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

Package.swift

@@ -142,9 +142,20 @@ let package = Package(
       exclude: ["CMakeLists.txt", "Testing.swiftcrossimport"],
       cxxSettings: .packageSettings,
       swiftSettings: .packageSettings + .enableLibraryEvolution(),
-      linkerSettings: [
-        .linkedLibrary("execinfo", .when(platforms: [.custom("freebsd"), .openbsd]))
-      ]
+      linkerSettings: {
+        var result = [LinkerSetting]()
+        result += [
+          .linkedLibrary("execinfo", .when(platforms: [.custom("freebsd"), .openbsd]))
+        ]
+#if compiler(>=6.3)
+        result += [
+          .linkedFramework("_TestingInterop", .whenApple()),
+          .linkedLibrary("_TestingInterop", .whenApple(false)),
+        ]
+#endif
+
+        return result
+      }()
     ),
     .testTarget(
       name: "TestingTests",

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 +1,48 @@
+//
+// 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 {
+  private static let _fallbackEventHandler: FallbackEventHandler? = {
+      _swift_testing_getFallbackEventHandler()
+  }()
+
+  /// 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 = Self._fallbackEventHandler 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/Testing/Events/Event.swift

@@ -326,6 +326,8 @@ extension Event {
       if configuration.eventHandlingOptions.shouldHandleEvent(self) {
         configuration.handleEvent(self, in: context)
       }
+    } else if postToFallbackHandler(in: context) {
+      // The fallback event handler handled this event.
     } else {
       // The current task does NOT have an associated configuration. This event
       // will be lost! Post it to every registered event handler to avoid that.

Sources/_TestingInternals/include/Stubs.h

@@ -180,6 +180,32 @@ static int swt_setfdflags(int fd, int flags) {
 }
 #endif
 
+// __SWIFT_COMPILER_VERSION is a packed value. The comparison below is >= 6.3
+#if __SWIFT_COMPILER_VERSION >= 6003000000000 && !SWT_NO_INTEROP
+
+/// A type describing a fallback event handler that testing API can invoke as an
+/// alternate method of reporting test events to the current test runner.
+/// Shadows the type with the same name in _TestingInterop.
+///
+/// - Parameters:
+///   - recordJSONSchemaVersionNumber: The JSON schema version used to encode
+///     the event record.
+///   - recordJSONBaseAddress: A pointer to the first byte of the encoded event.
+///   - recordJSONByteCount: The size of the encoded event in bytes.
+///   - reserved: Reserved for future use.
+typedef void (* FallbackEventHandler)(const char *recordJSONSchemaVersionNumber,
+                                      const void *recordJSONBaseAddress,
+                                      long recordJSONByteCount,
+                                      const void *_Nullable reserved);
+
+/// Get the current fallback event handler.
+/// Shadows the function with the same name in _TestingInterop.
+///
+/// - Returns: The currently-set handler function, if any.
+SWT_EXTERN FallbackEventHandler _Nullable _swift_testing_getFallbackEventHandler(void);
+
+#endif
+
 SWT_ASSUME_NONNULL_END
 
 #endif

Sources/_TestingInterop/FallbackEventHandler.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-#if !SWT_NO_INTEROP
+#if compiler(>=6.3) && !SWT_NO_INTEROP
 #if SWT_TARGET_OS_APPLE && !SWT_NO_OS_UNFAIR_LOCK && !hasFeature(Embedded)
 private import _TestingInternals
 #else