Implement graceful shutdown of listen function

Before when pressing 'ctrl+c' and stopping the go program non of the
deferred functions in listen.go were called. A standard procedure for
stopping goroutines with context was implemented which shuts down the
program gracefully. Logs were added to notify the user that the shutdown
was successful.

Signed-off-by: Stanislav Jakuschevskij <stas@two-giants.com>

off_chain_data/application-go/listen.go

@@ -7,6 +7,10 @@ import (
 	"offChainData/processor"
 	"offChainData/store"
 	"offChainData/utils"
+	"os"
+	"os/signal"
+	"sync"
+	"syscall"
 
 	"github.com/hyperledger/fabric-gateway/pkg/client"
 	"google.golang.org/grpc"
@@ -18,14 +22,20 @@ func listen(clientConnection *grpc.ClientConn) {
 	if err != nil {
 		panic(err)
 	}
-	defer gateway.Close()
+	defer func() {
+		gateway.Close()
+		fmt.Println("Gateway closed.")
+	}()
 
 	checkpointFile := utils.EnvOrDefault("CHECKPOINT_FILE", "checkpoint.json")
 	checkpointer, err := client.NewFileCheckpointer(checkpointFile)
 	if err != nil {
 		panic(err)
 	}
-	defer checkpointer.Close()
+	defer func() {
+		checkpointer.Close()
+		fmt.Println("Checkpointer closed.")
+	}()
 
 	fmt.Println("Start event listening from block", checkpointer.BlockNumber())
 	fmt.Println("Last processed transaction ID within block:", checkpointer.TransactionID())
@@ -33,9 +43,11 @@ func listen(clientConnection *grpc.ClientConn) {
 		fmt.Printf("Simulating a write failure every %d transactions\n", store.SimulatedFailureCount)
 	}
 
-	// TODO put into infinite loop like in public docs example
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer func() {
+		stop()
+		fmt.Println("Context closed.")
+	}()
 
 	network := gateway.GetNetwork(channelName)
 	blocks, err := network.BlockEvents(
@@ -47,13 +59,28 @@ func listen(clientConnection *grpc.ClientConn) {
 		panic(err)
 	}
 
-	for blockProto := range blocks {
-		blockProcessor := processor.NewBlock(
-			parser.ParseBlock(blockProto),
-			checkpointer,
-			store.ApplyWritesToOffChainStore,
-			channelName,
-		)
-		blockProcessor.Process()
-	}
+	var wg sync.WaitGroup
+	wg.Add(1)
+
+	go func() {
+		defer wg.Done()
+
+		for blockProto := range blocks {
+			select {
+			case <-ctx.Done():
+				return
+			default:
+				blockProcessor := processor.NewBlock(
+					parser.ParseBlock(blockProto),
+					checkpointer,
+					store.ApplyWritesToOffChainStore,
+					channelName,
+				)
+				blockProcessor.Process()
+			}
+		}
+	}()
+
+	wg.Wait()
+	fmt.Println("\nReceived 'SIGTERM' signal. Shutting down listener gracefully...")
 }