[hurd] Fix unconditional use of PATH_MAX

Patch by Samuel Thibault

The GNU/Hurd system does not define an arbitrary PATH_MAX limitation, the POSIX 2001 realpath
extension can be used instead, and the size of symlinks can be determined.

Reviewed as https://reviews.llvm.org/D54677

llvm-svn: 351414

Author: EricWF

libcxx/src/filesystem/operations.cpp

@@ -543,11 +543,19 @@ path __canonical(path const& orig_p, error_code* ec) {
   ErrorHandler<path> err("canonical", ec, &orig_p, &cwd);
 
   path p = __do_absolute(orig_p, &cwd, ec);
+#if _POSIX_VERSION >= 200112
+  std::unique_ptr<char, decltype(&::free)>
+    hold(::realpath(p.c_str(), nullptr), &::free);
+  if (hold.get() == nullptr)
+    return err.report(capture_errno());
+  return {hold.get()};
+#else
   char buff[PATH_MAX + 1];
   char* ret;
   if ((ret = ::realpath(p.c_str(), buff)) == nullptr)
     return err.report(capture_errno());
   return {ret};
+#endif
 }
 
 void __copy(const path& from, const path& to, copy_options options,
@@ -1089,16 +1097,27 @@ void __permissions(const path& p, perms prms, perm_options opts,
 path __read_symlink(const path& p, error_code* ec) {
   ErrorHandler<path> err("read_symlink", ec, &p);
 
-  char buff[PATH_MAX + 1];
-  error_code m_ec;
-  ::ssize_t ret;
-  if ((ret = ::readlink(p.c_str(), buff, PATH_MAX)) == -1) {
+#ifdef PATH_MAX
+  struct NullDeleter { void operator()(void*) const {} };
+  const size_t size = PATH_MAX + 1;
+  char stack_buff[size];
+  auto buff = std::unique_ptr<char[], NullDeleter>(stack_buff);
+#else
+  StatT sb;
+  if (::lstat(p.c_str(), &sb) == -1) {
     return err.report(capture_errno());
   }
-  _LIBCPP_ASSERT(ret <= PATH_MAX, "TODO");
+  const size_t size = sb.st_size + 1;
+  auto buff = unique_ptr<char[]>(new char[size]);
+#endif
+  ::ssize_t ret;
+  if ((ret = ::readlink(p.c_str(), buff.get(), size)) == -1)
+    return err.report(capture_errno());
   _LIBCPP_ASSERT(ret > 0, "TODO");
+  if (static_cast<size_t>(ret) >= size)
+    return err.report(errc::value_too_large);
   buff[ret] = 0;
-  return {buff};
+  return {buff.get()};
 }
 
 bool __remove(const path& p, error_code* ec) {