fix(api): Don't send multiple responses for one request (#21651)

fix(api): Don't send responses twice.

In some cases multiple responses are being sent for one request, causing
the API server to incorrectly drop connections.

This change introduces a ResponseWriter which tracks whether a
response has already been sent. This is used to prevent a response being
sent if something already has (e.g. by a preconditions check function).

Fixes #21633.

Co-authored-by: Menno Finlay-Smits <hello@menno.io>

cmd/api-response_test.go

@@ -18,8 +18,12 @@
 package cmd
 
 import (
+	"io"
 	"net/http"
+	"net/http/httptest"
 	"testing"
+
+	"github.com/klauspost/compress/gzhttp"
 )
 
 // Tests object location.
@@ -122,3 +126,89 @@ func TestGetURLScheme(t *testing.T) {
 		t.Errorf("Expected %s, got %s", httpsScheme, gotScheme)
 	}
 }
+
+func TestTrackingResponseWriter(t *testing.T) {
+	rw := httptest.NewRecorder()
+	trw := &trackingResponseWriter{ResponseWriter: rw}
+	trw.WriteHeader(123)
+	if !trw.headerWritten {
+		t.Fatal("headerWritten was not set by WriteHeader call")
+	}
+
+	_, err := trw.Write([]byte("hello"))
+	if err != nil {
+		t.Fatalf("Write unexpectedly failed: %v", err)
+	}
+
+	// Check that WriteHeader and Write were called on the underlying response writer
+	resp := rw.Result()
+	if resp.StatusCode != 123 {
+		t.Fatalf("unexpected status: %v", resp.StatusCode)
+	}
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatalf("reading response body failed: %v", err)
+	}
+	if string(body) != "hello" {
+		t.Fatalf("response body incorrect: %v", string(body))
+	}
+
+	// Check that Unwrap works
+	if trw.Unwrap() != rw {
+		t.Fatalf("Unwrap returned wrong result: %v", trw.Unwrap())
+	}
+}
+
+func TestHeadersAlreadyWritten(t *testing.T) {
+	rw := httptest.NewRecorder()
+	trw := &trackingResponseWriter{ResponseWriter: rw}
+
+	if headersAlreadyWritten(trw) {
+		t.Fatal("headers have not been written yet")
+	}
+
+	trw.WriteHeader(123)
+	if !headersAlreadyWritten(trw) {
+		t.Fatal("headers were written")
+	}
+}
+
+func TestHeadersAlreadyWrittenWrapped(t *testing.T) {
+	rw := httptest.NewRecorder()
+	trw := &trackingResponseWriter{ResponseWriter: rw}
+	wrap1 := &gzhttp.NoGzipResponseWriter{ResponseWriter: trw}
+	wrap2 := &gzhttp.NoGzipResponseWriter{ResponseWriter: wrap1}
+
+	if headersAlreadyWritten(wrap2) {
+		t.Fatal("headers have not been written yet")
+	}
+
+	wrap2.WriteHeader(123)
+	if !headersAlreadyWritten(wrap2) {
+		t.Fatal("headers were written")
+	}
+}
+
+func TestWriteResponseHeadersNotWritten(t *testing.T) {
+	rw := httptest.NewRecorder()
+	trw := &trackingResponseWriter{ResponseWriter: rw}
+
+	writeResponse(trw, 299, []byte("hello"), "application/foo")
+
+	resp := rw.Result()
+	if resp.StatusCode != 299 {
+		t.Fatal("response wasn't written")
+	}
+}
+
+func TestWriteResponseHeadersWritten(t *testing.T) {
+	rw := httptest.NewRecorder()
+	rw.Code = -1
+	trw := &trackingResponseWriter{ResponseWriter: rw, headerWritten: true}
+
+	writeResponse(trw, 200, []byte("hello"), "application/foo")
+
+	if rw.Code != -1 {
+		t.Fatalf("response was written when it shouldn't have been (Code=%v)", rw.Code)
+	}
+}