From: Eric Wong <normalperson@...>
Date: 2013-07-12T03:55:16+09:00
Subject: [ruby-core:55956] Re: [ruby-trunk - Feature #5138] Add nonblocking IO that does not use exceptions for EOF and EWOULDBLOCK

Aaron Patterson <tenderlove@ruby-lang.org> wrote:
> It occurs to me that this is pretty similar to IO#readpartial.  The main
> difference being that readpartial automatically retries on EWOULDBLOCK
> and raises an EOF error.  Maybe there should be a `try_readpartial` in
> addition to the proposed `try_read_nonblock`?

It's always bothered me that IO#readpartial raises EOFError, too
(especially when IO#read returns nil).  So yes, try_readpartial
would be good, but try_*nonblock is much more important since
EAGAIN is more common than EOF.

Also, I also wonder if there's a generic way for us to implement
"expected exceptions":

- without needing to introduce additional methods
- without allocating new objects for common exceptions

Perhaps similar to catch/throw...

Maybe:

  begin_expect(Errno::ENOENT)
    # cause the _class_ Errno::ENOENT to be raised,
    # not an instance of Errno::ENOENT
    File.open("/possibly/non-existent/file")
  rescue Errno::ENOENT => e
    e == Errno::ENOENT # note the '==' (not '===')
  end

Otherwise, we might end up needing try_open, try_stat, try_lstat,
try_link, try_unlink, try_rename, etc to avoid racy/wasteful
File.exist? calls...