From cdef1397102ae3d00a7656d2f71dc216442e219a Mon Sep 17 00:00:00 2001
From: SpecificProtagonist <specificprotagonist@posteo.org>
Date: Mon, 10 Mar 2025 22:16:14 +0100
Subject: [PATCH] Backtrace: std and threadsafe bevy_error_panic_hook (#18235)

# Objective

Make `bevy_error_panic_hook` threadsafe. As it relies on a global
variable, it fails when multiple threads panic.

## Solution

Switch from a global variable for storing whether an error message was
printed to a thread-local one.

`thread_local` is in `std`; the `backtrace` already relies on `std`
APIs. It didn't depend on the `std` feature though, so I've added that.
I've also put `bevy_error_panic_hook` behind the `backtrace` feature,
since it relies on the thread local variable, which fixes #18231.

## Testing

The following now loops instead of crashing:

```rust
std::thread::scope(|s| {
    use bevy_ecs::error::*;

    #[derive(Debug)]
    struct E;
    impl std::fmt::Display for E {
        fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            todo!()
        }
    }
    impl std::error::Error for E {}

    std::panic::set_hook(Box::new(bevy_error_panic_hook(|_| {
        unreachable!();
    })));
    for _ in 0..2 {
        s.spawn(|| {
            loop {
                let _ = std::panic::catch_unwind(|| {
                    panic!("{:?}", BevyError::from(E));
                });
            }
        });
    }
});
```
---
 crates/bevy_ecs/Cargo.toml              |  2 +-
 crates/bevy_ecs/src/error/bevy_error.rs | 13 +++++++------
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/crates/bevy_ecs/Cargo.toml b/crates/bevy_ecs/Cargo.toml
index 9a9c15de9f..2b8d2f0d54 100644
--- a/crates/bevy_ecs/Cargo.toml
+++ b/crates/bevy_ecs/Cargo.toml
@@ -37,7 +37,7 @@ reflect_functions = ["bevy_reflect", "bevy_reflect/functions"]
 configurable_error_handler = []
 
 ## Enables automatic backtrace capturing in BevyError
-backtrace = []
+backtrace = ["std"]
 
 # Debugging Features
 
diff --git a/crates/bevy_ecs/src/error/bevy_error.rs b/crates/bevy_ecs/src/error/bevy_error.rs
index 73777bad77..9632d89bc7 100644
--- a/crates/bevy_ecs/src/error/bevy_error.rs
+++ b/crates/bevy_ecs/src/error/bevy_error.rs
@@ -117,7 +117,7 @@ impl Debug for BevyError {
                 }
                 if !full_backtrace {
                     if std::thread::panicking() {
-                        SKIP_NORMAL_BACKTRACE.store(1, core::sync::atomic::Ordering::Relaxed);
+                        SKIP_NORMAL_BACKTRACE.set(true);
                     }
                     writeln!(f, "{FILTER_MESSAGE}")?;
                 }
@@ -132,22 +132,23 @@ impl Debug for BevyError {
 const FILTER_MESSAGE: &str = "note: Some \"noisy\" backtrace lines have been filtered out. Run with `BEVY_BACKTRACE=full` for a verbose backtrace.";
 
 #[cfg(feature = "backtrace")]
-static SKIP_NORMAL_BACKTRACE: core::sync::atomic::AtomicUsize =
-    core::sync::atomic::AtomicUsize::new(0);
+std::thread_local! {
+    static SKIP_NORMAL_BACKTRACE: core::cell::Cell<bool> =
+        const { core::cell::Cell::new(false) };
+}
 
 /// When called, this will skip the currently configured panic hook when a [`BevyError`] backtrace has already been printed.
-#[cfg(feature = "std")]
+#[cfg(feature = "backtrace")]
 pub fn bevy_error_panic_hook(
     current_hook: impl Fn(&std::panic::PanicHookInfo),
 ) -> impl Fn(&std::panic::PanicHookInfo) {
     move |info| {
-        if SKIP_NORMAL_BACKTRACE.load(core::sync::atomic::Ordering::Relaxed) > 0 {
+        if SKIP_NORMAL_BACKTRACE.replace(false) {
             if let Some(payload) = info.payload().downcast_ref::<&str>() {
                 std::println!("{payload}");
             } else if let Some(payload) = info.payload().downcast_ref::<alloc::string::String>() {
                 std::println!("{payload}");
             }
-            SKIP_NORMAL_BACKTRACE.store(0, core::sync::atomic::Ordering::Relaxed);
             return;
         }