[libcxx] Map ERROR_BAD_PATHNAME to errc::no_such_file_or_directory on windows

[mstorsjo]

Summary

Opening a path like \\server (without a trailing share name and path) produces this error, while opening e.g. \\server\share (for a nonexistent server/share) produces ERROR_BAD_NETPATH (which already is mapped).

This happens in some testcases (in fs.op.proximate); as proximate() calls weakly_canonical() on the inputs, weakly_canonical() checks whether the path exists or not. When the error code wasn't recognized (it mapped to errc::invalid_argument), the stat operation wasn't conclusive and weakly_canonical() errored out. With the proper error code mapping, this isn't considered an error, just a nonexistent path, and weakly_canonical() can proceed.

This roughly matches what MS STL does - it doesn't have ERROR_BAD_PATHNAME in its error code mapping table, but it checks for this error code specifically in the return of their correspondence of the stat function.

[curdeius]

LGTM.

libcxx/src/filesystem/operations.cpp

 errc __win_err_to_errc(int err) {
   constexpr struct {
     DWORD win;
     errc errc;
   } win_error_mapping[] = {
       {ERROR_ACCESS_DENIED, errc::permission_denied},
       {ERROR_ALREADY_EXISTS, errc::file_exists},
       {ERROR_BAD_NETPATH, errc::no_such_file_or_directory},
+      {ERROR_BAD_PATHNAME, errc::no_such_file_or_directory},
       {ERROR_BAD_UNIT, errc::no_such_device},
       {ERROR_BROKEN_PIPE, errc::broken_pipe},
       {ERROR_BUFFER_OVERFLOW, errc::filename_too_long},
       {ERROR_BUSY, errc::device_or_resource_busy},
       {ERROR_BUSY_DRIVE, errc::device_or_resource_busy},
       {ERROR_CANNOT_MAKE, errc::permission_denied},
       {ERROR_CANTOPEN, errc::io_error},
       {ERROR_CANTREAD, errc::io_error},