Re: reopen: can't change access mode from "w+" to "w"?

On 2008.02.06 04:30, Eero Saynatkari wrote:
> On 2008.02.06 03:00, Sam Ruby wrote:
> > require 'tmpdir'
> > 
> > save = STDOUT.dup
> > STDOUT.reopen File.open(File.join(Dir.tmpdir, "should_output_#{$$}"), "w+")
> > puts 'hi'
> > out = STDOUT.dup
> > STDOUT.reopen save
> > 
> > out.rewind
> > p out.read
> > 
> > reopen.rb:7:in `reopen': /tmp/should_output_8694 can't change access 
> > mode from "w+" to "w" (ArgumentError)
> >         from reopen.rb:7:in `<main>'
> > 
> > Is this a bug in Ruby 1.9?  
> 
> The error message is ambiguous at least. It means that for some reason,
> the backend is trying to apply the existing mode 'w+' to the stream to
> be opened and since STDOUT is 'w', failing to do so because that would
> "widen" the stream. It is a bug, probably caused by 'incorrect' use of
> freopen().
> 
> Logically, there is no reason why reopen should care about the previous
> mode of the old stream since it is just closed/discarded. freopen() does
> take a new mode which is not allowed to widen the existing mode on the
> stream but that only affects the new stream (or pointer), not the previous
> one.

Blah, exactly the wrong angle of explanation and the "analysis" was
inaccurate, too, after reading the io.c code. My apologies. The issue
is a special protection mechanism for the stdio streams.

Looks like the original change was this:

  Fri Dec 24 02:52:52 2004  Nobuyoshi Nakada  <[e-mail]>

    * io.c (io_reopen, rb_io_reopen): prohibit to change access mode for
      special IO ports.  [ruby-dev:25225]


And Matz reverted it later--but only in the 1.8 branch.

  Sat Dec 25 01:28:23 2004  Yukihiro Matsumoto  <[e-mail]>

    * io.c: cancel io_reopen() change on Dec. 24th.

1.9 does not have that revert.

Anyway, the larger point stands: in my opinion, there is no reason why
#reopen should care about the existing file mode under any circumstances
and that the 1.8 behaviour is the correct one. This is a buyer beware
situation--if you #reopen stdio streams, you better make sure that
you have them stored somewhere if you want them back.

If anyone can explain where the 1.9 behaviour is desirable, I would
certainly be happy to listen. The only post I found is the ruby-dev
one that seems to reference a now-defunct or changed site (my Japanese
is not exactly stellar.)


--
Eero