[Bug?]: Race condition when modifying workspace directory and calling `yarn pack` in parallel causes yarn to crash

[GeorgeTaveras1231]

Self-service

• I'd be willing to implement a fix

Describe the bug

Scenario

Two parallel jobs are running.
Job 1 - Creates temporary files that are deleted after the process ends
Job 2 - Runs yarn pack or calls genPackList from @yarnpkg/plugin-pack

Explanation

genPackList calls walk - a function that traverses the folder structure to determine which files are part of the pack. In this process, walk calls cwdFs.lstatPromise(here) to determine wether the path belongs to a directory or a normal file. If its a directory, it calls cwdFs.readdirPromise(here) to get a list of all paths, which are added to the DFS traversal stack; each discovered path goes through the same process.

The race condition manifests within a traversal loop, after readdir. In the scenario where Job 1 created a file, in a directory being processed by the walk function, the file will be added to the DFS traversal stack. However, by the time that this path is popped, before lstat is called, it may have already been deleted by Job 1

Result

This produces a failure with the following error

> yarn pack
...
Error: ENOENT: no such file or directory, lstat '$PATH_TO_TEMPORARY_FILE'] {
  errno: -2,
  code: 'ENOENT',
  syscall: 'lstat',
  path: '$PATH_TO_TEMPORARY_FILE'
}

To reproduce

1. Create a workspace
2. Create a script that adds files -- wait 1 second -- then deletes the file
3. Create another script that calls the script on step 2, as well as yarn pack, executing them in parallel
4. Call the script created in step 3

The results may be mixed and may require tweaking the 1 second variable. When reproduced correctly, you should see the lstat error mentioned above.

Environment

System:
    OS: macOS 26.4.1
    CPU: (12) arm64 Apple M3 Pro
  Binaries:
    Node: 22.20.0 - /Users/georgetaveras/.asdf/installs/nodejs/22.20.0/bin/node
    Yarn: 4.11.0 - /opt/homebrew/bin/yarn
    npm: 10.9.3 - /Users/georgetaveras/.asdf/plugins/nodejs/shims/npm

Additional context

Proposal

Catch errors from lstat, ignore if the file no longer exists.

[arcanis]

Let's say we silence errors. Does that mean we'd end up with non-deterministic pack lists? That seems a problem.

[GeorgeTaveras1231]

@arcanis I wouldn't think of it like that.

First, I'd make sure that the only error we silence is ENOENT -- I think its safe would be safe to assume that such a file was never meant to be part of the packlist.

Secondly, the act of silencing the error doesn't make it more or less deterministic. Regardless of which approach is taken, the code produces non-deterministic results: either seemingly arbitrary crashes, or a non-deterministic packlist.

Silencing the error will just lean into the reality that there is nothing this process can do to enforce consistent results when other processes are manipulating files.

We can also considering emitting a non-crashing warning when this happens so developers are made aware of the non-deterministic results created by their operations.

[arcanis]

Can you detail the actual use case of modifying a directory at the same time as you're running the pack operation? You mention temporary folders / directories - isn't the fix for you to cover them via npmignore patterns ?