Fix a race condition where a concurrent user-issued Up or Down command (#5418)

could interleave with a sleep/wake event causing out-of-order state
transitions. The mutex now covers the full duration of each handler
including the status check, the Up/Down call, and the flag update.

Note: if Up or Down commands are triggered in parallel with sleep/wake
events, the overall ordering of up/down/sleep/wake operations is still
not guaranteed beyond what the mutex provides within the handler itself.

client/internal/sleep/handler/handler.go

@@ -0,0 +1,80 @@
+package handler
+
+import (
+	"context"
+	"sync"
+
+	log "github.com/sirupsen/logrus"
+
+	"github.com/netbirdio/netbird/client/internal"
+)
+
+type Agent interface {
+	Up(ctx context.Context) error
+	Down(ctx context.Context) error
+	Status() (internal.StatusType, error)
+}
+
+type SleepHandler struct {
+	agent Agent
+
+	mu sync.Mutex
+	// sleepTriggeredDown indicates whether the sleep handler triggered the last client down, to avoid unnecessary up on wake
+	sleepTriggeredDown bool
+}
+
+func New(agent Agent) *SleepHandler {
+	return &SleepHandler{
+		agent: agent,
+	}
+}
+
+func (s *SleepHandler) HandleWakeUp(ctx context.Context) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if !s.sleepTriggeredDown {
+		log.Info("skipping up because wasn't sleep down")
+		return nil
+	}
+
+	// avoid other wakeup runs if sleep didn't make the computer sleep
+	s.sleepTriggeredDown = false
+
+	log.Info("running up after wake up")
+	err := s.agent.Up(ctx)
+	if err != nil {
+		log.Errorf("running up failed: %v", err)
+		return err
+	}
+
+	log.Info("running up command executed successfully")
+	return nil
+}
+
+func (s *SleepHandler) HandleSleep(ctx context.Context) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	status, err := s.agent.Status()
+	if err != nil {
+		return err
+	}
+
+	if status != internal.StatusConnecting && status != internal.StatusConnected {
+		log.Infof("skipping setting the agent down because status is %s", status)
+		return nil
+	}
+
+	log.Info("running down after system started sleeping")
+
+	if err = s.agent.Down(ctx); err != nil {
+		log.Errorf("running down failed: %v", err)
+		return err
+	}
+
+	s.sleepTriggeredDown = true
+
+	log.Info("running down executed successfully")
+	return nil
+}