Use GetFileAttributesW in symlink_status if CreateFileW fails.

For some system files and folders like "System Volume Information"
CreateFileW fails with ERROR_ACCESS_DENIED while GetFileAttributesW
succeeds. GetFileAttributesW doesn't allow to discover whether the
file is a symlink or some other kind of a reparse point, so this
fallback will only work for files that are not reparse points,
symlinks or not. For reparse points continue to report error.

Closes boostorg#234.

Author: Lastique

doc/release_history.html

@@ -39,6 +39,11 @@
     </td>
 </table>
 
+<h2>1.80.0</h2>
+<ul>
+  <li>On Windows, added a fallback implementation for querying file attributes in case if the file cannot be opened with <code>ERROR_ACCESS_DENIED</code> error. This may allow <code>status</code> and <code>symlink_status</code> to succeed for system files and directories that are not reparse points or symlinks. (<a href="https://github.com/boostorg/filesystem/issues/234">#234</a>)</li>
+</ul>
+
 <h2>1.79.0</h2>
 <ul>
   <li><b>v3:</b> <code>path::replace_extension</code> now works in terms of <b>v3</b> definition of <code>path::extension</code> rather than <b>v4</b>.</li>

src/operations.cpp

@@ -1284,9 +1284,8 @@ inline std::size_t get_full_path_name(path const& src, std::size_t len, wchar_t*
     return static_cast< std::size_t >(::GetFullPathNameW(src.c_str(), static_cast< DWORD >(len), buf, p));
 }
 
-inline fs::file_status process_status_failure(path const& p, error_code* ec)
+inline fs::file_status process_status_failure(DWORD errval, path const& p, error_code* ec)
 {
-    DWORD errval = ::GetLastError();
     if (ec)                                    // always report errval, even though some
         ec->assign(errval, system_category()); // errval values are not status_errors
 
@@ -1305,6 +1304,11 @@ inline fs::file_status process_status_failure(path const& p, error_code* ec)
     return fs::file_status(fs::status_error);
 }
 
+inline fs::file_status process_status_failure(path const& p, error_code* ec)
+{
+    return process_status_failure(::GetLastError(), p, ec);
+}
+
 //! remove() implementation for Windows XP and older
 bool remove_nt5_impl(path const& p, DWORD attrs, error_code* ec)
 {
@@ -3855,21 +3859,38 @@ file_status symlink_status(path const& p, error_code* ec)
         OPEN_EXISTING,
         FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OPEN_REPARSE_POINT));
 
+    DWORD attrs;
     if (h.handle == INVALID_HANDLE_VALUE)
     {
-    return_status_failure:
-        return process_status_failure(p, ec);
+        // For some system files and folders like "System Volume Information" CreateFileW fails
+        // with ERROR_ACCESS_DENIED. GetFileAttributesW succeeds for such files, so try that.
+        // Though this will only help if the file is not a reparse point (symlink or not).
+        DWORD err = GetLastError();
+        if (err == ERROR_ACCESS_DENIED)
+        {
+            attrs = GetFileAttributesW(p.c_str());
+            if (attrs != INVALID_FILE_ATTRIBUTES)
+            {
+                if ((attrs & FILE_ATTRIBUTE_REPARSE_POINT) == 0u)
+                    return file_status((attrs & FILE_ATTRIBUTE_DIRECTORY) ? fs::directory_file : fs::regular_file, make_permissions(p, attrs));
+            }
+            else
+            {
+                err = GetLastError();
+            }
+        }
+
+        return process_status_failure(err, p, ec);
     }
 
-    DWORD attrs;
     fs::perms permissions;
     GetFileInformationByHandleEx_t* get_file_information_by_handle_ex = filesystem::detail::atomic_load_relaxed(get_file_information_by_handle_ex_api);
     if (BOOST_LIKELY(get_file_information_by_handle_ex != NULL))
     {
         file_attribute_tag_info info;
         BOOL res = get_file_information_by_handle_ex(h.handle, file_attribute_tag_info_class, &info, sizeof(info));
         if (BOOST_UNLIKELY(!res))
-            goto return_status_failure;
+            return process_status_failure(p, ec);
 
         attrs = info.FileAttributes;
         permissions = make_permissions(p, attrs);
@@ -3882,7 +3903,7 @@ file_status symlink_status(path const& p, error_code* ec)
         BY_HANDLE_FILE_INFORMATION info;
         BOOL res = ::GetFileInformationByHandle(h.handle, &info);
         if (BOOST_UNLIKELY(!res))
-            goto return_status_failure;
+            return process_status_failure(p, ec);
 
         attrs = info.dwFileAttributes;
         permissions = make_permissions(p, attrs);

test/operations_test.cpp

@@ -1069,6 +1069,18 @@ void predicate_and_status_tests()
     BOOST_TEST(!fs::is_regular_file(stat));
     BOOST_TEST(!fs::is_other(stat));
     BOOST_TEST(!fs::is_symlink(stat));
+
+#ifdef BOOST_WINDOWS_API
+    stat = fs::status(L"\\System Volume Information");
+    BOOST_TEST(fs::type_present(stat));
+    BOOST_TEST(fs::permissions_present(stat));
+    BOOST_TEST(fs::status_known(stat));
+    BOOST_TEST(fs::exists(stat));
+    BOOST_TEST(fs::is_directory(stat));
+    BOOST_TEST(!fs::is_regular_file(stat));
+    BOOST_TEST(!fs::is_other(stat));
+    BOOST_TEST(!fs::is_symlink(stat));
+#endif // BOOST_WINDOWS_API
 }
 
 //  create_directory_tests  ----------------------------------------------------------//