fix: empty querystring must be cached (eggjs#3535)

app/extend/request.js

@@ -139,11 +139,7 @@ module.exports = {
   // How to read query safely
   // https://github.com/koajs/qs/issues/5
   _customQuery(cacheName, filter) {
-    const str = this.querystring;
-    if (!str) {
-      return {};
-    }
-
+    const str = this.querystring || '';
     let c = this[cacheName];
     if (!c) {
       c = this[cacheName] = {};
@@ -153,7 +149,7 @@ module.exports = {
       cacheQuery = c[str] = {};
       const isQueries = cacheName === _queriesCache;
       // `querystring.parse` CANNOT parse something like `a[foo]=1&a[bar]=2`
-      const query = querystring.parse(str);
+      const query = str ? querystring.parse(str) : {};
       for (const key in query) {
         if (!key) {
           // key is '', like `a=b&`

test/app/extend/request.test.js

@@ -165,6 +165,28 @@ describe('test/app/extend/request.test.js', () => {
       });
     });
 
+    describe('this.query && this.queries can be modified', () => {
+      it('should success with querystring present', () => {
+        req.querystring = 'a=a&b=b1&b=b2';
+        assert.deepEqual(req.query, { a: 'a', b: 'b1' });
+        assert.deepEqual(req.queries, { a: [ 'a' ], b: [ 'b1', 'b2' ] });
+        req.query.a = 'aa';
+        req.queries.b = [ 'bb' ];
+        assert.deepEqual(req.query, { a: 'aa', b: 'b1' });
+        assert.deepEqual(req.queries, { a: [ 'a' ], b: [ 'bb' ] });
+      });
+
+      it('should success with empty querystring', () => {
+        req.querystring = '';
+        assert.deepEqual(req.query, {});
+        assert.deepEqual(req.queries, {});
+        req.query.a = 'aa';
+        req.queries.b = [ 'bb' ];
+        assert.deepEqual(req.query, { a: 'aa' });
+        assert.deepEqual(req.queries, { b: [ 'bb' ] });
+      });
+    });
+
     describe('this.query[key] => String', () => {
       function expectQuery(querystring, query) {
         mm(req, 'querystring', querystring);