message: file-backed-mmap: comment + clean up FD
diff: @@ -89,6 +89,8 @@ int copyfile(int dstFd, int srcFd, size_t sz) {
  return result;
  }
  +// wraps an integer file descriptor to provide close-on-destruct
  +// semantics (so that we can use a shared_ptr to refcount a FD)
  class FD {
  private:
  DISALLOW_COPY_AND_ASSIGN(FD);
  @@ -98,10 +100,8 @@ public:
  }
  ~FD() {
  - if (_fd >= 0) {
  - debug("closed fd %d", _fd);
  + if (_fd >= 0)
  close(_fd);
  - }
  _fd = -2;
  }
  