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


On 12/09/15 12:07 -0600, Martin Sebor wrote:
On 09/12/2015 04:09 AM, Jonathan Wakely wrote:
On 11 September 2015 at 18:39, Martin Sebor wrote:
On 09/11/2015 08:21 AM, Jonathan Wakely wrote:

Solaris 10 doesn't follow POSIX in accepting a null pointer as the
second argument to realpath(), so allocate a buffer for it.


FWIW, the NULL requirement is new in Issue 7. In Issue 6, the behavior
is implementation-defined, and before then it was an error. Solaris 10
claims conformance to SUSv2 and its realpath fails with EINVAL.
Solaris 11 says it conforms to Issue 6 but according to the man page
its realpath already implements the Issue 7 requirement.

Thanks.

I suspect the same problem will come up on other systems so checking
the POSIX version might be better than testing for each OS.

The problem with doing that is that many BSD systems have supported
passing NULL as an extension long before issue 7, so if we just check
something like _POSIX_VERSION >= 200809L then we can only canonicalize
paths up to PATH_MAX on many systems where the extension is available
but _POSIX_VERSION implies conformance to an older standard.

You're right. I agree that just checking the POSIX version may not
lead to optimal results. Some implementations also support multiple
versions and the one in effect may not be the one most recent. To
get the most out of those, it's usually recommended to set
_POSIX_C_SOURCE to the latest version before including any headers,
then test the supported version, and when the supported version is
less than the one requested and involves implementation defined
behavior (as in Issue 6) or undefined behavior that's known to be
used to provide extensions (as in SUSv2), check the implementation
version just as the patch does.


So maybe we want an autoconf macro saying whether realpath() accepts
NULL, and just hardcode it for the targets known to support it, and
only check _POSIX_VERSION for the unknowns.

That will work for Issue 6 where the realpath behavior is
implementation-defined. The test wouldn't yield reliable results
for SUSv2 implementations where the behavior is undefined. There,
the result would have to be hardcoded based on what the manual says.
An autoconf test won't help with the ENAMETOOLONG problem since it
might depend on the filesystem. To overcome that, libstdc++ would
need to do the path traversal itself.

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.

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).

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.

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.

What do you think?

Tested powerpc64le-linux and x86_64-dragonfly4.2.


commit e79ad2dbcb14d1e66f6edead4ff87b62e575a8e7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 16 14:07:54 2015 +0100

    Implement filesystem::canonical() without realpath
    
    	PR libstdc++/67173
    	* acinclude.m4 (GLIBCXX_CHECK_FILESYSTEM_DEPS): Check _XOPEN_VERSION
    	and PATH_MAX for _GLIBCXX_USE_REALPATH.
    	* config.h.in: Regenerate.
    	* configure: Regenerate.
    	* src/filesystem/dir.cc: Define _XOPEN_VERSION.
    	* src/filesystem/ops.cc: Likewise.
    	(canonical) [!_GLIBCXX_USE_REALPATH]: Add alternative implementation.
    	* testsuite/experimental/filesystem/operations/canonical.cc: New.

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 64c9b7e..364a7d2 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -3926,7 +3926,7 @@ dnl
   AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_save_CXXFLAGS="$CXXFLAGS"
-  CXXFLAGS="$CXXFLAGS -fno-exceptions"
+  CXXFLAGS="$CXXFLAGS -fno-exceptions -D_XOPEN_SOURCE=700"
 dnl
   AC_MSG_CHECKING([for struct dirent.d_type])
   AC_CACHE_VAL(glibcxx_cv_dirent_d_type, [dnl
@@ -3947,13 +3947,24 @@ dnl
   AC_MSG_CHECKING([for realpath])
   AC_CACHE_VAL(glibcxx_cv_realpath, [dnl
     GCC_TRY_COMPILE_OR_LINK(
-      [#include <stdlib.h>],
-      [char *tmp = realpath((const char*)NULL, (char*)NULL);],
+      [
+       #include <stdlib.h>
+       #include <unistd.h>
+      ],
+      [
+       #if _XOPEN_VERSION < 500
+       #error
+       #elif _XOPEN_VERSION >= 700 || defined(PATH_MAX)
+       char *tmp = realpath((const char*)NULL, (char*)NULL);
+       #else
+       #error
+       #endif
+      ],
       [glibcxx_cv_realpath=yes],
       [glibcxx_cv_realpath=no])
   ])
   if test $glibcxx_cv_realpath = yes; then
-    AC_DEFINE(_GLIBCXX_USE_REALPATH, 1, [Define if realpath is available in <stdlib.h>.])
+    AC_DEFINE(_GLIBCXX_USE_REALPATH, 1, [Define if usable realpath is available in <stdlib.h>.])
   fi
   AC_MSG_RESULT($glibcxx_cv_realpath)
 dnl
diff --git a/libstdc++-v3/src/filesystem/dir.cc b/libstdc++-v3/src/filesystem/dir.cc
index 016a78d..cfa44b1 100644
--- a/libstdc++-v3/src/filesystem/dir.cc
+++ b/libstdc++-v3/src/filesystem/dir.cc
@@ -22,6 +22,8 @@
 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 // <http://www.gnu.org/licenses/>.
 
