[relay] Gate the quick reconnect on network availability

Replace the fixed 1.5s pre-dial sleep with a wait driven by the OS
availability verdict: while online only a 200ms settle window is spent
(the offline flag lands a few milliseconds after the disconnect that
triggered the reconnect), and while offline the guard waits for the
network to return, bounded by the same 1.5s budget as before. Without
an injected netState the behavior is unchanged.

Measured on Android: relay recovery after a network switch drops from
1.7-1.9s to under 500ms after the cut.

shared/relay/client/guard.go

@@ -11,7 +11,18 @@ import (
 	"github.com/netbirdio/netbird/client/netstate"
 )
 
-const defaultMaxBackoffInterval = 60 * time.Second
+const (
+	defaultMaxBackoffInterval = 60 * time.Second
+
+	// quickReconnectBudget bounds how long a quick reconnect waits for the
+	// network before handing the retry over to the ticker.
+	quickReconnectBudget = 1500 * time.Millisecond
+
+	// verdictSettleWindow is how long an online verdict must hold before it
+	// is trusted: the disconnect often precedes the OS offline flag by a few
+	// milliseconds.
+	verdictSettleWindow = 200 * time.Millisecond
+)
 
 // Guard manage the reconnection tries to the Relay server in case of disconnection event.
 type Guard struct {
@@ -118,13 +129,11 @@ func (g *Guard) tryToQuickReconnect(parentCtx context.Context, rc *Client) bool
 		return false
 	}
 
-	if cancelled := waiteBeforeRetry(parentCtx); !cancelled {
+	if ok := g.waitForNetwork(parentCtx); !ok {
 		return false
 	}
 
-	// Re-check after the wait: the disconnect that triggered this reconnect
-	// is often the first symptom of the network going away, so the
-	// availability flag typically arrives while we sleep here.
+	// Still offline after the budget: leave the retry to the ticker.
 	if !g.netState.IsOnline() {
 		return false
 	}
@@ -191,14 +200,40 @@ func (g *Guard) exponentTicker(ctx context.Context) *backoff.Ticker {
 	return backoff.NewTicker(bo)
 }
 
-func waiteBeforeRetry(ctx context.Context) bool {
-	timer := time.NewTimer(1500 * time.Millisecond)
-	defer timer.Stop()
+// waitForNetwork waits out the settle window while online, or waits for the
+// network to return while offline, within the budget. Returns false when ctx
+// is cancelled. Without an injected netState it degrades to a fixed
+// budget-long sleep, the pre-netstate behavior.
+func (g *Guard) waitForNetwork(ctx context.Context) bool {
+	budget := time.NewTimer(quickReconnectBudget)
+	defer budget.Stop()
 
-	select {
-	case <-timer.C:
-		return true
-	case <-ctx.Done():
-		return false
+	settleWindow := verdictSettleWindow
+	if g.netState == nil {
+		settleWindow = quickReconnectBudget
+	}
+	settle := time.NewTimer(settleWindow)
+	defer settle.Stop()
+
+	for {
+		// Channel first, flag second: a flip in between still fires the channel.
+		changedCh := g.netState.Changed()
+		if g.netState.IsOnline() {
+			select {
+			case <-settle.C:
+				return true
+			case <-changedCh:
+				continue
+			case <-ctx.Done():
+				return false
+			}
+		}
+		select {
+		case <-budget.C:
+			return true
+		case <-changedCh:
+		case <-ctx.Done():
+			return false
+		}
 	}
 }