[Bug libstdc++/99311] New: std::filesystem::exists & std::filesystem::create_directories don't work correctly with UNC paths

[moritz at bunkus dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99311

            Bug ID: 99311
           Summary: std::filesystem::exists &
                    std::filesystem::create_directories don't work
                    correctly with UNC paths
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: moritz at bunkus dot org
  Target Milestone: ---

Created attachment 50270
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50270&action=edit
test case showing the issues

These might be two separate issues, but my gut feeling is that they're both due
to the same underlying problem, especially if create_directories() uses
exists() to determine which components it has to create.

Using std::filesystem::exists on a UNC path with mingw 10.20.0 that points to a
share (not a file or directory in a share) returns false. For example:

• exists() for the existing file "\\server\share\this_exists.txt" returns true
proving that the share itself exists & the user has access to it. However,
exists() on "\\server\share" (retrieved via ".parent_path()" from the full
path) returns false. By comparison, boost::filesystem::exists() returns true
for both paths.

• Using the same server & same share, trying to create a sub-directory via
create_directories() fails. For example, trying to create
"\\server\share\this_does_not_exist_yet" fails. Yes, I'm 100% sure that I have
write access to the share in question. By comparison,
boost::filesystem::create_directories() works fine with such a path.

I'll a sample program that shows the problem. Of course you'll have to adjust
server & share names. Compiled with gcc 10.20.0 from the mxe project. Command
line:

x86_64-w64-mingw32.static-g++ -std=c++17 -o
std_filesystem_exists_and_create_directories.exe
std_filesystem_exists_and_create_directories.cpp

The program outputs the following:

-----
sanity check, should be true: 1
should return true but doesn't: 0
directory \\disky\mosu\does-not-exist doesn't exist yet, should be false: 0
tried to create directory \\disky\mosu\does-not-exist, have error code? 1
-----

The first is the check for the existing file. It does return true, showing that
exists() works correctly for existing files.

The second line tries exists() for the parent_path() of the file tested in line
1. Of course it should return true, too, but doesn't. The path points to the
share.

Line three just establishes that the directory really doesn't exist yet.
exists() should return false here and does so. Great.

Line four shows the result of the create_directories() call, or rather, whether
the error_code is indicating an error. It does, which it shouldn't.