Fix locking soundness in PersistentPlugin

[devyn]

Description

There were two problems in PersistentPlugin which could cause a deadlock:

1. There were two mutexes being used, and get() could potentially hold both simultaneously if it had to spawn. This won't necessarily cause a deadlock on its own, but it does mean that lock order is sensitive

2. set_gc_config() called flush() while still holding the lock, meaning that the GC thread had to proceed before the lock was released. However, waiting for the GC thread to proceed could mean waiting for the GC thread to call stop(), which itself would try to lock the mutex. So, it's not safe to wait for the GC thread while the lock is held. This is fixed now.

I've also reverted #12177, as @IanManske reported that this was also happening for him on Linux, and it seems to be this problem which should not be platform-specific at all. I believe this solves it.

User-Facing Changes

None

Tests + Formatting

• 🟢 toolkit fmt
• 🟢 toolkit clippy
• 🟢 toolkit test
• 🟢 toolkit test stdlib

After Submitting

[fdncred]

Thanks for digging in and figuring it out!

[fdncred]

Not that it's related but right after this PR I get this error compiling on macos now. So, back to using --locked :(

error[E0658]: use of unstable library feature 'ptr_from_ref'
   --> /Users/fdncred/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libproc-0.14.5/src/libproc/proc_pid.rs:241:22
    |
241 |     let buffer_ptr = std::ptr::from_mut::<T>(&mut pidinfo).cast::<c_void>();
    |                      ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #106116 <https://github.com/rust-lang/rust/issues/106116> for more information

error[E0658]: use of unstable library feature 'ptr_from_ref'
   --> /Users/fdncred/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libproc-0.14.5/src/libproc/pid_rusage.rs:389:22
    |
389 |     let buffer_ptr = std::ptr::from_mut::<T>(&mut pidrusage).cast::<c_void>();
    |                      ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #106116 <https://github.com/rust-lang/rust/issues/106116> for more information

error[E0658]: use of unstable library feature 'ptr_from_ref'
   --> /Users/fdncred/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libproc-0.14.5/src/libproc/file_info.rs:168:22
    |
168 |     let buffer_ptr = std::ptr::from_mut::<T>(&mut pidinfo).cast::<c_void>();
    |                      ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #106116 <https://github.com/rust-lang/rust/issues/106116> for more information

[devyn]

I haven't made any dependency changes today, so unfortunately I think it's probably upstream again

[devyn]

rust-lang/rust#106116

Looks to me like that got stabilized with rustc 1.76.0 fyi, so it's using something newer than it should

[fdncred]

yup, agreed, it's upstream. A little aggravating that we're a couple rust releases from that being in stable for us. We try to stay two versions behind.

[devyn]

Issue submitted

[WindSoilder]

@devyn Thank you for tackling all of plugin stuff! It's hard to get it right with bug free at once. I'm happy to see all improvements on plugin system