wgengine: drop userspaceEngine.peerSequence

Another baby step toward removing slices of peers from the engine.

getStatus iterated peerSequence (a key snapshot built in Reconfig
from cfg.Peers) and then asked wgdev for each peer's stats; peers
that weren't active in wgdev silently fell out. Iterate active wgdev
peers directly via RemoveMatchingPeers(returnFalse) instead.

Updates #12542

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I3abd348abc30db706db29b3a785179259e48abda

wgengine/userspace.go

@@ -142,7 +142,6 @@ type userspaceEngine struct {
 	netMap         *netmap.NetworkMap // or nil
 	closing        bool               // Close was called (even if we're still closing)
 	statusCallback StatusCallback
-	peerSequence   views.Slice[key.NodePublic]
 	endpoints      []tailcfg.Endpoint
 	pendOpen       map[flowtrackTuple]*pendingOpenFlow // see pendopen.go
 
@@ -829,15 +828,11 @@ func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, routerCfg *router.Config,
 	e.tundev.SetWGConfig(cfg)
 
 	peerSet := make(set.Set[key.NodePublic], len(cfg.Peers))
-
-	e.mu.Lock()
-	seq := make([]key.NodePublic, 0, len(cfg.Peers))
 	for _, p := range cfg.Peers {
-		seq = append(seq, p.PublicKey)
 		peerSet.Add(p.PublicKey)
 	}
-	e.peerSequence = views.SliceOf(seq)
 
+	e.mu.Lock()
 	nm := e.netMap
 	e.mu.Unlock()
 
@@ -1155,7 +1150,6 @@ func (e *userspaceEngine) getStatus() (*Status, error) {
 
 	e.mu.Lock()
 	closing := e.closing
-	peerKeys := e.peerSequence
 	localAddrs := slices.Clone(e.endpoints)
 	e.mu.Unlock()
 
@@ -1163,9 +1157,20 @@ func (e *userspaceEngine) getStatus() (*Status, error) {
 		return nil, ErrEngineClosing
 	}
 
-	peers := make([]ipnstate.PeerStatusLite, 0, peerKeys.Len())
-	for _, key := range peerKeys.All() {
-		if status, ok := e.getPeerStatusLite(key); ok {
+	// Snapshot the set of active wgdev peers. wireguard-go has no
+	// read-only iterator over its peer map; RemoveMatchingPeers with
+	// a callback that always returns false is the cheap equivalent
+	// (the callback can't itself call LookupActivePeer, though, as
+	// RemoveMatchingPeers holds the wireguard device's mutex).
+	var peerKeys []key.NodePublic
+	e.wgdev.RemoveMatchingPeers(func(pk device.NoisePublicKey) bool {
+		peerKeys = append(peerKeys, key.NodePublicFromRaw32(mem.B(pk[:])))
+		return false
+	})
+
+	peers := make([]ipnstate.PeerStatusLite, 0, len(peerKeys))
+	for _, k := range peerKeys {
+		if status, ok := e.getPeerStatusLite(k); ok {
 			peers = append(peers, status)
 		}
 	}