Reader

Memoizes key information about a file handle such as:

• The size from calling stat, or the error that occurred therein.
• The current seek position.
• The error that occurred when trying to seek.
• Whether reading should be done positionally or streaming.
• Whether reading should be done via fd-to-fd syscalls (e.g. sendfile) versus plain variants (e.g. read).

Fulfills the std.Io.Reader interface.