wgengine/magicsock: don't use endpoint debug ringbuffer on mobile.

Save some memory.

Updates tailscale/corp#18514

Change-Id: Ibcaf3c6d8e5cc275c81f04141d0f176e2249509b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/ringbuffer/ringbuffer.go

@@ -24,7 +24,12 @@ type RingBuffer[T any] struct {
 
 // Add appends a new item to the RingBuffer, possibly overwriting the oldest
 // item in the buffer if it is already full.
+//
+// It does nothing if rb is nil.
 func (rb *RingBuffer[T]) Add(t T) {
+	if rb == nil {
+		return
+	}
 	rb.mu.Lock()
 	defer rb.mu.Unlock()
 	if len(rb.buf) < rb.max {
@@ -37,6 +42,8 @@ func (rb *RingBuffer[T]) Add(t T) {
 
 // GetAll returns a copy of all the entries in the ring buffer in the order they
 // were added.
+//
+// It returns nil if rb is nil.
 func (rb *RingBuffer[T]) GetAll() []T {
 	if rb == nil {
 		return nil

wgengine/magicsock/magicsock.go

@@ -1815,9 +1815,11 @@ func debugRingBufferSize(numPeers int) int {
 	}
 	var maxRingBufferSize int
 	if runtime.GOOS == "ios" || runtime.GOOS == "android" {
-		maxRingBufferSize = 1 * 1024 * 1024
+		maxRingBufferSize = 1 << 20
+		// But as of 2024-03-20, we now just disable the ring buffer entirely
+		// on mobile as it hadn't proven useful enough to justify even 1 MB.
 	} else {
-		maxRingBufferSize = 4 * 1024 * 1024
+		maxRingBufferSize = 4 << 20
 	}
 	if v := debugRingBufferMaxSizeBytes(); v > 0 {
 		maxRingBufferSize = v
@@ -1984,7 +1986,6 @@ func (c *Conn) SetNetworkMap(nm *netmap.NetworkMap) {
 
 		ep = &endpoint{
 			c:                 c,
-			debugUpdates:      ringbuffer.New[EndpointChange](entriesPerBuffer),
 			nodeID:            n.ID(),
 			publicKey:         n.Key(),
 			publicKeyHex:      n.Key().UntypedHexString(),
@@ -1993,6 +1994,14 @@ func (c *Conn) SetNetworkMap(nm *netmap.NetworkMap) {
 			heartbeatDisabled: flags.heartbeatDisabled,
 			isWireguardOnly:   n.IsWireGuardOnly(),
 		}
+		switch runtime.GOOS {
+		case "ios", "android":
+			// Omit, to save memory. Prior to 2024-03-20 we used to limit it to
+			// ~1MB on mobile but we never used the data so the memory was just
+			// wasted.
+		default:
+			ep.debugUpdates = ringbuffer.New[EndpointChange](entriesPerBuffer)
+		}
 		if n.Addresses().Len() > 0 {
 			ep.nodeAddr = n.Addresses().At(0).Addr()
 		}