Bug#1094270: std::filesystem::weakly_canonical throws if the file disappears during its execution

[Enrico Zini]

Package: libstdc++-14-dev
Version: 14.2.0-12
Severity: normal

Hello,

thank you for maintaining libstdc++.

The function std::filesystem::weakly_canonical is defined as working
even if the path points to a file that does not exist.

If however the path disappears while weakly_canonical does its thing, we
surprisingly get an exception.

I'm attaching a reproducer:

    $ g++ weakly_canonical.cc -o weakly_canonical
    $ ./weakly_canonical
    Canonicalized path for missing file: "…/tmp/does-not-exist"
    weakly_canonical failed at iteration 36: filesystem error: cannot make 
canonical path: No such file or directory [tmp/test]

Enrico

-- System Information:
Debian Release: trixie/sid
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: amd64 (x86_64)

Kernel: Linux 6.12.6-amd64 (SMP w/16 CPU threads; PREEMPT)
Kernel taint flags: TAINT_WARN
Locale: LANG=en_IE.UTF-8, LC_CTYPE=en_IE.UTF-8 (charmap=UTF-8), 
LANGUAGE=en_IE:en
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages libstdc++-14-dev depends on:
ii  gcc-14-base    14.2.0-12
ii  libc6-dev      2.40-5
ii  libgcc-14-dev  14.2.0-12
ii  libstdc++6     14.2.0-12

libstdc++-14-dev recommends no packages.

Versions of packages libstdc++-14-dev suggests:
pn  libstdc++-14-doc  <none>

-- no debconf information

#include <filesystem>
#include <thread>
#include <iostream>

std::filesystem::path workdir("tmp");
auto oddpath = workdir / "test";
bool done = false;

void glitch()
{
    while (! done)
    {
        std::filesystem::remove(oddpath);
        std::filesystem::create_directory(oddpath);
    }
}

void canonical()
{
    unsigned iteration = 0;
    try {
        while (true)
        {
            ++iteration;
            (void)std::filesystem::weakly_canonical(oddpath);
        }
    } catch (std::exception& e) {
        done = true;
        std::cerr << "weakly_canonical failed at iteration " << iteration << ": 
" << e.what() << std::endl;
    }
}


int main(int argc, const char* argv[])
{
    std::filesystem::create_directory(workdir);
    auto missing = std::filesystem::weakly_canonical(workdir / 
"does-not-exist");
    std::cerr << "Canonicalized path for missing file: " << missing << 
std::endl;

    std::thread glitcher(glitch);

    canonical();

    glitcher.join();

    std::filesystem::remove_all(workdir);

    return 0;
}