net/http: return ErrNoCookie from Request.Cookie when name is ""

Request.Cookie(name string) will return the first cookie
when cookie name is "". Since readCookies in
file net/http/cookie.go at line 247 return all cookies
when second parameter is a empty string.

To fix it, Return ErrNoCookie from Request.Cookie(""),
instead of the first cookie in the request.

Fixes #53181

Change-Id: Ie623ca4c53da64ef7623a7863292a2d771f76832
GitHub-Last-Rev: 01098cd
GitHub-Pull-Request: #53183
Reviewed-on: https://go-review.googlesource.com/c/go/+/409754
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/net/http/request.go

@@ -416,6 +416,9 @@ var ErrNoCookie = errors.New("http: named cookie not present")
 // If multiple cookies match the given name, only one cookie will
 // be returned.
 func (r *Request) Cookie(name string) (*Cookie, error) {
+	if name == "" {
+		return nil, ErrNoCookie
+	}
 	for _, c := range readCookies(r.Header, name) {
 		return c, nil
 	}

src/net/http/request_test.go

@@ -1177,6 +1177,47 @@ func testMultipartFile(t *testing.T, req *Request, key, expectFilename, expectCo
 	return f
 }
 
+// Issue 53181: verify Request.Cookie return the correct Cookie.
+// Return ErrNoCookie instead of the first cookie when name is "".
+func TestRequestCookie(t *testing.T) {
+	for _, tt := range []struct {
+		name        string
+		value       string
+		expectedErr error
+	}{
+		{
+			name:        "foo",
+			value:       "bar",
+			expectedErr: nil,
+		},
+		{
+			name:        "",
+			expectedErr: ErrNoCookie,
+		},
+	} {
+		req, err := NewRequest("GET", "http://example.com/", nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		req.AddCookie(&Cookie{Name: tt.name, Value: tt.value})
+		c, err := req.Cookie(tt.name)
+		if err != tt.expectedErr {
+			t.Errorf("got %v, want %v", err, tt.expectedErr)
+		}
+
+		// skip if error occured.
+		if err != nil {
+			continue
+		}
+		if c.Value != tt.value {
+			t.Errorf("got %v, want %v", c.Value, tt.value)
+		}
+		if c.Name != tt.name {
+			t.Errorf("got %s, want %v", tt.name, c.Name)
+		}
+	}
+}
+
 const (
 	fileaContents = "This is a test file."
 	filebContents = "Another test file."