update readFile() to open files with O_CLOEXEC

Summary:
Files opened by `readFile()` should not be propagated to children processes
if another thread happens to spawn a subprocess while `readFile()` is running.

Reviewed By: yfeldblum, meyering

Differential Revision: D9553354

fbshipit-source-id: 2a6028b3a920bb1e76cd0b07f82e6af57d01c2a4

folly/FileUtil.h

@@ -172,7 +172,7 @@ bool readFile(
     size_t num_bytes = std::numeric_limits<size_t>::max()) {
   DCHECK(file_name);
 
-  const auto fd = openNoInt(file_name, O_RDONLY);
+  const auto fd = openNoInt(file_name, O_RDONLY | O_CLOEXEC);
   if (fd == -1) {
     return false;
   }