:db8:2:2::1/128
+!
+interface eth0
+ ipv6 address 2001:db8:12::2/64
+!
+segment-routing
+ srv6
+  locators
+   locator loc1
+    prefix 2001:db8:3::/48 block-len 32 node-len 16 func-bits 16
+   !
+  !
+ !
+!
+ip forwarding
+ipv6 forwarding
+!
+! Underlay reachability to r1's loopback
+ipv6 route 2001:db8:1:1::/64 2001:db8:12::1
+!
+! The SRv6 VPN destination address: exported to r1 with an SRv6 SID so that
+! r1's recursive resolution of 198.0.0.150/32 finds a nexthop with nh_srv6.
+ipv6 route 2600:1000::101/128 Null0 vrf test1
+!
+line vty
+!
+router bgp 65002
+ bgp router-id 2.2.2.2
+ no bgp ebgp-requires-policy
+ no bgp default ipv4-unicast
+ neighbor 2001:db8:12::1 remote-as 65001
+ neighbor 2001:db8:12::1 timers 3 10
+ neighbor 2001:db8:12::1 timers connect 1
+ neighbor 2001:db8:12::1 capability extended-nexthop
+ !
+ segment-routing srv6
+  locator loc1
+ !
+ address-family ipv6 vpn
+  neighbor 2001:db8:12::1 activate
+ exit-address-family
+!
+router bgp 65002 vrf test1
+ bgp router-id 2.2.2.2
+ no bgp ebgp-requires-policy
+ !
+ address-family ipv6 unicast
+  redistribute static
+  sid vpn export 1
+  rd vpn export 65002:20
+  rt vpn both 0:20
+  export vpn
+ exit-address-family
+!