implement transparent gzip compression in the webserver

we only compress if applicable (content-type indicates likely compressible),
client supports it, response doesn't already have a content-encoding).

for internal handlers, we always enable compression.  for reverse proxied and
static files, compression must be enabled per handler.

for internal & reverse proxy handlers, we do streaming compression at
"bestspeed" quality (probably level 1).

for static files, we have a cache based on mtime with fixed max size, where we
evict based on least recently used. we compress with the default level (more
cpu, better ratio).

webmail/webmail.go

@@ -224,6 +224,7 @@ func (m *merged) serve(ctx context.Context, log *mlog.Log, w http.ResponseWriter
 	gz := acceptsGzip(r)
 	var out []byte
 	var mtime time.Time
+	var origSize int64
 
 	func() {
 		m.Lock()
@@ -265,13 +266,14 @@ func (m *merged) serve(ctx context.Context, log *mlog.Log, w http.ResponseWriter
 				xcheckf(ctx, err, "gzipping combined html")
 				m.combinedGzip = b.Bytes()
 			}
+			origSize = int64(len(out))
 			out = m.combinedGzip
 		}
 		mtime = m.mtime
 	}()
 
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
-	http.ServeContent(gzipInjector{w, gz}, r, "", mtime, bytes.NewReader(out))
+	http.ServeContent(gzipInjector{w, gz, origSize}, r, "", mtime, bytes.NewReader(out))
 }
 
 // gzipInjector is a http.ResponseWriter that optionally injects a
@@ -283,6 +285,7 @@ func (m *merged) serve(ctx context.Context, log *mlog.Log, w http.ResponseWriter
 type gzipInjector struct {
 	http.ResponseWriter // Keep most methods.
 	gz                  bool
+	origSize            int64
 }
 
 // WriteHeader adds a Content-Encoding: gzip header before actually writing the
@@ -290,6 +293,9 @@ type gzipInjector struct {
 func (w gzipInjector) WriteHeader(statusCode int) {
 	if w.gz && statusCode == http.StatusOK {
 		w.ResponseWriter.Header().Set("Content-Encoding", "gzip")
+		if lw, ok := w.ResponseWriter.(interface{ SetUncompressedSize(int64) }); ok {
+			lw.SetUncompressedSize(w.origSize)
+		}
 	}
 	w.ResponseWriter.WriteHeader(statusCode)
 }