From a0c424622ab2698cc9eb7d8a0902136f7b89a928 Mon Sep 17 00:00:00 2001
From: Alastair Houghton <ahoughton@apple.com>
Date: Mon, 3 Nov 2025 10:24:05 +0000
Subject: [PATCH] [Observation] Add threading fatal error handler.

When the Threading library experiences a fatal error, we need to
tell it what to do.
---
 .../Sources/Observation/CMakeLists.txt        |  1 +
 .../Sources/Observation/ThreadingError.cpp    | 26 +++++++++++++++++++
 2 files changed, 27 insertions(+)
 create mode 100644 stdlib/public/Observation/Sources/Observation/ThreadingError.cpp

diff --git a/stdlib/public/Observation/Sources/Observation/CMakeLists.txt b/stdlib/public/Observation/Sources/Observation/CMakeLists.txt
index 2e7259a1421..09bc8524d5c 100644
--- a/stdlib/public/Observation/Sources/Observation/CMakeLists.txt
+++ b/stdlib/public/Observation/Sources/Observation/CMakeLists.txt
@@ -19,6 +19,7 @@ add_swift_target_library(swiftObservation ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS
   ObservationRegistrar.swift
   ObservationTracking.swift
   Observations.swift
+  ThreadingError.cpp
   ThreadLocal.cpp
   ThreadLocal.swift
 
diff --git a/stdlib/public/Observation/Sources/Observation/ThreadingError.cpp b/stdlib/public/Observation/Sources/Observation/ThreadingError.cpp
new file mode 100644
index 00000000000..d6ff578d6f2
--- /dev/null
+++ b/stdlib/public/Observation/Sources/Observation/ThreadingError.cpp
@@ -0,0 +1,26 @@
+//===--- ThreadingError.cpp - Error handling support code -----------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/Threading/Errors.h"
+#include <cstdio>
+#include <cstdlib>
+
+// Handle fatal errors from the threading library
+SWIFT_ATTRIBUTE_NORETURN
+SWIFT_FORMAT(1, 2)
+void swift::threading::fatal(const char *format, ...) {
+  va_list val;
+
+  va_start(val, format);
+  vfprintf(stderr, format, val);
+  abort();
+}