fix interleaved panic output

previously, we only held a lock for printing the backtrace itself. since all threads were printing to the same file descriptor, that meant random output in the default panic hook would be interleaved with the backtrace. now, we hold the lock for the full duration of the hook, and the output is ordered.

1 files changed, 6 insertions, 2 deletions

diff --git a/library/std/src/panicking.rs b/library/std/src/panicking.rs
index ebd05415695..418a855fb72 100644
--- a/library/std/src/panicking.rs
+++ b/library/std/src/panicking.rs
@@ -253,16 +253,20 @@ fn default_hook(info: &PanicHookInfo<'_>) {
     let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");
 
     let write = |err: &mut dyn crate::io::Write| {
+        // Use a lock to prevent mixed output in multithreading context.
+        // Some platforms also require it when printing a backtrace, like `SymFromAddr` on Windows.
+        let mut lock = backtrace::lock();
         let _ = writeln!(err, "thread '{name}' panicked at {location}:\n{msg}");
 
         static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
 
         match backtrace {
+            // SAFETY: we took out a lock just a second ago.
             Some(BacktraceStyle::Short) => {
-                drop(backtrace::print(err, crate::backtrace_rs::PrintFmt::Short))
+                drop(lock.print(err, crate::backtrace_rs::PrintFmt::Short))
             }
             Some(BacktraceStyle::Full) => {
-                drop(backtrace::print(err, crate::backtrace_rs::PrintFmt::Full))
+                drop(lock.print(err, crate::backtrace_rs::PrintFmt::Full))
             }
             Some(BacktraceStyle::Off) => {
                 if FIRST_PANIC.swap(false, Ordering::Relaxed) {