#19 add mutex operation for caching

cache.go

@@ -3,9 +3,8 @@ package htmlquery
 import (
 	"sync"
 
-	"github.com/golang/groupcache/lru"
-
 	"github.com/antchfx/xpath"
+	"github.com/golang/groupcache/lru"
 )
 
 // DisableSelectorCache will disable caching for the query selector if value is true.
@@ -16,8 +15,9 @@ var DisableSelectorCache = false
 var SelectorCacheMaxEntries = 50
 
 var (
-	cacheOnce sync.Once
-	cache     *lru.Cache
+	cacheOnce  sync.Once
+	cache      *lru.Cache
+	cacheMutex sync.RWMutex
 )
 
 func getQuery(expr string) (*xpath.Expr, error) {
@@ -27,9 +27,14 @@ func getQuery(expr string) (*xpath.Expr, error) {
 	cacheOnce.Do(func() {
 		cache = lru.New(50)
 	})
+	cacheMutex.RLock()
 	if v, ok := cache.Get(expr); ok {
+		cacheMutex.RUnlock()
 		return v.(*xpath.Expr), nil
 	}
+	cacheMutex.RUnlock()
+	cacheMutex.Lock()
+	defer cacheMutex.Unlock()
 	v, err := xpath.Compile(expr)
 	if err != nil {
 		return nil, err