Move common function into existing file

middleware/middleware.go

@@ -4,6 +4,7 @@ package middleware
 import (
 	"net/http"
 	"path"
+	"time"
 )
 
 type (
@@ -78,3 +79,30 @@ func IndexFile(root http.FileSystem, fpath string, indexFiles []string) (string,
 	}
 	return "", false
 }
+
+// SetLastModifiedHeader checks if the provided modTime is valid and if it is sets it
+// as a Last-Modified header to the ResponseWriter. If the modTime is in the future
+// the current time is used instead.
+func SetLastModifiedHeader(w http.ResponseWriter, modTime time.Time) {
+	if modTime.IsZero() || modTime.Equal(time.Unix(0, 0)) {
+		// the time does not appear to be valid. Don't put it in the response
+		return
+	}
+
+	// RFC 2616 - Section 14.29 - Last-Modified:
+	// An origin server MUST NOT send a Last-Modified date which is later than the
+	// server's time of message origination. In such cases, where the resource's last
+	// modification would indicate some time in the future, the server MUST replace
+	// that date with the message origination date.
+	now := currentTime()
+	if modTime.After(now) {
+		modTime = now
+	}
+
+	w.Header().Set("Last-Modified", modTime.UTC().Format(http.TimeFormat))
+}
+
+// currentTime returns time.Now() everytime it's called. It's used for mocking in tests.
+var currentTime = func() time.Time {
+	return time.Now()
+}