[Runtime] Disable prespecialized metadata if we have overridden images.

We need to check for overridden images on every image load, otherwise
XCTest (among others) may `dlopen()` an image that pulls in something
that is overridden, at which point the prespecialized metadata won't
match the image we loaded.

rdar://125727356

Author: al45tair

include/swift/Runtime/LibPrespecialized.h

@@ -48,6 +48,7 @@ struct LibPrespecializedData {
 const LibPrespecializedData<InProcess> *getLibPrespecializedData();
 Metadata *getLibPrespecializedMetadata(const TypeContextDescriptor *description,
                                        const void *const *arguments);
+void libPrespecializedImageLoaded();
 
 } // namespace swift
 

stdlib/public/runtime/LibPrespecialized.cpp

@@ -16,6 +16,8 @@
 #include "swift/Runtime/EnvironmentVariables.h"
 #include "swift/Runtime/Metadata.h"
 
+#include <atomic>
+
 #if SWIFT_STDLIB_HAS_DLADDR && __has_include(<dlfcn.h>)
 #include <dlfcn.h>
 #define USE_DLOPEN 1
@@ -27,6 +29,8 @@
 
 using namespace swift;
 
+static std::atomic<bool> disablePrespecializedMetadata = false;
+
 static const LibPrespecializedData<InProcess> *findLibPrespecialized() {
   if (!runtime::environment::SWIFT_DEBUG_ENABLE_LIB_PRESPECIALIZED())
     return nullptr;
@@ -91,10 +95,23 @@ static bool hasNonTypeGenericArguments(const TypeContextDescriptor *description)
 
 static bool disableForValidation = false;
 
+void
+swift::libPrespecializedImageLoaded() {
+  #if DYLD_GET_SWIFT_PRESPECIALIZED_DATA_DEFINED
+  // A newly loaded image might have caused us to load images that are
+  // overriding images in the shared cache.  If we do that, turn off
+  // prespecialized metadata.
+  if (dyld_shared_cache_some_image_overridden())
+    disablePrespecializedMetadata.store(true, std::memory_order_release);
+  #endif
+}
+
 Metadata *
 swift::getLibPrespecializedMetadata(const TypeContextDescriptor *description,
                                     const void *const *arguments) {
-  if (disableForValidation)
+  if (SWIFT_UNLIKELY(
+        disableForValidation
+        || disablePrespecializedMetadata.load(std::memory_order_acquire)))
     return nullptr;
 
   auto *data = getLibPrespecializedData();

stdlib/public/runtime/MetadataLookup.cpp

@@ -29,6 +29,7 @@
 #include "swift/Runtime/Debug.h"
 #include "swift/Runtime/EnvironmentVariables.h"
 #include "swift/Runtime/HeapObject.h"
+#include "swift/Runtime/LibPrespecialized.h"
 #include "swift/Runtime/Metadata.h"
 #include "swift/Strings.h"
 #include "swift/Threading/Mutex.h"
@@ -347,6 +348,8 @@ void swift::addImageTypeMetadataRecordBlockCallbackUnsafe(
   assert(recordsSize % sizeof(TypeMetadataRecord) == 0
          && "weird-sized type metadata section?!");
 
+  libPrespecializedImageLoaded();
+
   // If we have a section, enqueue the type metadata for lookup.
   auto recordBytes = reinterpret_cast<const char *>(records);
   auto recordsBegin