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Fix panic when restricted-std is enabled. #104971

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Fix panic when restricted-std is enabled. #104971

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16 changes: 11 additions & 5 deletions library/std/src/panicking.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -336,20 +336,26 @@ pub mod panic_count {
pub fn increase() -> (bool, usize) {
let global_count = GLOBAL_PANIC_COUNT.fetch_add(1, Ordering::Relaxed);
let must_abort = global_count & ALWAYS_ABORT_FLAG != 0;
let panics = if must_abort {
global_count & !ALWAYS_ABORT_FLAG
} else {
LOCAL_PANIC_COUNT.with(|c| {
let mut panics = global_count & !ALWAYS_ABORT_FLAG;

// In a restricted_std environment, we likely don't have thread_local.
// If we attempt to use LOCAL_PANIC_COUNT here, it is probable that another panic will
// occur, sending us into an infinite panic loop that never calls the panic handler.
#[cfg(not(feature = "restricted-std"))]
if !must_abort {
panics = LOCAL_PANIC_COUNT.with(|c| {
Comment on lines +341 to +346
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What does "likely" mean here? Your PR description says

When restricted-std is in use, we do not have access to thread local storage, as sys/unsupported does not implement it.

Is that always the case when using restricted-std? and if not, when can the behavior be different?

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Looking at it a bit more, it seems it's true most of the time, but not necessarily always. The restricted-std requirement is defined here if a platform match is not found, and since thread_local is unimplemented in sys/unsupported, a panic loop occurs.

There do appear to be some cases where this may not always be true - i.e. in the case of aarch64-apple-tvos, which is not allowed to use regular unrestricted std, despite being part of the unix family and thus using the unix system module which does properly implement thread-local support. I would imagine this same situation would also apply to any of the other targets which are called out in the same area in build.rs and also use a non-unsupported system module.

I'm not really sure how big of an impact this would be; since restricted-std is infectious, requiring itself to be in every std-dependent crate throughout the entire dependency tree in order to be used, it seems like a minor detail, especially since a double panic on these platforms will still trigger an abort via the global panic counter.

let next = c.get() + 1;
c.set(next);
next
})
});
};

(must_abort, panics)
}

pub fn decrease() {
GLOBAL_PANIC_COUNT.fetch_sub(1, Ordering::Relaxed);
#[cfg(not(feature = "restricted-std"))]
LOCAL_PANIC_COUNT.with(|c| {
let next = c.get() - 1;
c.set(next);
Expand Down