root-dev-ino.c limitation

[Eric Blake]

I just intentionally crippled the use of getcwd in a cygwin compilation
(gl_cv_func_getcwd_null=no gl_cv_func_getcwd_path_max=no
when configuring), to stress-test a pending patch to cygwin readdir
to provide d_ino.  In the process, I discovered a fundamental
flaw in root-dev-ino.c - it expects that all pathnames derive from
/.  But on cygwin, // is a distinct root (//.. is //, and // cannot
be listed when doing readdir on /).  Therefore, programs
such as pwd, when using the readdir fallback rather than
getcwd, are unable to print directory listings when in a
subdirectory of //.

I'm wondering whether it is better to augment struct dev_ino in
lib/dev-ino.h to have a struct root_dev_ino that holds two
inodes on platforms known to have a distinct //, or whether
it is better to alter algorithms that compare to root to see
if '$dir' and '$dir/..' are the same file for arbitrary $dir.  The
potential problem with the latter, on platforms where
directory hard-links are permitted, is that a corrupted
filesystem might have 'dir/..' pointing to 'dir' without
any path back to /, in which case you DO want an error
about pwd not being able to trace back to /.

-- 
Eric Blake