Interop: send events to fallback event handler if active configuration not present

WIP, needs some additional work to get build working locally. Although
the `_swift_testing_getFallbackEventHandler` is forward declared, I
don't think it will be present at link time since we're not linking
against _TestingInterop.

Author: jerryjrchen

Sources/Testing/CMakeLists.txt

@@ -31,6 +31,7 @@ add_library(Testing
   Attachments/Attachment.swift
   Events/Clock.swift
   Events/Event.swift
+  Events/Event+FallbackHandler.swift
   Events/Recorder/Event.AdvancedConsoleOutputRecorder.swift
   Events/Recorder/Event.ConsoleOutputRecorder.swift
   Events/Recorder/Event.HumanReadableOutputRecorder.swift

Sources/Testing/Events/Event+FallbackHandler.swift

@@ -0,0 +1,44 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 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 Swift project authors
+//
+
+private import _TestingInternals
+
+extension Event {
+  /// Post this event to the currently-installed fallback event handler.
+  ///
+  /// - Parameters:
+  ///   - context: The context associated with this event.
+  ///
+  /// - Returns: Whether or not the fallback event handler was invoked. If the
+  ///   currently-installed handler belongs to the testing library, returns
+  ///   `false`.
+  borrowing func postToFallbackHandler(in context: borrowing Context) -> Bool {
+    #if canImport(_TestingInterop)
+      guard let fallbackEventHandler = _swift_testing_getFallbackEventHandler() else {
+        return false
+      }
+
+      // Encode the event as JSON and pass it to the handler.
+      let encodeAndInvoke = ABI.CurrentVersion.eventHandler(encodeAsJSONLines: false) {
+        recordJSON in
+        fallbackEventHandler(
+          String(describing: ABI.CurrentVersion.versionNumber),
+          recordJSON.baseAddress!,
+          recordJSON.count,
+          nil
+        )
+      }
+      encodeAndInvoke(self, context)
+      return true
+    #else
+      return false
+    #endif
+  }
+}

Sources/_TestingInternals/include/Stubs.h

@@ -180,6 +180,17 @@ static int swt_setfdflags(int fd, int flags) {
 }
 #endif
 
+#if !SWT_NO_INTEROP
+
+typedef void (*FallbackEventHandler)(const char *recordJSONSchemaVersionNumber,
+                                     const void *recordJSONBaseAddress,
+                                     long recordJSONByteCount,
+                                     const void *_Nullable reserved);
+
+FallbackEventHandler _Nullable _swift_testing_getFallbackEventHandler();
+
+#endif
+
 SWT_ASSUME_NONNULL_END
 
 #endif