Actually make SseWriter be spec compliant

- Change inbound data type from byte slice to dedicated event message
  type
- Include data validation for id and event type
- Better documentation

http/sse.go

@@ -4,18 +4,36 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
 )
 
+type SseEvent struct {
+	Type *string
+	Id   *string
+	Data string
+}
+
 var (
 	ErrWriterNotFlushable = errors.New("response writer not flushable")
+	ErrEventTypeNoNewline = errors.New("event type contained a newline")
+	ErrEventIdNoNewline   = errors.New("event id contained a newline")
 )
 
 // SseWriter provides a simple implementation for sending data via Server Side Events
-// to the client. The function runs until the dataStream channel is closed.
-// The ResponseWriter must not be used after calling this function
+// to the client. The function returns once the dataStream channel is closed.
+// The ResponseWriter should not be used after calling this function
+//
+// Event rules:
+// - SseEvent.Type must not contain any newlines
+// - SseEvent.Id must not contain any newlines
+// - "\r\n" in SseEvent.Data will be replaced with "\n", afterwards "\r" will also be replaced with "\n"
 //
 // Inspired by and partially copied from https://medium.com/@rian.eka.cahya/server-sent-event-sse-with-go-10592d9c2aa1
-func SseWriter(w http.ResponseWriter, dataStream chan []byte) error {
+// Also why this and not github.com/tmaxmax/go-sse?
+// go-sse is not flexible in its use case. You have one handler that doesn't differentiate between clients.
+// This, of course, does not work when you have to send individualised notifications as everyone would
+// get the same messages, causing confusion in the best case and data leaks in the worst case
+func SseWriter(w http.ResponseWriter, dataStream chan SseEvent) error {
 	w.Header().Set("Access-Control-Expose-Headers", "Content-Type")
 
 	w.Header().Set("Content-Type", "text/event-stream")
@@ -27,11 +45,34 @@ func SseWriter(w http.ResponseWriter, dataStream chan []byte) error {
 		return ErrWriterNotFlushable
 	}
 	for data := range dataStream {
-		_, err := fmt.Fprintf(w, "%s\n\n", string(data))
-		if err != nil {
+		if err := ValidateSseEvent(&data); err != nil {
 			return err
 		}
+		if data.Type != nil {
+			_, _ = fmt.Fprintf(w, "event: %s\n", *data.Type)
+		}
+		if data.Id != nil {
+			_, _ = fmt.Fprintf(w, "id: %s\n", *data.Id)
+		}
+		noCr := strings.ReplaceAll(data.Data, "\r\n", "\n")
+		noCr = strings.ReplaceAll(noCr, "\r", "\n")
+		for _, line := range strings.Split(noCr, "\n") {
+			_, _ = fmt.Fprintf(w, "data: %s\n", line)
+		}
+		_, _ = fmt.Fprint(w, "\n\n")
 		flusher.Flush()
 	}
 	return nil
 }
+
+// Check if a given event is valid for sending via SseWriter
+// This function is also internally called by SseWriter for each outbound event
+func ValidateSseEvent(e *SseEvent) error {
+	if e.Type != nil && strings.ContainsRune(*e.Type, '\n') {
+		return ErrEventTypeNoNewline
+	}
+	if e.Id != nil && strings.ContainsRune(*e.Id, '\n') {
+		return ErrEventIdNoNewline
+	}
+	return nil
+}