OnceMap unsound if Hash panics in such a way as to prevent HashMap/HashTable rehashing to succeed

[steffahn]

Adapted from

• FrozenMap unsound if Hash panics in such a way as to prevent HashMap rehashing to succeed. Manishearth/elsa#48

also relevant for context

• Consider not dropping entries on panicky rehashes rust-lang/hashbrown#444

Here’s the reproduction:

use std::{
    panic::{catch_unwind, AssertUnwindSafe},
    sync::Mutex,
};

#[derive(PartialEq, Eq, Debug)]
struct H(u32);

impl std::hash::Hash for H {
    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
        if PANIC_ON.lock().unwrap().as_ref() == Some(&self.0) {
            panic!();
        }
        0_u32.hash(state);
    }
}

static PANIC_ON: Mutex<Option<u32>> = Mutex::new(None);

fn main() {
    let mut map = once_map::sync::OnceMap::new();
    for i in 1..=28 {
        map.insert(H(i), |k| {
            if *k == H(28) {
                String::from("Hello World!")
            } else {
                String::new()
            }
        });
    }
    for i in 1..=27 {
        map.remove(&H(i));
    }

    let hello_world = map.get(&H(28)).unwrap();

    println!("exists: {hello_world}");

    let _ = catch_unwind(AssertUnwindSafe(|| {
        *PANIC_ON.lock().unwrap() = Some(28);
        map.insert(H(1), |_| String::new());
    }));

    println!("gone: {hello_world}");
}

$ cargo run
   Compiling libc v0.2.150
   Compiling cfg-if v1.0.0
   Compiling version_check v0.9.4
   Compiling autocfg v1.1.0
   Compiling parking_lot_core v0.9.9
   Compiling smallvec v1.11.2
   Compiling scopeguard v1.2.0
   Compiling zerocopy v0.7.26
   Compiling once_cell v1.18.0
   Compiling stable_deref_trait v1.2.0
   Compiling lock_api v0.4.11
   Compiling ahash v0.8.6
   Compiling getrandom v0.2.11
   Compiling parking_lot v0.12.1
   Compiling hashbrown v0.14.2
   Compiling once_map v0.4.12
   Compiling local_playground v0.1.0 (/path/to/local_playground)
    Finished dev [unoptimized + debuginfo] target(s) in 2.06s
     Running `target/debug/local_playground`
exists: Hello World!
thread 'main' panicked at src/main.rs:12:13:
explicit panic
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
gone: ���X���F

See also here and here in the hashbrown source-code for more information.

[a1phyr]

Thanks for this report and this repro !

This is quite unexpected from hashbrown to be honest. I'll add code to make sure that hashing never unwind.