Log the exact size sent over the wire in host and open an API for it

Previously we were logging the full file size not just the parts of the
file actually sent. This size also includes headers and generated pages
like the directory listing.

host/embedhost/host.go

@@ -82,11 +82,14 @@ func Run(flags *flag.FlagSet, args []string) {
 				return // Auth failed!
 			}
 			if *logRequests {
-				size, _ := dir.FileSize(req.URL.Path)
+				w = tools.NewSizeTrackerHTTPResponseWriter(w)
+			}
+			fileServer.ServeHTTP(w, req)
+			if *logRequests {
+				size := w.(tools.SizeTrackerHTTPResponseWriter).BytesRead()
 				log.Printf("%v requested and was served %v (%v bytes)",
 					clientName, fullPath, size)
 			}
-			fileServer.ServeHTTP(w, req)
 		},
 	)))
 

tools/http.go

@@ -53,6 +53,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -516,3 +517,31 @@ func (r RedirectToHTTPS) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	url.Host = url.Hostname() // strip the port if one exists
 	http.Redirect(w, req, url.String(), http.StatusSeeOther)
 }
+
+type SizeTrackerHTTPResponseWriter struct {
+	http.ResponseWriter
+	bytesRead *atomic.Uint64
+}
+
+func NewSizeTrackerHTTPResponseWriter(w http.ResponseWriter) SizeTrackerHTTPResponseWriter {
+	return SizeTrackerHTTPResponseWriter{
+		ResponseWriter: w,
+		bytesRead:      &atomic.Uint64{},
+	}
+}
+
+func (w SizeTrackerHTTPResponseWriter) Write(input []byte) (n int, err error) {
+	n, err = w.ResponseWriter.Write(input)
+	w.bytesRead.Add(uint64(n))
+	return
+}
+
+func (w SizeTrackerHTTPResponseWriter) Flush() {
+	if f, ok := w.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}
+
+func (w SizeTrackerHTTPResponseWriter) BytesRead() uint64 {
+	return w.bytesRead.Load()
+}