markdown: fix race in accessing map

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

middleware/markdown/markdown.go

@@ -126,8 +126,11 @@ func (md Markdown) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error
 					}
 				}
 
+				cfg.RLock()
+				filepath, ok := cfg.StaticFiles[fpath]
+				cfg.RUnlock()
 				// if static site is generated, attempt to use it
-				if filepath, ok := cfg.StaticFiles[fpath]; ok {
+				if ok {
 					if fs1, err := os.Stat(filepath); err == nil {
 						// if markdown has not been modified since static page
 						// generation, serve the static page

middleware/markdown/process.go

@@ -153,7 +153,9 @@ func (md Markdown) generatePage(c *Config, requestPath string, content []byte) e
 			return err
 		}
 
+		c.Lock()
 		c.StaticFiles[requestPath] = filePath
+		c.Unlock()
 	}
 
 	return nil