dynamolock/v2: fix lock race condition

Addresses #184

v2/client_test.go

@@ -26,7 +26,6 @@ import (
 	"net"
 	"os"
 	"os/exec"
-	"path/filepath"
 	"strings"
 	"sync"
 	"testing"
@@ -45,10 +44,11 @@ func TestMain(m *testing.M) {
 	if err != nil {
 		panic("cannot execute tests without Java")
 	}
-	_ = os.Remove(filepath.Join("local-dynamodb", "shared-local-instance.db")) // unconditionally remove state file
 	ctx, cancel := context.WithCancel(context.Background())
-	cmd := exec.CommandContext(ctx, javaPath, "-Djava.library.path=./DynamoDBLocal_lib", "-jar", "DynamoDBLocal.jar", "-sharedDb")
+	cmd := exec.CommandContext(ctx, javaPath, "-Djava.library.path=./DynamoDBLocal_lib", "-jar", "DynamoDBLocal.jar", "-sharedDb", "-inMemory")
 	cmd.Dir = "local-dynamodb"
+	cmd.Stderr = os.Stderr
+	cmd.Stdout = os.Stdout
 	if err := cmd.Start(); err != nil {
 		panic("cannot start local dynamodb:" + err.Error())
 	}
@@ -61,6 +61,7 @@ func TestMain(m *testing.M) {
 		c.Close()
 		break
 	}
+	time.Sleep(1 * time.Second)
 	exitCode := m.Run()
 	cancel()
 	cmd.Wait()

v2/lock.go

@@ -121,6 +121,11 @@ func (l *Lock) AdditionalAttributes() map[string]types.AttributeValue {
 // "danger zone". It returns false if the lock has not been released and the
 // lock has not yet entered the "danger zone"
 func (l *Lock) IsAlmostExpired() (bool, error) {
+	if l == nil {
+		return false, ErrLockAlreadyReleased
+	}
+	l.semaphore.Lock()
+	defer l.semaphore.Unlock()
 	t, err := l.timeUntilDangerZoneEntered()
 	if err != nil {
 		return false, err
@@ -143,7 +148,7 @@ func (l *Lock) timeUntilDangerZoneEntered() (time.Duration, error) {
 	if l.sessionMonitor == nil {
 		return 0, ErrSessionMonitorNotSet
 	}
-	if l.IsExpired() {
+	if l.isExpired() {
 		return 0, ErrLockAlreadyReleased
 	}
 	return l.sessionMonitor.timeUntilLeaseEntersDangerZone(l.lookupTime), nil