backend/main.go

@@ -179,7 +179,7 @@ func newRouter(store *Store, cfg Config) http.Handler {
 		web.register(mux)
 	}
 
-	return withCORS(cfg.AllowedOrigins, guardEmptyUserscriptToken(mux))
+	return withCORS(cfg.AllowedOrigins, withGzip(guardEmptyUserscriptToken(mux)))
 }
 
 // guardEmptyUserscriptToken heads off ServeMux's own path-cleaning redirect:

backend/main_test.go

@@ -1,8 +1,10 @@
 package main
 
 import (
+	"compress/gzip"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -196,3 +198,46 @@ func TestPutOmittedStatusPreservesArchivedAndAppliesProgress(t *testing.T) {
 		t.Fatalf("stored last_chapter_num = %v, want 12", got.LastChapterNum)
 	}
 }
+
+func TestGzipCompressesTextNotFonts(t *testing.T) {
+	srv, _ := newWebTestServer(t, webConfig())
+
+	cases := []struct {
+		path string
+		want bool
+	}{
+		{"/static/style.css", true},
+		{"/static/filter.js", true},
+		{"/static/htmx.min.js", true},
+		{"/static/fonts/dm-sans-var-latin.woff2", false},
+	}
+	for _, tc := range cases {
+		req := httptest.NewRequest(http.MethodGet, tc.path, nil)
+		req.Header.Set("Accept-Encoding", "gzip")
+		rr := httptest.NewRecorder()
+		srv.ServeHTTP(rr, req)
+		if rr.Code != http.StatusOK {
+			t.Fatalf("GET %s = %d, want 200", tc.path, rr.Code)
+		}
+		got := rr.Header().Get("Content-Encoding") == "gzip"
+		if got != tc.want {
+			t.Errorf("GET %s Content-Encoding gzip = %v, want %v", tc.path, got, tc.want)
+		}
+		if got {
+			zr, err := gzip.NewReader(rr.Body)
+			if err != nil {
+				t.Fatalf("GET %s: body is not gzip: %v", tc.path, err)
+			}
+			if _, err := io.ReadAll(zr); err != nil {
+				t.Fatalf("GET %s: gzip body did not decode: %v", tc.path, err)
+			}
+		}
+	}
+
+	// A client that does not ask still gets plain bytes.
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, httptest.NewRequest(http.MethodGet, "/static/style.css", nil))
+	if enc := rr.Header().Get("Content-Encoding"); enc != "" {
+		t.Errorf("Content-Encoding without Accept-Encoding = %q, want empty", enc)
+	}
+}

backend/middleware.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"compress/gzip"
 	"crypto/subtle"
 	"net/http"
 	"strings"
@@ -26,6 +27,71 @@ func withAuth(token string, next http.Handler) http.Handler {
 	})
 }
 
+// gzipWriter compresses the body and drops Content-Length, which no longer
+// describes what goes on the wire. WriteHeader is where the decision is made:
+// only then is Content-Type known.
+type gzipWriter struct {
+	http.ResponseWriter
+	gz      *gzip.Writer
+	decided bool
+}
+
+// compressible covers what this server actually serves in bulk: HTML, CSS, JS
+// and JSON. Fonts are woff2, which is already compressed — gzipping them costs
+// CPU to add bytes.
+func compressible(contentType string) bool {
+	ct, _, _ := strings.Cut(contentType, ";")
+	switch strings.TrimSpace(ct) {
+	case "text/html", "text/css", "text/javascript", "application/javascript",
+		"application/json", "text/plain":
+		return true
+	}
+	return false
+}
+
+func (w *gzipWriter) WriteHeader(status int) {
+	if !w.decided {
+		w.decided = true
+		if compressible(w.Header().Get("Content-Type")) {
+			w.Header().Set("Content-Encoding", "gzip")
+			w.Header().Del("Content-Length")
+			w.gz = gzip.NewWriter(w.ResponseWriter)
+		}
+	}
+	w.ResponseWriter.WriteHeader(status)
+}
+
+func (w *gzipWriter) Write(b []byte) (int, error) {
+	if !w.decided {
+		w.WriteHeader(http.StatusOK)
+	}
+	if w.gz != nil {
+		return w.gz.Write(b)
+	}
+	return w.ResponseWriter.Write(b)
+}
+
+// withGzip compresses text responses for clients that ask. The templates,
+// stylesheet and htmx together are ~120 KB uncompressed and roughly a quarter
+// of that gzipped, which is the difference between a fast and a slow first load
+// on mobile data.
+func withGzip(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
+		}
+		w.Header().Add("Vary", "Accept-Encoding")
+		gw := &gzipWriter{ResponseWriter: w}
+		defer func() {
+			if gw.gz != nil {
+				gw.gz.Close()
+			}
+		}()
+		next.ServeHTTP(gw, r)
+	})
+}
+
 // withCORS reflects the request Origin only when it is in allowed, answers
 // preflight OPTIONS with 204, and passes everything else through. It wraps the
 // auth middleware so preflight (which carries no Authorization header) is never