[12/N] [libcxx] Sanitize paths before creating symlinks on windows

[mstorsjo]

Summary

The STL does even more cleanup (corresponding to lexically_normal I think), but this seems to be the very minimum needed for making the symlinks work when the target path contains non-native paths.

I guess this could be squashed into patch 11, but this adds a test case for making sure the created symlinks really work.

[ldionne]

@EricWF can you have a look at this?

[amccarth]

LGTM

[mstorsjo]

Rebased, rerunning CI before pushing

libcxx/src/filesystem/posix_compat.h

 
 int mkdir(const wchar_t *path, int permissions) {
   (void)permissions;
   return _wmkdir(path);
 }
 
 int symlink_file_dir(const wchar_t *oldname, const wchar_t *newname,
                      bool is_dir) {
+  path dest(oldname);
+  dest.make_preferred();
+  oldname = dest.c_str();
   DWORD flags = is_dir ? SYMBOLIC_LINK_FLAG_DIRECTORY : 0;
   if (CreateSymbolicLinkW(newname, oldname,
                           flags | SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE))
     return 0;
   int e = GetLastError();
   if (e != ERROR_INVALID_PARAMETER)
     return set_errno(e);
   if (CreateSymbolicLinkW(newname, oldname, flags))

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.create_symlink/create_symlink.pass.cpp

const path dir_sym = env.create_directory_symlink(dir, "dir_sym");
         std::error_code ec;
         fs::create_directory_symlink(dir_sym, dest, ec);
         TEST_REQUIRE(!ec);
         TEST_CHECK(is_symlink(dest));
         TEST_CHECK(equivalent(dest, dir));
     }
 }
 
+TEST_CASE(create_symlink_dest_cleanup)
+{
+    scoped_test_env env;
+    const path dir = env.create_dir("dir");
+    const path file = env.create_file("file", 42);
+    const path sym = dir / "link";
+    // The target path has to be normalized to backslashes before creating
+    // the link on windows, otherwise the link isn't dereferencable.
+    const path sym_target = "../file";
+    path sym_target_normalized = sym_target;
+    sym_target_normalized.make_preferred();
+    std::error_code ec;
+    fs::create_symlink(sym_target, sym, ec);
+    TEST_REQUIRE(!ec);
+    TEST_CHECK(equivalent(sym, file, ec));
+    const path ret = fs::read_symlink(sym, ec);
+    TEST_CHECK(!ec);
+    TEST_CHECK(ret.native() == sym_target_normalized.native());
+}
+
 
 TEST_SUITE_END()