cat bug on cygwin

[Eric Blake]

This code in cat.c, line 669:

      if (O_BINARY && ! isatty (STDOUT_FILENO))
        freopen (NULL, "wb", stdout);
  
is buggy.  If the stream was originally opened in append mode, this converts it 
to non-append mode, such that output overwrites at the current file offset 
instead of appending on the end in something like:

cat extra >> existing

It has not been discovered until now on cygwin, because cygwin also had a bug 
that lseek(open("existing",O_WRONLY|O_APPEND),0,SEEK_CUR) returned the end of 
file offset instead of 0, contrary to POSIX; and that bug was masking the bug 
in cat since the non-append writes still happened to take place at the correct 
offset to appear as though it were an append.

The code in cat.c needs to be something like:

  freopen (NULL, (fcntl (STDOUT_FILENO, F_GETFL) & O_APPEND) ? "ab" : "wb",
           stdout);

I'm wondering how many of the other coreutils that use this freopen trick are 
affected, and whether we should use a wrapper function rather than duplicating 
all the logic.  At one point, we even had a wrapper SET_BINARY() that attempted 
to do this (prior to commit 8770c00ef...), except that it used the non-standard 
setmode() function from the non-standard header <io.h>, not to mention that it 
interfered with proper stdio operation; so I am in favor of sticking with the 
standardized freopen interface.  I'm also wondering if such a wrapper belongs 
in the binary-io gnulib module.

-- 
Eric Blake