feat: Add ServerTrace instrumentation hooks to the SSE server

encoder.go

@@ -67,7 +67,10 @@ func (enc *Encoder) Encode(ec eventOrComment) error {
 			return fmt.Errorf("eventsource encode: %v", err)
 		}
 	default:
-		return fmt.Errorf("unexpected parameter to Encode: %v", ec)
+		// %T, not %v: an unexpected value must not have its contents -- which
+		// could include an event payload -- rendered into an error string that
+		// flows to WriteError consumers and logs.
+		return fmt.Errorf("unexpected parameter to Encode: %T", ec)
 	}
 	if enc.compressed {
 		return enc.w.(*gzip.Writer).Flush()

encoder_test.go

@@ -65,6 +65,16 @@ func TestEncoderComment(t *testing.T) {
 	assert.Equal(t, ":hello\n", string(buf.Bytes()))
 }
 
+func TestEncoderRejectsUnknownTypeWithoutRenderingValue(t *testing.T) {
+	buf := bytes.NewBuffer(nil)
+	err := NewEncoder(buf, false).Encode("do-not-disclose")
+	assert.Error(t, err)
+	// The error names the type only: its contents could be a payload, and the
+	// error flows to WriteError consumers and logs.
+	assert.NotContains(t, err.Error(), "do-not-disclose")
+	assert.Contains(t, err.Error(), "string")
+}
+
 func TestEncoderGzipCompression(t *testing.T) {
 	uncompressedBuf, compressedBuf, expectedCompressedBuf := bytes.NewBuffer(nil), bytes.NewBuffer(nil), bytes.NewBuffer(nil)
 

interface.go

@@ -18,7 +18,9 @@ type Event interface {
 	Id() string
 	// The name of the event. Return empty string if not required.
 	Event() string
-	// The payload of the event.
+	// The payload of the event. Repeated calls must return the same value:
+	// the server may read it more than once, for example when accounting for
+	// payload sizes in addition to encoding.
 	Data() string
 }