Gzip large JSON API responses to cut transcript download time

internal/server/compress.go

@@ -0,0 +1,64 @@
+package server
+
+import (
+	"compress/gzip"
+	"log/slog"
+	"net/http"
+	"strings"
+)
+
+const gzipMin = 8 << 10
+
+type gzipWriter struct {
+	http.ResponseWriter
+	held int
+	gz   *gzip.Writer
+}
+
+func (w *gzipWriter) WriteHeader(status int) {
+	if status == http.StatusOK && w.Header().Get("Content-Type") == "application/json" {
+		w.held = status
+		return
+	}
+	w.ResponseWriter.WriteHeader(status)
+}
+
+func (w *gzipWriter) Write(b []byte) (int, error) {
+	if w.held != 0 {
+		if len(b) >= gzipMin {
+			w.Header().Set("Content-Encoding", "gzip")
+			w.Header().Del("Content-Length")
+			w.gz = gzip.NewWriter(w.ResponseWriter)
+		}
+		w.ResponseWriter.WriteHeader(w.held)
+		w.held = 0
+	}
+	if w.gz != nil {
+		return w.gz.Write(b)
+	}
+	return w.ResponseWriter.Write(b)
+}
+
+func (w *gzipWriter) Unwrap() http.ResponseWriter {
+	return w.ResponseWriter
+}
+
+func compress(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if !strings.Contains(r.Header.Get("Accept-Encoding"), "gzip") {
+			next.ServeHTTP(w, r)
+			return
+		}
+		gw := &gzipWriter{ResponseWriter: w}
+		next.ServeHTTP(gw, r)
+		if gw.held != 0 {
+			w.WriteHeader(gw.held)
+		}
+		if gw.gz != nil {
+			err := gw.gz.Close()
+			if err != nil {
+				slog.Warn("write response", "err", err)
+			}
+		}
+	})
+}

internal/server/server.go

@@ -135,7 +135,7 @@ func (s *Server) Handler() http.Handler {
 			writeError(w, http.StatusBadRequest, err)
 		},
 	})
-	return timing(http.NewCrossOriginProtection().Handler(mux))
+	return timing(compress(http.NewCrossOriginProtection().Handler(mux)))
 }
 
 func (s *Server) LoopbackHandler() http.Handler {