all: Store assets as strings (#6611)

Storing assets as []byte requires every compiled-in asset to be copied
into writable memory at program startup. That currently takes up 1.6MB
per syncthing process. Strings stay in the RODATA section and should be
shared between processes running the same binary.

lib/api/api_statics.go

@@ -7,14 +7,14 @@
 package api
 
 import (
-	"bytes"
 	"compress/gzip"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"mime"
 	"net/http"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"time"
 
@@ -27,7 +27,7 @@ const themePrefix = "theme-assets/"
 
 type staticsServer struct {
 	assetDir        string
-	assets          map[string][]byte
+	assets          map[string]string
 	availableThemes []string
 
 	mut             sync.RWMutex
@@ -168,16 +168,15 @@ func (s *staticsServer) serveAsset(w http.ResponseWriter, r *http.Request) {
 	}
 	if strings.Contains(r.Header.Get("Accept-Encoding"), "gzip") {
 		w.Header().Set("Content-Encoding", "gzip")
+		w.Header().Set("Content-Length", strconv.Itoa(len(bs)))
+		io.WriteString(w, bs)
 	} else {
 		// ungzip if browser not send gzip accepted header
 		var gr *gzip.Reader
-		gr, _ = gzip.NewReader(bytes.NewReader(bs))
-		bs, _ = ioutil.ReadAll(gr)
+		gr, _ = gzip.NewReader(strings.NewReader(bs))
+		io.Copy(w, gr)
 		gr.Close()
 	}
-	w.Header().Set("Content-Length", fmt.Sprintf("%d", len(bs)))
-
-	w.Write(bs)
 }
 
 func (s *staticsServer) serveThemes(w http.ResponseWriter, r *http.Request) {

lib/api/api_test.go

@@ -152,19 +152,19 @@ func TestAssetsDir(t *testing.T) {
 	gw := gzip.NewWriter(buf)
 	gw.Write([]byte("default"))
 	gw.Close()
-	def := buf.Bytes()
+	def := buf.String()
 
 	buf = new(bytes.Buffer)
 	gw = gzip.NewWriter(buf)
 	gw.Write([]byte("foo"))
 	gw.Close()
-	foo := buf.Bytes()
+	foo := buf.String()
 
 	e := &staticsServer{
 		theme:    "foo",
 		mut:      sync.NewRWMutex(),
 		assetDir: "testdata",
-		assets: map[string][]byte{
+		assets: map[string]string{
 			"foo/a":     foo, // overridden in foo/a
 			"foo/b":     foo,
 			"default/a": def, // overridden in default/a (but foo/a takes precedence)