[llvm-bugs] [Bug 49493] New: std::filesystem::weakly_canonical race condition when the underlying path is removed

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https://bugs.llvm.org/show_bug.cgi?id=49493

            Bug ID: 49493
           Summary: std::filesystem::weakly_canonical race condition when
                    the underlying path is removed
           Product: libc++
           Version: 10.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: xavier.roche at algolia.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

Created attachment 24609
  --> https://bugs.llvm.org/attachment.cgi?id=24609&action=edit
Test aimed to reproduce the race issue

The std::filesystem::weakly_canonical function is normally able to canonize
both non-existing path(es) and existing path(es).

However, the current implementation is having issues with canonizing a path
that disappears while the function is being called, leading to randomly
returning errors (and/or raising filesystem_error).

Important Note: this MAY be an expected behavior, as the official specification
of std::filesystem::weakly_canonical does not appear to be completely clear
towards impact of filesystem volatility.

The attached sample demonstrate the issue:
-----------------------------------------

clang++ -g3 -pthread -std=c++20 filesystem_weakly_canonical-race-canonical.cpp
-o filesystem_weakly_canonical-race-canonical
./filesystem_weakly_canonical-race-canonical

Starting computing
terminate called after throwing an instance of
'std::filesystem::__cxx11::filesystem_error'
  what():  filesystem error: cannot make canonical path: No such file or
directory [foo]
Aborted (core dumped)

Our understanding is that std::filesystem::weakly_canonical calls
std::filesystem::canonical under the hood, if the path segment exist:

  while (PP.State != PathParser::PS_BeforeBegin) {
    tmp.assign(createView(p.native().data(), &PP.RawEntry.back()));
    error_code m_ec;
    file_status st = __status(tmp, &m_ec);
    if (!status_known(st)) {
      return err.report(m_ec);
    } else if (exists(st)) {
      result = __canonical(tmp, ec);
      break;
    }
    DNEParts.push_back(*PP);
    --PP;
  }

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