Merge pull request #939 from pascaldekloe/master

FIX: WATCH example stack protection (with retry count)

example_test.go

@@ -1,8 +1,8 @@
 package redis_test
 
 import (
+	"errors"
 	"fmt"
-	"strconv"
 	"sync"
 	"time"
 
@@ -278,45 +278,55 @@ func ExampleClient_TxPipeline() {
 }
 
 func ExampleClient_Watch() {
-	var incr func(string) error
+	const routineCount = 100
 
 	// Transactionally increments key using GET and SET commands.
-	incr = func(key string) error {
-		err := redisdb.Watch(func(tx *redis.Tx) error {
-			n, err := tx.Get(key).Int64()
+	increment := func(key string) error {
+		txf := func(tx *redis.Tx) error {
+			// get current value or zero
+			n, err := tx.Get(key).Int()
 			if err != nil && err != redis.Nil {
 				return err
 			}
 
+			// actual opperation (local in optimistic lock)
+			n++
+
+			// runs only if the watched keys remain unchanged
 			_, err = tx.Pipelined(func(pipe redis.Pipeliner) error {
-				pipe.Set(key, strconv.FormatInt(n+1, 10), 0)
+				// pipe handles the error case
+				pipe.Set(key, n, 0)
 				return nil
 			})
 			return err
-		}, key)
-		if err == redis.TxFailedErr {
-			return incr(key)
 		}
+
+		for retries := routineCount; retries > 0; retries-- {
+			err := redisdb.Watch(txf, key)
+			if err != redis.TxFailedErr {
 				return err
 			}
+			// optimistic lock lost
+		}
+		return errors.New("increment reached maximum number of retries")
+	}
 
 	var wg sync.WaitGroup
-	for i := 0; i < 100; i++ {
-		wg.Add(1)
+	wg.Add(routineCount)
+	for i := 0; i < routineCount; i++ {
 		go func() {
 			defer wg.Done()
 
-			err := incr("counter3")
-			if err != nil {
-				panic(err)
+			if err := increment("counter3"); err != nil {
+				fmt.Println("increment error:", err)
 			}
 		}()
 	}
 	wg.Wait()
 
-	n, err := redisdb.Get("counter3").Int64()
-	fmt.Println(n, err)
-	// Output: 100 <nil>
+	n, err := redisdb.Get("counter3").Int()
+	fmt.Println("ended with", n, err)
+	// Output: ended with 100 <nil>
 }
 
 func ExamplePubSub() {