[patch] libstdc++/67173 Fix filesystem::canonical for Solaris 10.

[Martin Sebor]

> It turns out I was wrong about BSD traditionally supporting
> realpath(path, NULL), it first appeared in these relatively recent
> versions:
>
> FreeBSD 9.0
> OpenBSD 5.0
> NetBSD 6.1
>
> Like Solaris 11, some of them still define _POSIX_VERSION=200112L even
> though they support passing NULL,  so we could hardcode the targets
> that are known to support it, but we'll still need a fallback for lots
> of slightly older targets anyway.
>
> So here's a new implementation of canonical() which only uses
> realpath() when this autoconf compile-or-link test passes:
>
> #if _XOPEN_VERSION < 500
> #error
> #elif _XOPEN_VERSION >= 700 || defined(PATH_MAX)
> char *tmp = realpath((const char*)NULL, (char*)NULL);
> #else
> #error
> #endif
>
> i.e. I use realpath() for Issue 7, or for Issue 6 if PATH_MAX is
> defined.

I probably wouldn't code the PATH_MAX test quite the same way.
I would expect it to be mainly Issue 6 implementations that don't
define the macro to want to provide the null extension since there
would otherwise be no safe way to use the function.

I didn't test it at all but I'd be inclined to write the conditional
to look more along these lines:

   #if _XOPEN_VERSION >= 700
     // Issue 7 and better -- null resolved_name means allocate
     char *tmp = realpath (file_name, (char*)NULL);
   #elif _XOPEN_VERSION == 600
      // Issue 6 -- null resolved_name is implementation-defined
   #  if FreeBSD 9.0 or OpenBSD 5.0 or NetBSD 6.1
     char *tmp = realpath (file_name, (char*)NULL);
   #  elif PATH_MAX
     char *tmp = realpath (file_name, malloc (PATH_MAX));
   #  else
   #    error No safe way to call realpath
   #  endif
   #elif _XOPEN_VERSION && PATH_MAX
     // SUSv2 -- null resolved_name is an error
     char *tmp = realpath (file_name, malloc (PATH_MAX));
   #else
   #  error realpath not supported or no safe way to call it
   #endif

>
> Then in filesystem::canonical() if _GLIBCXX_USE_REALPATH is set I use
> it, passing NULL for Issue 7, or allocating a buffer of PATH_MAX
> otherwise. If realpath isn't supported or fails with ENAMETOOLONG then
> I do the path traversal by hand (which can be done entirely using the
> std::experimental::filesystem operations).

FWIW, to work across filesystems with different _PC_PATH_MAX, I
suspect the operations might need to use readlinkat. I.e., they
might need to descend into each individual subdirectory to avoid
forming a temporary pathname that's too long for the file system
being traversed, even though both the initial and the final
pathnames are under the limit. (I haven't actually tested this
but I don't see where GLIBC handles this case so it might not
do the right thing either.)

>
> Only passing NULL for Issue 7 is quite conservative. It means we don't
> do it for targets that support it as an implementation-defined
> extension to Issue 6, which includes Solaris 11, the BSDs and even
> older GNU systems (including RHEL6). But that's OK, we have a fallback
> now so it means no loss of functionality, just efficiency.  We can
> tweak the config later for targets known to handle NULL.

Does the config test actually run? If not, I don't see the point
(it doesn't tell us anything the POSIX feature tests macros don't).
If it did run, it would fail since the first argument is null.

>
> The new testcase is not very thorough. I've run a few more involved
> tests that aren't suitable to check in until I figure out a good way
> of running filesystem tests that can create/remove arbitrary files and
> symlinks.

Yeah, writing good tests is always the hard part. If you need help
I can try to put some together in my spare time.

Martin