bug#62385: POSIX behavior of readlink, realpath

[Eric Blake]

POSIX will be standardizing readlink(1) and realpath(1):

https://www.austingroupbugs.net/view.php?id=1457

Among other things, I can see the following changes that coreutils
will need to make to become compliant, or else we need to push back on
the POSIX folks if we have strong reasons to complain that their
specification will break things:

POSIX wants 'readlink non-symlink' to output a diagnostic; that is, it
looks like POSIX wants us to behave like '-v' is enabled by default
(our current behavior of -q by default will be non-compliant).

POSIX wants us to support 'realpath -E file'.  I'm not quite sure if
it is matches our existing behavior when -e is omitted (in which case,
all the more we have to do is have -E coming later than -e disable the
earlier -e).  In particular, it gives a convincing example:

"The behavior with the -E option when file does not resolve (with
symbolic links followed) to an existing file is not the same as simply
calling realpath() with the path prefix of the file operand and
writing the resulting pathname, a <slash>, and the last component of
file to standard output. For example, if /tmp/nofile does not exist,
and file is A/B where A is an existing directory and B is a symbolic
link to /tmp/nofile, realpath with -E will output /tmp/nofile, but if
B is a symbolic link to /tmp/nofile/foo, realpath with -E will treat
this as an error. In both cases <tt>realpath("A/B")</tt> would fail
with errno set to [ENOENT]. Even though <tt>realpath("A")</tt> would
succeed, in neither case is anything ending /B the result."

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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