feat: SSE reader and flushing writers

sse.go

@@ -0,0 +1,167 @@
+package traefikllmgateway
+
+import (
+	"bufio"
+	"io"
+	"net/http"
+	"strings"
+)
+
+// sseScannerInitialBufSize is bufio.Scanner's starting token buffer for
+// readSSE. It grows on demand up to sseScannerMaxLineSize.
+const sseScannerInitialBufSize = 64 * 1024
+
+// sseScannerMaxLineSize caps a single text/event-stream field line at
+// 4MiB. Upstream LLM SSE events can carry large data lines — a
+// base64-encoded image, for instance — so the cap is generous, but a line
+// beyond it makes bufio.Scanner return bufio.ErrTooLong instead of
+// growing its buffer without bound.
+const sseScannerMaxLineSize = 4 * 1024 * 1024
+
+// sseDataPrefix is the "data:" field prefix sseWriter writes before every
+// event payload, per the text/event-stream wire format.
+const sseDataPrefix = "data: "
+
+// sseEvent is one parsed text/event-stream event: its "event:" field
+// (empty when the stream omits it) and its "data:" field, with multi-line
+// data joined by "\n" per the SSE spec.
+type sseEvent struct {
+	event string
+	data  []byte
+}
+
+// readSSE parses r as a text/event-stream body and calls fn once per
+// event. It accumulates "event:" and "data:" field lines — multiple
+// "data:" lines join with "\n" — and dispatches fn on a blank line, then
+// resets the accumulated fields. Comment lines (starting with ":") and
+// "id:"/"retry:" fields are recognized and silently ignored. Line endings
+// may be "\n" or "\r\n": bufio.ScanLines, the scanner's split function,
+// strips an optional trailing "\r" itself.
+//
+// readSSE stops and returns fn's error the first time fn returns one. On
+// a clean EOF it returns nil, first dispatching a final un-terminated
+// event (fields accumulated with no trailing blank line) if the stream
+// ended with one pending. A field line longer than sseScannerMaxLineSize
+// makes the underlying scanner fail; that error (bufio.ErrTooLong) is
+// returned as-is.
+func readSSE(r io.Reader, fn func(sseEvent) error) error {
+	scanner := bufio.NewScanner(r)
+	scanner.Buffer(make([]byte, sseScannerInitialBufSize), sseScannerMaxLineSize)
+
+	var (
+		event     string
+		dataLines []string
+		pending   bool
+	)
+
+	dispatch := func() error {
+		if !pending {
+			return nil
+		}
+		ev := sseEvent{event: event, data: []byte(strings.Join(dataLines, "\n"))}
+		event, dataLines, pending = "", nil, false
+		return fn(ev)
+	}
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			if err := dispatch(); err != nil {
+				return err
+			}
+			continue
+		}
+		if strings.HasPrefix(line, ":") {
+			continue // comment line, per the SSE spec
+		}
+
+		field, value, _ := strings.Cut(line, ":")
+		value = strings.TrimPrefix(value, " ")
+		switch field {
+		case "event":
+			event = value
+			pending = true
+		case "data":
+			dataLines = append(dataLines, value)
+			pending = true
+		case "id", "retry":
+			// recognized fields, intentionally ignored
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return err
+	}
+	return dispatch()
+}
+
+// sseWriter writes Server-Sent Events over an http.ResponseWriter,
+// flushing after each event so a proxied client sees each chunk as it is
+// produced instead of buffered until the response closes.
+type sseWriter struct {
+	w http.ResponseWriter
+	f http.Flusher
+}
+
+// newSSEWriter returns an sseWriter over w, setting the SSE response
+// headers before anything is written: Content-Type: text/event-stream,
+// Cache-Control: no-cache, and X-Accel-Buffering: no (disables buffering
+// in a reverse proxy such as nginx sitting in front of the gateway). w
+// need not implement http.Flusher — the Gateway's statusTrackingWriter
+// always does, since it delegates — but writeData/writeDone tolerate its
+// absence by skipping the flush.
+func newSSEWriter(w http.ResponseWriter) *sseWriter {
+	h := w.Header()
+	h.Set("Content-Type", "text/event-stream")
+	h.Set("Cache-Control", "no-cache")
+	h.Set("X-Accel-Buffering", "no")
+
+	f, _ := w.(http.Flusher)
+	return &sseWriter{w: w, f: f}
+}
+
+// writeData writes one SSE data event — "data: " + b + "\n\n" — as a
+// single Write call, then flushes when the underlying writer supports it.
+func (s *sseWriter) writeData(b []byte) error {
+	buf := make([]byte, 0, len(sseDataPrefix)+len(b)+2)
+	buf = append(buf, sseDataPrefix...)
+	buf = append(buf, b...)
+	buf = append(buf, '\n', '\n')
+
+	if _, err := s.w.Write(buf); err != nil {
+		return err
+	}
+	if s.f != nil {
+		s.f.Flush()
+	}
+	return nil
+}
+
+// writeDone writes the SSE stream's terminal "data: [DONE]\n\n" sentinel
+// (the OpenAI streaming convention) and flushes. Write errors are not
+// returned — writeDone is the last thing a handler does before returning,
+// and the connection is about to close either way.
+func (s *sseWriter) writeDone() {
+	_, _ = s.w.Write([]byte(sseDataPrefix + "[DONE]\n\n"))
+	if s.f != nil {
+		s.f.Flush()
+	}
+}
+
+// flushWriter wraps an io.Writer, flushing after every Write. Passthrough
+// and MCP proxying use it as the destination for io.Copy, so a streamed
+// reverse-proxy body reaches the client incrementally instead of sitting
+// in a buffer until the copy finishes.
+type flushWriter struct {
+	w io.Writer
+	f http.Flusher
+}
+
+// Write delegates to the wrapped io.Writer, then flushes when f is
+// non-nil.
+func (fw *flushWriter) Write(b []byte) (int, error) {
+	n, err := fw.w.Write(b)
+	if fw.f != nil {
+		fw.f.Flush()
+	}
+	return n, err
+}