optimized render.JSON (#929)

* api/render: render JSON directly to the underlying writer

* also consider json.MarshalerError a panic

api/render/render.go

@@ -2,7 +2,6 @@
 package render
 
 import (
-	"bytes"
 	"encoding/json"
 	"errors"
 	"net/http"
@@ -24,14 +23,25 @@ func JSON(w http.ResponseWriter, v interface{}) {
 // JSONStatus sets the Content-Type of w to application/json unless one is
 // specified.
 func JSONStatus(w http.ResponseWriter, v interface{}, status int) {
-	var b bytes.Buffer
-	if err := json.NewEncoder(&b).Encode(v); err != nil {
+	setContentTypeUnlessPresent(w, "application/json")
+	w.WriteHeader(status)
+
+	if err := json.NewEncoder(w).Encode(v); err != nil {
+		var errUnsupportedType *json.UnsupportedTypeError
+		if errors.As(err, &errUnsupportedType) {
 			panic(err)
 		}
 
-	setContentTypeUnlessPresent(w, "application/json")
-	w.WriteHeader(status)
-	_, _ = b.WriteTo(w)
+		var errUnsupportedValue *json.UnsupportedValueError
+		if errors.As(err, &errUnsupportedValue) {
+			panic(err)
+		}
+
+		var errMarshalError *json.MarshalerError
+		if errors.As(err, &errMarshalError) {
+			panic(err)
+		}
+	}
 
 	log.EnabledResponse(w, v)
 }

api/render/render_test.go

@@ -1,8 +1,10 @@
 package render
 
 import (
+	"encoding/json"
 	"fmt"
 	"io"
+	"math"
 	"net/http"
 	"net/http/httptest"
 	"strconv"
@@ -26,10 +28,43 @@ func TestJSON(t *testing.T) {
 	assert.Empty(t, rw.Fields())
 }
 
-func TestJSONPanics(t *testing.T) {
-	assert.Panics(t, func() {
-		JSON(httptest.NewRecorder(), make(chan struct{}))
-	})
+func TestJSONPanicsOnUnsupportedType(t *testing.T) {
+	jsonPanicTest[json.UnsupportedTypeError](t, make(chan struct{}))
+}
+
+func TestJSONPanicsOnUnsupportedValue(t *testing.T) {
+	jsonPanicTest[json.UnsupportedValueError](t, math.NaN())
+}
+
+func TestJSONPanicsOnMarshalerError(t *testing.T) {
+	var v erroneousJSONMarshaler
+	jsonPanicTest[json.MarshalerError](t, v)
+}
+
+type erroneousJSONMarshaler struct{}
+
+func (erroneousJSONMarshaler) MarshalJSON() ([]byte, error) {
+	return nil, assert.AnError
+}
+
+func jsonPanicTest[T json.UnsupportedTypeError | json.UnsupportedValueError | json.MarshalerError](t *testing.T, v any) {
+	t.Helper()
+
+	defer func() {
+		var err error
+		if r := recover(); r == nil {
+			t.Fatal("expected panic")
+		} else if e, ok := r.(error); !ok {
+			t.Fatalf("did not panic with an error (%T)", r)
+		} else {
+			err = e
+		}
+
+		var e *T
+		assert.ErrorAs(t, err, &e)
+	}()
+
+	JSON(httptest.NewRecorder(), v)
 }
 
 type renderableError struct {