[client] Reconnect management and signal promptly after a network change

Three pieces, each verified on Android across WiFi/cellular switches:

- A sweep-aware backoff wrapper: the first retry after a disconnect
  that follows a recent network-change mark comes after 200ms instead
  of the randomized [0..1.6s] interval. Any other failure keeps the
  unchanged spread, so the clients of a restarted server still scatter
  their reconnects.
- The retry sleep wakes on OS network availability transitions
  (nbgrpc.Retry): a disconnect that precedes the offline flag by a few
  milliseconds no longer sleeps blindly through the recovery - the
  loop parks on the netstate gate and resumes the moment the network
  returns.
- The connection state is re-checked after WaitForStateChange: a dial
  settling in Ready proceeds immediately instead of burning another
  backoff round on an already-usable channel.

Measured after a network switch: management and signal recover in
270-470ms deterministically, down from a 312-1593ms lottery.

client/grpc/retry.go

@@ -0,0 +1,49 @@
+package grpc
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"github.com/cenkalti/backoff/v4"
+
+	"github.com/netbirdio/netbird/client/netstate"
+)
+
+// Retry mirrors backoff.Retry, but the sleep between attempts also wakes on
+// OS network availability transitions: an operation cut down by a network
+// change retries the moment the network settles instead of sleeping through
+// the recovery. A nil netState never fires, leaving plain backoff.Retry
+// behavior.
+func Retry(ctx context.Context, operation backoff.Operation, bo backoff.BackOff, netState *netstate.State) error {
+	bo.Reset()
+	for {
+		err := operation()
+		if err == nil {
+			return nil
+		}
+
+		var permanent *backoff.PermanentError
+		if errors.As(err, &permanent) {
+			return permanent.Err
+		}
+
+		next := bo.NextBackOff()
+		if next == backoff.Stop {
+			if cerr := ctx.Err(); cerr != nil {
+				return cerr
+			}
+			return err
+		}
+
+		timer := time.NewTimer(next)
+		select {
+		case <-timer.C:
+		case <-netState.Changed():
+			timer.Stop()
+		case <-ctx.Done():
+			timer.Stop()
+			return ctx.Err()
+		}
+	}
+}

client/grpc/retry_test.go

@@ -0,0 +1,91 @@
+package grpc
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/cenkalti/backoff/v4"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/netbirdio/netbird/client/netstate"
+)
+
+func TestRetryWakesOnNetworkChange(t *testing.T) {
+	ns := netstate.New()
+	attempts := 0
+	operation := func() error {
+		attempts++
+		if attempts == 1 {
+			return errors.New("cut by network change")
+		}
+		return nil
+	}
+
+	go func() {
+		time.Sleep(20 * time.Millisecond)
+		ns.Set(false)
+	}()
+
+	start := time.Now()
+	err := Retry(context.Background(), operation, backoff.NewConstantBackOff(time.Minute), ns)
+
+	require.NoError(t, err)
+	assert.Equal(t, 2, attempts)
+	assert.Less(t, time.Since(start), time.Second, "the transition must cut the minute-long sleep short")
+}
+
+func TestRetryPermanentError(t *testing.T) {
+	sentinel := errors.New("permission denied")
+	operation := func() error {
+		return backoff.Permanent(sentinel)
+	}
+
+	err := Retry(context.Background(), operation, backoff.NewConstantBackOff(time.Millisecond), nil)
+	assert.ErrorIs(t, err, sentinel)
+}
+
+func TestRetryNilNetState(t *testing.T) {
+	attempts := 0
+	operation := func() error {
+		attempts++
+		if attempts < 3 {
+			return errors.New("transient")
+		}
+		return nil
+	}
+
+	err := Retry(context.Background(), operation, backoff.NewConstantBackOff(time.Millisecond), nil)
+	require.NoError(t, err)
+	assert.Equal(t, 3, attempts)
+}
+
+func TestRetryStops(t *testing.T) {
+	failure := errors.New("still failing")
+	operation := func() error {
+		return failure
+	}
+
+	err := Retry(context.Background(), operation, &backoff.StopBackOff{}, nil)
+	assert.ErrorIs(t, err, failure)
+}
+
+func TestRetryCtxCancelDuringSleep(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	operation := func() error {
+		return errors.New("failing")
+	}
+
+	go func() {
+		time.Sleep(20 * time.Millisecond)
+		cancel()
+	}()
+
+	start := time.Now()
+	err := Retry(ctx, operation, backoff.NewConstantBackOff(time.Minute), netstate.New())
+
+	assert.ErrorIs(t, err, context.Canceled)
+	assert.Less(t, time.Since(start), time.Second)
+}

client/netsweep/netsweep.go

@@ -13,13 +13,18 @@ import (
 	"sync"
 	"time"
 
+	"github.com/cenkalti/backoff/v4"
 	log "github.com/sirupsen/logrus"
+
+	"github.com/netbirdio/netbird/client/netstate"
 )
 
 // DefaultSweepDelay absorbs network flapping while the OS settles on a
 // default network before the stale registrations are cut.
 const DefaultSweepDelay = 500 * time.Millisecond
 
