net/http: log Readdir error to Server.ErrorLog

Now that issue #12438 is resolved, this TODO can be completed.
Create a logf helper, which is similar to Server.logf method,
but takes a *Request to infer the *Server and its ErrorLog from.

Update documentation of Server.ErrorLog to mention a new type
of errors that may be logged to it.

Also update a statement in documentation of Server.ErrorLog from:

	// If nil, logging goes to os.Stderr via the log package's
	// standard logger.

To:

	// If nil, logging is done via the log package's standard logger.

The motivation for doing so is to avoid making inaccurate claims.
Logging may not go to os.Stderr if anyone overrides the log package's
default output via https://godoc.org/log#SetOutput. Saying that
the standard logger is used should be sufficient to explain the
behavior, and users can infer that os.Stderr is used by default,
unless it's changed.

Updates #12438.

Change-Id: I3a4b0db51d652fd25fb2065fbc2157a3dec4dd38
Reviewed-on: https://go-review.googlesource.com/53950
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/fs.go

@@ -98,12 +98,10 @@ type File interface {
 	Stat() (os.FileInfo, error)
 }
 
-func dirList(w ResponseWriter, f File) {
+func dirList(w ResponseWriter, r *Request, f File) {
 	dirs, err := f.Readdir(-1)
 	if err != nil {
-		// TODO: log err.Error() to the Server.ErrorLog, once it's possible
-		// for a handler to get at its Server via the ResponseWriter. See
-		// Issue 12438.
+		logf(r, "http: error reading directory: %v", err)
 		Error(w, "Error reading directory", StatusInternalServerError)
 		return
 	}
@@ -615,7 +613,7 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 			return
 		}
 		w.Header().Set("Last-Modified", d.ModTime().UTC().Format(TimeFormat))
-		dirList(w, f)
+		dirList(w, r, f)
 		return
 	}
 

src/net/http/server.go

@@ -2400,9 +2400,9 @@ type Server struct {
 	ConnState func(net.Conn, ConnState)
 
 	// ErrorLog specifies an optional logger for errors accepting
-	// connections and unexpected behavior from handlers.
-	// If nil, logging goes to os.Stderr via the log package's
-	// standard logger.
+	// connections, unexpected behavior from handlers, and
+	// underlying FileSystem errors.
+	// If nil, logging is done via the log package's standard logger.
 	ErrorLog *log.Logger
 
 	disableKeepAlives int32     // accessed atomically.
@@ -2853,6 +2853,18 @@ func (s *Server) logf(format string, args ...interface{}) {
 	}
 }
 
+// logf prints to the ErrorLog of the *Server associated with request r
+// via ServerContextKey. If there's no associated server, or if ErrorLog
+// is nil, logging is done via the log package's standard logger.
+func logf(r *Request, format string, args ...interface{}) {
+	s, _ := r.Context().Value(ServerContextKey).(*Server)
+	if s != nil && s.ErrorLog != nil {
+		s.ErrorLog.Printf(format, args...)
+	} else {
+		log.Printf(format, args...)
+	}
+}
+
 // ListenAndServe listens on the TCP network address addr
 // and then calls Serve with handler to handle requests
 // on incoming connections.