Apply advisory locks when building source distributions

[charliermarsh]

Summary

I don't love this, but it turns out that setuptools is not robust to parallel builds: pypa/setuptools#3119. As a result, if you run uv from multiple processes, and they each attempt to build the same source distribution, you can hit failures.

This PR applies an advisory lock to the source distribution directory. We apply it unconditionally, even if we ultimately find something in the cache and don't do a build, which helps ensure that we only build the distribution once (and wait for that build to complete) rather than kicking off builds from each thread.

Closes #3512.

Test Plan

Ran:

#!/bin/bash
make_venv(){
    target/debug/uv venv $1
    source $1/bin/activate
    target/debug/uv pip install opentracing --no-deps --verbose
}

for i in {1..8}
do
   make_venv ./$1/$i &
done

[charliermarsh]

Realizing... I think I need this to be async? Is that right @ibraheemdev?

[zanieb]

Looks like you hold it across await points, so yeah.

[charliermarsh]

It’s not a mutex though, it’s a flock. I’d need to do some research.

[zanieb]

Hm.. aren't you still yielding control? So some other task could hit this lock and block without awaiting and returning control to the event loop and consequently to this task? e.g.

• Task A takes lock
• Task A yields to event loop
• Task B attempts to take lock
• Lock is not available so Task B is blocked but has not yielded to the event loop
• Task A is never returned to
• Deadlock

[charliermarsh]

Wrapped it in a tokio task.

[BurntSushi]

Yeah I agree with this.

[BurntSushi]

LGTM.