+const recentMarkWindow = 3 * time.Second
+
 // Config customizes a Sweeper. The zero value applies the defaults.
 type Config struct {
 	// SweepDelay overrides DefaultSweepDelay when positive.
@@ -96,6 +101,7 @@ type Sweeper struct {
 	gen        uint64
 	timer      *time.Timer
 	sweepDelay time.Duration
+	lastMark   time.Time
 }
 
 // New creates an empty sweeper with the default configuration.
@@ -179,6 +185,7 @@ func (s *Sweeper) MarkNetworkChange() {
 	s.mu.Lock()
 	s.gen++
 	cutoff := s.gen
+	s.lastMark = time.Now()
 	if s.timer != nil {
 		s.timer.Stop()
 	}
@@ -189,6 +196,27 @@ func (s *Sweeper) MarkNetworkChange() {
 	s.mu.Unlock()
 }
 
+// QuickRetryBackoff wraps bo so that after each Reset the first retry comes
+// quickly when the disconnect followed a recent network change and the
+// network is online. Any other failure keeps bo's spread, so the clients of
+// a restarted server still scatter their reconnects. A nil sweeper returns
+// bo unchanged.
+func (s *Sweeper) QuickRetryBackoff(ctx context.Context, bo backoff.BackOff, netState *netstate.State) backoff.BackOff {
+	if s == nil {
+		return bo
+	}
+	return backoff.WithContext(newQuickRetryBackoff(bo, s, netState), ctx)
+}
+
+func (s *Sweeper) markedRecently() bool {
+	if s == nil {
+		return false
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return !s.lastMark.IsZero() && time.Since(s.lastMark) < recentMarkWindow
+}
+
 // sweep closes the registered connections and aborts the in-flight dials
 // older than cutoff, and returns how many connections it closed. A dial
 // whose connection was not yet handed to WrapConn is marked, so the late

client/netsweep/quick_retry.go

@@ -0,0 +1,39 @@
+package netsweep
+
+import (
+	"time"
+
+	"github.com/cenkalti/backoff/v4"
+
+	"github.com/netbirdio/netbird/client/netstate"
+)
+
+const quickRetryDelay = 200 * time.Millisecond
+
+type quickRetryBackoff struct {
+	backoff.BackOff
+	sweeper  *Sweeper
+	netState *netstate.State
+	used     bool
+}
+
+func newQuickRetryBackoff(bo backoff.BackOff, sweeper *Sweeper, netState *netstate.State) *quickRetryBackoff {
+	return &quickRetryBackoff{
+		BackOff:  bo,
+		sweeper:  sweeper,
+		netState: netState,
+	}
+}
+
+func (b *quickRetryBackoff) NextBackOff() time.Duration {
+	if !b.used && b.sweeper.markedRecently() && b.netState.IsOnline() {
+		b.used = true
+		return quickRetryDelay
+	}
+	return b.BackOff.NextBackOff()
+}
+
+func (b *quickRetryBackoff) Reset() {
+	b.used = false
+	b.BackOff.Reset()
+}

client/netsweep/quick_retry_test.go

@@ -0,0 +1,58 @@
+package netsweep
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/cenkalti/backoff/v4"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestQuickRetryAfterRecentMark(t *testing.T) {
+	sweeper := New()
+	sweeper.MarkNetworkChange()
+
+	slow := backoff.NewConstantBackOff(5 * time.Second)
+	bo := sweeper.QuickRetryBackoff(context.Background(), slow, nil)
+
+	assert.Equal(t, quickRetryDelay, bo.NextBackOff(), "first retry after a mark must be quick")
+	assert.Equal(t, 5*time.Second, bo.NextBackOff(), "second retry must fall back to the wrapped backoff")
+
+	bo.Reset()
+	assert.Equal(t, quickRetryDelay, bo.NextBackOff(), "reset must re-arm the quick retry")
+}
+
+func TestQuickRetryWithoutMarkKeepsSpread(t *testing.T) {
+	sweeper := New()
+
+	slow := backoff.NewConstantBackOff(5 * time.Second)
+	bo := sweeper.QuickRetryBackoff(context.Background(), slow, nil)
+
+	assert.Equal(t, 5*time.Second, bo.NextBackOff(), "without a mark the wrapped backoff decides")
+
+	sweeper.mu.Lock()
+	sweeper.lastMark = time.Now().Add(-recentMarkWindow)
+	sweeper.mu.Unlock()
+	assert.Equal(t, 5*time.Second, bo.NextBackOff(), "a stale mark must not trigger the quick retry")
+}
+
+func TestQuickRetryNilSweeperPassthrough(t *testing.T) {
+	var sweeper *Sweeper
+
+	slow := backoff.NewConstantBackOff(5 * time.Second)
+	bo := sweeper.QuickRetryBackoff(context.Background(), slow, nil)
+
+	assert.Equal(t, backoff.BackOff(slow), bo, "nil sweeper must return the backoff unchanged")
+}
+
+func TestQuickRetryHonorsContext(t *testing.T) {
+	sweeper := New()
+	sweeper.MarkNetworkChange()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	bo := sweeper.QuickRetryBackoff(ctx, backoff.NewConstantBackOff(time.Millisecond), nil)
+
+	assert.Equal(t, backoff.Stop, bo.NextBackOff(), "cancelled context must stop the retry loop")
+}