Runtime: add a non-Darwin error message storage

This introduces a non-Darwin (non-CrashReporter) storage for error
messages to allow extraction for crash reporting. This is initially
meant to be used on Windows, though it is generic enough to be used on
any platform.

Author: compnerd

include/swift/Runtime/Debug.h

@@ -127,6 +127,9 @@ void swift_reportError(uint32_t flags, const char *message);
 SWIFT_RUNTIME_EXPORT
 void swift_reportWarning(uint32_t flags, const char *message);
 
+SWIFT_RUNTIME_EXPORT
+const char **swift_getFatalErrorMessageBuffer();
+
 // Halt due to an overflow in swift_retain().
 SWIFT_RUNTIME_ATTRIBUTE_NORETURN SWIFT_RUNTIME_ATTRIBUTE_NOINLINE
 void swift_abortRetainOverflow();

stdlib/public/runtime/Errors.cpp

@@ -34,6 +34,7 @@
 
 #include "ImageInspection.h"
 #include "swift/Demangling/Demangle.h"
+#include "swift/Runtime/Atomic.h"
 #include "swift/Runtime/Debug.h"
 #include "swift/Runtime/Portability.h"
 #include "swift/Runtime/Win32.h"
@@ -66,8 +67,8 @@
 #ifdef SWIFT_HAVE_CRASHREPORTERCLIENT
 #include <atomic>
 #include <malloc/malloc.h>
-
-#include "swift/Runtime/Atomic.h"
+#else
+static char *kFatalErrorMessage;
 #endif // SWIFT_HAVE_CRASHREPORTERCLIENT
 
 #include "BacktracePrivate.h"
@@ -297,7 +298,23 @@ reportOnCrash(uint32_t flags, const char *message)
              std::memory_order_release,
              SWIFT_MEMORY_ORDER_CONSUME));
 #else
-  // empty
+  char *previous = nullptr, *current = nullptr;
+  previous =
+      std::atomic_load_explicit((const char * _Atomic *)&kFatalErrorMessage,
+                                SWIFT_MEMORY_ORDER_CONSUME);
+
+  do {
+    free(current);
+    current = nullptr;
+
+    if (previous)
+      swift_asprintf(&current, "%s%s", current, message);
+    else
+      current = strdup(message);
+  } while (!std::atomic_compare_exchange_strong_explicit((const char * _Atomic *)&kFatalErrorMessage,
+                                                         &previous, current,
+                                                         std::memory_order_release,
+                                                         SWIFT_MEMORY_ORDER_CONSUME));
 #endif // SWIFT_HAVE_CRASHREPORTERCLIENT
 }
 
@@ -421,6 +438,10 @@ void swift::swift_reportWarning(uint32_t flags, const char *message) {
   warning(flags, "%s", message);
 }
 
+const char **swift::swift_getFatalErrorMessageBuffer() {
+  return &kFatalErrorMessage;
+}
+
 // Crash when a deleted method is called by accident.
 SWIFT_RUNTIME_EXPORT SWIFT_NORETURN void swift_deletedMethodError() {
   swift::fatalError(/* flags = */ 0,