Fix panic on gateway.Query() with a nil QueryInput.QueryDocument (#195)

This can occur when calling graphql.IntrospectAPI() on the gateway itself.

internal.go

@@ -5,7 +5,9 @@ import (
 
 	"github.com/99designs/gqlgen/graphql/introspection"
 	"github.com/mitchellh/mapstructure"
+	"github.com/vektah/gqlparser/v2"
 	"github.com/vektah/gqlparser/v2/ast"
+	"github.com/vektah/gqlparser/v2/gqlerror"
 
 	"github.com/nautilus/graphql"
 )
@@ -48,6 +50,18 @@ func (g *Gateway) Query(ctx context.Context, input *graphql.QueryInput, receiver
 	// wrap the schema in something capable of introspection
 	introspectionSchema := introspection.WrapSchema(g.schema)
 
+	if input.QueryDocument == nil && input.Query != "" {
+		internalSchema, err := g.internalSchema()
+		if err != nil {
+			return err
+		}
+		var loadErr gqlerror.List
+		input.QueryDocument, loadErr = gqlparser.LoadQuery(internalSchema, input.Query)
+		if len(loadErr) > 0 {
+			return loadErr
+		}
+	}
+
 	// for local stuff we don't care about fragment directives
 	querySelection, err := graphql.ApplyFragments(input.QueryDocument.Operations[0].SelectionSet, input.QueryDocument.Fragments)
 	if err != nil {

internal_test.go

@@ -258,6 +258,48 @@ func TestSchemaIntrospection_missingType(t *testing.T) {
 	assert.Nil(t, result.Type)
 }
 
+func TestSchemaIntrospection_missingQueryDocument(t *testing.T) {
+	t.Parallel()
+	schema, err := graphql.LoadSchema("")
+	require.NoError(t, err)
+	gateway, err := New([]*graphql.RemoteSchema{
+		{Schema: schema, URL: "url1"},
+	})
+	require.NoError(t, err)
+
+	t.Run("valid query", func(t *testing.T) {
+		t.Parallel()
+		var result interface{}
+		err := gateway.Query(context.Background(), &graphql.QueryInput{
+			Query: `
+			{
+				__type(name: "Query") {
+					name
+				}
+			}
+		`,
+			QueryDocument: nil,
+		}, &result)
+		assert.NoError(t, err)
+		name := "Query"
+		assert.Equal(t, map[string]interface{}{
+			"__type": map[string]interface{}{
+				"name": &name,
+			},
+		}, result)
+	})
+
+	t.Run("invalid query", func(t *testing.T) {
+		t.Parallel()
+		var result interface{}
+		err := gateway.Query(context.Background(), &graphql.QueryInput{
+			Query:         "garbage",
+			QueryDocument: nil,
+		}, &result)
+		assert.EqualError(t, err, "input:1: Unexpected Name \"garbage\"\n")
+	})
+}
+
 func TestSchema_resolveNodeInlineID(t *testing.T) {
 	t.Parallel()
 	type Result struct {