Fix race in locker call to dec()

Can't safely use uint32 for locker since we need to decrement the count,
which requires loading the unit and doing some math, which is inherintly
racey.
Instead use Int32 which we can safely use with atomic and AddInt32 with
`-1`

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

locker/locker.go

@@ -32,22 +32,23 @@ type Locker struct {
 type lockCtr struct {
 	mu sync.Mutex
 	// waiters is the number of waiters waiting to acquire the lock
-	waiters uint32
+	// this is int32 instead of uint32 so we can add `-1` in `dec()`
+	waiters int32
 }
 
 // inc increments the number of waiters waiting for the lock
 func (l *lockCtr) inc() {
-	atomic.AddUint32(&l.waiters, 1)
+	atomic.AddInt32(&l.waiters, 1)
 }
 
 // dec decrements the number of waiters wating on the lock
 func (l *lockCtr) dec() {
-	atomic.AddUint32(&l.waiters, ^uint32(l.waiters-1))
+	atomic.AddInt32(&l.waiters, -1)
 }
 
 // count gets the current number of waiters
-func (l *lockCtr) count() uint32 {
-	return atomic.LoadUint32(&l.waiters)
+func (l *lockCtr) count() int32 {
+	return atomic.LoadInt32(&l.waiters)
 }
 
 // Lock locks the mutex

locker/locker_test.go

@@ -1,6 +1,7 @@
 package locker
 
 import (
+	"sync"
 	"testing"
 	"time"
 )
@@ -89,3 +90,35 @@ func TestLockerUnlock(t *testing.T) {
 		t.Fatalf("lock should not be blocked")
 	}
 }
+
+func TestLockerConcurrency(t *testing.T) {
+	l := New()
+
+	var wg sync.WaitGroup
+	for i := 0; i <= 10000; i++ {
+		wg.Add(1)
+		go func() {
+			l.Lock("test")
+			// if there is a concurrency issue, will very likely panic here
+			l.Unlock("test")
+			wg.Done()
+		}()
+	}
+
+	chDone := make(chan struct{})
+	go func() {
+		wg.Wait()
+		close(chDone)
+	}()
+
+	select {
+	case <-chDone:
+	case <-time.After(10 * time.Second):
+		t.Fatal("timeout waiting for locks to complete")
+	}
+
+	// Since everything has unlocked this should not exist anymore
+	if ctr, exists := l.locks["test"]; exists {
+		t.Fatalf("lock should not exist: %v", ctr)
+	}
+}