Runtime: Add precondition check to swift_updateClassMetadata()

This initialization pattern can only be used if there is a backward deployment
layout (IRGen calls this ClassMetadataStrategy::FixedOrUpdate) or if we are
running on a newer Objective-C runtime that supports class metadata update
hooks (IRGen calls this ClassMetadataStrategy::Update).

If neither condition holds, we must trap here to avoid undefined behavior.

Author: slavapestov

stdlib/public/runtime/Metadata.cpp

@@ -2749,6 +2749,14 @@ _swift_updateClassMetadataImpl(ClassMetadata *self,
   // If we're running on a older Objective-C runtime, just realize
   // the class.
   if (!requiresUpdate) {
+    // If we don't have a backward deployment layout, we cannot proceed here.
+    if (self->getInstanceSize() == 0 ||
+        self->getInstanceAlignMask() == 0) {
+      fatalError(0, "class %s does not have a fragile layout; "
+                 "the deployment target was newer than this OS\n",
+                 self->getDescription()->Name.get());
+    }
+
     // Realize the class. This causes the runtime to slide the field offsets
     // stored in the field offset globals.
     //

validation-test/Runtime/old_runtime_crash_without_fixed_layout.swift

@@ -0,0 +1,50 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-build-swift-dylib(%t/%target-library-name(resilient_struct)) -Xfrontend -enable-resilience %S/../../test/Inputs/resilient_struct.swift -emit-module -emit-module-path %t/resilient_struct.swiftmodule -module-name resilient_struct
+// RUN: %target-codesign %t/%target-library-name(resilient_struct)
+
+// RUN: %target-build-swift %s -L %t -I %t -lresilient_struct -o %t/main %target-rpath(%t) -target x86_64-apple-macosx10.14.4
+// RUN: %target-codesign %t/main
+
+// RUN: %target-run %t/main %t/%target-library-name(resilient_struct) %t/libresilient_class%{target-shared-library-suffix}
+
+
+// REQUIRES: executable_test
+// REQUIRES: objc_interop
+// REQUIRES: OS=macosx
+// REQUIRES: CPU=x86_64
+
+// Test that on an old Objective-C runtime we crash if attempting to use a
+// class that requires the update callback and does not have a fragile layout.
+
+import StdlibUnittest
+import resilient_struct
+
+var ClassTestSuite = TestSuite("Update callback")
+
+public class ClassWithResilientField {
+  var x = ResilientInt(i: 0)
+}
+
+@_optimize(none) func blackHole<T>(_: T) {}
+
+@_optimize(none) func forceMetadata() {
+  blackHole(ClassWithResilientField.self)
+}
+
+if #available(macOS 10.14.4, iOS 12.2, tvOS 12.2, watchOS 5.2, *) {
+  ClassTestSuite.test("RealizeResilientClass") {  
+    print("Nothing to test. We have the new Objective-C runtime.")
+    forceMetadata()
+  }
+} else {
+  ClassTestSuite.test("RealizeResilientClass")
+    .crashOutputMatches("class ClassWithResilientField does not have a fragile layout")
+    .code {
+      expectCrashLater()
+      print("About to crash...")
+      forceMetadata()
+    }
+}
+
+runAllTests()