Add query cache

This commit adds a query cache with a configurable maximum size.
Past this size, queries are evicted from the cache on an LRU basis.

The default cache size is 1000, chosen fairly arbitrarily. If the size
is configured with a non-positive value, then the cache is disabled.

Also ran `dep ensure` to add the new dependency to `Gopkg.lock`.

handler/graphql.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/99designs/gqlgen/graphql"
 	"github.com/gorilla/websocket"
+	"github.com/hashicorp/golang-lru"
 	"github.com/vektah/gqlparser"
 	"github.com/vektah/gqlparser/ast"
 	"github.com/vektah/gqlparser/gqlerror"
@@ -23,6 +24,7 @@ type params struct {
 }
 
 type Config struct {
+	cacheSize      int
 	upgrader       websocket.Upgrader
 	recover        graphql.RecoverFunc
 	errorPresenter graphql.ErrorPresenterFunc
@@ -110,8 +112,19 @@ func RequestMiddleware(middleware graphql.RequestMiddleware) Option {
 	}
 }
 
+// CacheSize sets the maximum size of the query cache.
+// If size is less than or equal to 0, the cache is disabled.
+func CacheSize(size int) Option {
+	return func(cfg *Config) {
+		cfg.cacheSize = size
+	}
+}
+
+const DefaultCacheSize = 1000
+
 func GraphQL(exec graphql.ExecutableSchema, options ...Option) http.HandlerFunc {
 	cfg := Config{
+		cacheSize: DefaultCacheSize,
 		upgrader: websocket.Upgrader{
 			ReadBufferSize:  1024,
 			WriteBufferSize: 1024,
@@ -122,6 +135,13 @@ func GraphQL(exec graphql.ExecutableSchema, options ...Option) http.HandlerFunc
 		option(&cfg)
 	}
 
+	var cache *lru.Cache
+	if cfg.cacheSize > 0 {
+		// An error is only returned for non-positive cache size
+		// and we already checked for that.
+		cache, _ = lru.New(DefaultCacheSize)
+	}
+
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.Method == http.MethodOptions {
 			w.Header().Set("Allow", "OPTIONS, GET, POST")
@@ -157,10 +177,23 @@ func GraphQL(exec graphql.ExecutableSchema, options ...Option) http.HandlerFunc
 		}
 		w.Header().Set("Content-Type", "application/json")
 
-		doc, qErr := gqlparser.LoadQuery(exec.Schema(), reqParams.Query)
-		if len(qErr) > 0 {
-			sendError(w, http.StatusUnprocessableEntity, qErr...)
-			return
+		var doc *ast.QueryDocument
+		if cache != nil {
+			val, ok := cache.Get(reqParams.Query)
+			if ok {
+				doc = val.(*ast.QueryDocument)
+			}
+		}
+		if doc == nil {
+			var qErr gqlerror.List
+			doc, qErr = gqlparser.LoadQuery(exec.Schema(), reqParams.Query)
+			if len(qErr) > 0 {
+				sendError(w, http.StatusUnprocessableEntity, qErr...)
+				return
+			}
+			if cache != nil {
+				cache.Add(reqParams.Query, doc)
+			}
 		}
 
 		op := doc.Operations.ForName(reqParams.OperationName)