feat: added consumer.WaitForHandlers

Signed-off-by: Ben Meier <ben.meier@humanitec.com>

consume.go

@@ -1,11 +1,13 @@
 package rabbitmq
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"sync"
 
 	amqp "github.com/rabbitmq/amqp091-go"
+
 	"github.com/wagslane/go-rabbitmq/internal/channelmanager"
 )
 
@@ -33,6 +35,10 @@ type Consumer struct {
 	closeConnectionToManagerCh chan<- struct{}
 	options                    ConsumerOptions
 
+	// the handlerMux wraps handler execution in a read-lock and allows the write-lock to be used to wait for
+	// all handlers to complete their current messages.
+	handlerMux *sync.RWMutex
+
 	isClosedMux *sync.RWMutex
 	isClosed    bool
 }
@@ -78,6 +84,14 @@ func NewConsumer(
 		isClosed:                   false,
 	}
 
+	handler = func(d Delivery) (action Action) {
+		if !consumer.handlerMux.TryRLock() {
+			return NackRequeue
+		}
+		defer consumer.handlerMux.RUnlock()
+		return handler(d)
+	}
+
 	err = consumer.startGoroutines(
 		handler,
 		*options,
@@ -125,6 +139,30 @@ func (consumer *Consumer) Close() {
 	}()
 }
 
+// WaitForHandlers will wait for all handler goroutines to finish processing their current messages and may issue
+// NackRequeue for any remaining messages if auto-ack is not enabled. So this should only be called after Close has
+// called or may be used as a circuit breaker to interrupt processing.
+// If the context is cancelled before the handlers finish, the error from the context will be returned.
+func (consumer *Consumer) WaitForHandlers(ctx context.Context) error {
+	if ctx == nil {
+		ctx = context.Background()
+	} else if ctx.Err() != nil {
+		return ctx.Err()
+	}
+	c := make(chan struct{})
+	go func() {
+		consumer.handlerMux.Lock()
+		defer consumer.handlerMux.Unlock()
+		close(c)
+	}()
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case <-c:
+		return nil
+	}
+}
+
 // startGoroutines declares the queue if it doesn't exist,
 // binds the queue to the routing key(s), and starts the goroutines
 // that will consume from the queue

examples/consumer/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"context"
 	"fmt"
 	"log"
 	"os"
@@ -35,7 +36,13 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	defer consumer.Close()
+	defer func() {
+		consumer.Close()
+
+		// We can optionally wait for any long-running handlers to finish cleanly and avoid stopping the process
+		// while handlers are still closing files or performing actions that are vulnerable to corruption.
+		_ = consumer.WaitForHandlers(context.Background())
+	}()
 
 	// block main thread - wait for shutdown signal
 	sigs := make(chan os.Signal, 1)