+#define _XOPEN_SOURCE 700
+
 #include <experimental/filesystem>
 #include <utility>
 #include <stack>
diff --git a/libstdc++-v3/src/filesystem/ops.cc b/libstdc++-v3/src/filesystem/ops.cc
index cefb927..45daf34 100644
--- a/libstdc++-v3/src/filesystem/ops.cc
+++ b/libstdc++-v3/src/filesystem/ops.cc
@@ -22,6 +22,8 @@
 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 // <http://www.gnu.org/licenses/>.
 
+#define _XOPEN_SOURCE 700
+
 #include <experimental/filesystem>
 #include <functional>
 #include <stack>
@@ -96,23 +98,98 @@ namespace
 fs::path
 fs::canonical(const path& p, const path& base, error_code& ec)
 {
-  path can;
+  const path pa = absolute(p, base);
+  path result;
 #ifdef _GLIBCXX_USE_REALPATH
-  char* buffer = nullptr;
-#if defined(__SunOS_5_10) && defined(PATH_MAX)
-  buffer = (char*)::malloc(PATH_MAX);
-#endif
-  if (char_ptr rp = char_ptr{::realpath(absolute(p, base).c_str(), buffer)})
+  char_ptr buf{ nullptr };
+# if _XOPEN_VERSION < 700
+  // Not safe to call realpath(path, NULL)
+  buf.reset( (char*)::malloc(PATH_MAX) );
+# endif
+  if (char* rp = ::realpath(pa.c_str(), buf.get()))
     {
-      can.assign(rp.get());
+      if (buf == nullptr)
+	buf.reset(rp);
+      result.assign(rp);
       ec.clear();
+      return result;
+    }
+  if (errno != ENAMETOOLONG)
+    {
+      ec.assign(errno, std::generic_category());
+      return result;
     }
-  else
-    ec.assign(errno, std::generic_category());
-#else
-  ec = std::make_error_code(std::errc::not_supported);
 #endif
-  return can;
+
+  auto fail = [&ec, &result](int e) mutable {
+      if (!ec.value())
+	ec.assign(e, std::generic_category());
+      result.clear();
+  };
+
+  if (!exists(pa, ec))
+    {
+      fail(ENOENT);
+      return result;
+    }
+  // else we can assume no unresolvable symlink loops
+
+  result = pa.root_path();
+
+  deque<path> cmpts;
+  for (auto& f : pa.relative_path())
+    cmpts.push_back(f);
+
+  while (!cmpts.empty())
+    {
+      path f = std::move(cmpts.front());
+      cmpts.pop_front();
+
+      if (f.compare(".") == 0)
+	{
+	  if (!is_directory(result, ec))
+	    {
+	      fail(ENOTDIR);
+	      break;
+	    }
+	}
+      else if (f.compare("..") == 0)
+	{
+	  auto parent = result.parent_path();
+	  if (parent.empty())
+	    result = pa.root_path();
+	  else
+	    result.swap(parent);
+	}
+      else
+	{
+	  result /= f;
+
+	  if (is_symlink(result, ec))
+	    {
+	      path link = read_symlink(result, ec);
+	      if (!ec.value())
+		{
+		  if (link.is_absolute())
+		    {
+		      result = link.root_path();
+		      link = link.relative_path();
+		    }
+		  else
+		    result.remove_filename();
+
+		  cmpts.insert(cmpts.begin(), link.begin(), link.end());
+		}
+	    }
+
+	  if (ec.value() || !exists(result, ec))
+	    {
+	      fail(ENOENT);
+	      break;
+	    }
+	}
+    }
+  return result;
 }
 
 fs::path
diff --git a/libstdc++-v3/testsuite/experimental/filesystem/operations/canonical.cc b/libstdc++-v3/testsuite/experimental/filesystem/operations/canonical.cc
new file mode 100644
index 0000000..0bcb85b
--- /dev/null
+++ b/libstdc++-v3/testsuite/experimental/filesystem/operations/canonical.cc
@@ -0,0 +1,74 @@
+// Copyright (C) 2015 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++11 -lstdc++fs" }
+// { dg-require-filesystem-ts "" }
+
+#include <experimental/filesystem>
+#include <testsuite_hooks.h>
+#include <testsuite_fs.h>
+
+namespace fs = std::experimental::filesystem;
+
+void
+test01()
+{
+  std::error_code ec;
+  fs::path p = "";
+  canonical( p, ec );
+  VERIFY( !ec );
+
+  canonical( __gnu_test::nonexistent_path(), ec );
+  VERIFY( !ec );
+
+  p = "/";
+  p = canonical( p, ec );
+  VERIFY( p == "/" );
+  VERIFY( ec );
+
+  p = "/.";
+  p = canonical( p, ec );
+  VERIFY( p == "/" );
+  VERIFY( ec );
+
+  p = "/..";
+  p = canonical( p, ec );
+  VERIFY( p == "/" );
+  VERIFY( ec );
+
+  p = "/../.././.";
+  p = canonical( p, ec );
+  VERIFY( p == "/" );
+  VERIFY( ec );
+
+  p = "/dev/stdin";
+  if (exists(p))
+    {
+      auto p2 = canonical(p);
+      if (is_symlink(p))
+        VERIFY( p != p2 );
+      else
+        VERIFY( p == p2 );
+      VERIFY( canonical(p2) == p2 );
+    }
+}
+
+int
+main()
+{
+  test01();
+}

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