Prevention of Hijack() runtime panics

response_writer.go

@@ -6,6 +6,7 @@ package gin
 
 import (
 	"bufio"
+	"errors"
 	"io"
 	"net"
 	"net/http"
@@ -16,6 +17,8 @@ const (
 	defaultStatus = http.StatusOK
 )
 
+var errHijackAlreadyWritten = errors.New("gin: response already written")
+
 // ResponseWriter ...
 type ResponseWriter interface {
 	http.ResponseWriter
@@ -106,6 +109,9 @@ func (w *responseWriter) Written() bool {
 
 // Hijack implements the http.Hijacker interface.
 func (w *responseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	if w.Written() {
+		return nil, nil, errHijackAlreadyWritten
+	}
 	if w.size < 0 {
 		w.size = 0
 	}

response_writer_test.go

@@ -5,6 +5,8 @@
 package gin
 
 import (
+	"bufio"
+	"net"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -124,6 +126,74 @@ func TestResponseWriterHijack(t *testing.T) {
 	w.Flush()
 }
 
+type mockHijacker struct {
+	*httptest.ResponseRecorder
+	hijacked bool
+}
+
+// Hijack implements the http.Hijacker interface. It just records that it was called.
+func (m *mockHijacker) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	m.hijacked = true
+	return nil, nil, nil
+}
+
+func TestResponseWriterHijackAfterWrite(t *testing.T) {
+	tests := []struct {
+		name                      string
+		action                    func(w ResponseWriter) error // Action to perform before hijacking
+		expectWrittenBeforeHijack bool
+		expectHijackSuccess       bool
+		expectWrittenAfterHijack  bool
+		expectError               error
+	}{
+		{
+			name:                      "hijack before write should succeed",
+			action:                    func(w ResponseWriter) error { return nil },
+			expectWrittenBeforeHijack: false,
+			expectHijackSuccess:       true,
+			expectWrittenAfterHijack:  true, // Hijack itself marks the writer as written
+			expectError:               nil,
+		},
+		{
+			name: "hijack after write should fail",
+			action: func(w ResponseWriter) error {
+				_, err := w.Write([]byte("test"))
+				return err
+			},
+			expectWrittenBeforeHijack: true,
+			expectHijackSuccess:       false,
+			expectWrittenAfterHijack:  true,
+			expectError:               errHijackAlreadyWritten,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			hijacker := &mockHijacker{ResponseRecorder: httptest.NewRecorder()}
+			writer := &responseWriter{}
+			writer.reset(hijacker)
+			w := ResponseWriter(writer)
+
+			// Check initial state
+			assert.False(t, w.Written(), "should not be written initially")
+
+			// Perform pre-hijack action
+			require.NoError(t, tc.action(w), "unexpected error during pre-hijack action")
+
+			// Check state before hijacking
+			assert.Equal(t, tc.expectWrittenBeforeHijack, w.Written(), "unexpected w.Written() state before hijack")
+
+			// Attempt to hijack
+			_, _, hijackErr := w.Hijack()
+
+			// Check results
+			require.ErrorIs(t, hijackErr, tc.expectError, "unexpected error from Hijack()")
+			assert.Equal(t, tc.expectHijackSuccess, hijacker.hijacked, "unexpected hijacker.hijacked state")
+			assert.Equal(t, tc.expectWrittenAfterHijack, w.Written(), "unexpected w.Written() state after hijack")
+		})
+	}
+}
+
 func TestResponseWriterFlush(t *testing.T) {
 	testServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		writer := &responseWriter{}