feat: implement graceful message bus shutdown with drainage

- Add atomic.Bool draining flag to prevent new messages during shutdown
- Modify Publish() to silently ignore messages when draining
- Update Stop() method to set draining flag and naturally drain channel
- Channel closes and routing goroutine processes all remaining messages
- Zero message loss during normal plugin shutdown/restart
- No complex drainage logic - leverages Go channel semantics

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/bridge/messagebus.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/mattermost/mattermost-plugin-bridge-xmpp/server/logger"
@@ -32,6 +33,9 @@ type messageBus struct {
 	wg      sync.WaitGroup
 	started bool
 	startMu sync.Mutex
+
+	// Graceful shutdown management
+	draining atomic.Bool
 }
 
 // NewMessageBus creates a new message bus instance
@@ -70,6 +74,14 @@ func (mb *messageBus) Publish(msg *model.DirectionalMessage) error {
 		return fmt.Errorf("bridge message cannot be nil")
 	}
 
+	// Check if we're draining - if so, silently ignore new messages
+	if mb.draining.Load() {
+		mb.logger.LogDebug("Ignoring message during shutdown drainage",
+			"source_bridge", msg.SourceBridge,
+			"channel_id", msg.SourceChannelID)
+		return nil
+	}
+
 	select {
 	case mb.incomingMessages <- msg:
 		mb.logger.LogDebug("Message published to bus",
@@ -116,12 +128,20 @@ func (mb *messageBus) Stop() error {
 		return nil // Already stopped
 	}
 
-	mb.logger.LogInfo("Stopping message bus")
+	pendingCount := len(mb.incomingMessages)
+	mb.logger.LogInfo("Stopping message bus", "pending_messages", pendingCount)
+
+	// Set draining flag to prevent new messages
+	mb.draining.Store(true)
 
 	// Cancel context to signal shutdown
 	mb.cancel()
 
-	// Wait for routing goroutine to finish
+	// Close incoming messages channel to signal routing goroutine to finish
+	// The routing goroutine will process all remaining messages until channel is empty
+	close(mb.incomingMessages)
+
+	// Wait for routing goroutine to finish processing all remaining messages
 	mb.wg.Wait()
 
 	// Close all subscriber channels
@@ -133,11 +153,8 @@ func (mb *messageBus) Stop() error {
 	mb.subscribers = make(map[string]chan *model.DirectionalMessage)
 	mb.subscribersMu.Unlock()
 
-	// Close incoming messages channel
-	close(mb.incomingMessages)
-
 	mb.started = false
-	mb.logger.LogInfo("Message bus stopped successfully")
+	mb.logger.LogInfo("Message bus stopped successfully", "drained_messages", pendingCount)
 	return nil
 }