[Runtime] Fix race condition in protocol conformance lookups that caused false negatives.

In the uncached case, we'd scan conformances, cache them, then re-query the cache. This worked fine when the cache always grew, but now we clear the cache when loading new Swift images into the process. If that happens between the scan and the re-query, we lose the entry and return a false negative.

Instead, track what we've found in the scan in a separate local table, then query that after completing the scan.

While we're in there, fix a bug in TypeLookupError where operator= accidentally copied this->Context instead of other.Context. This caused the runtime to crash when trying to print error messages due to the false negative.

Add a no-parameter constructor to TypeLookupErrorOr<> to distinguish the case where it's being initialized with nothing from the case where it's being initialized with nullptr.

Author: mikeash

include/swift/Demangling/TypeLookupError.h

@@ -110,7 +110,7 @@ class TypeLookupError {
 
     Fn(Context, Command::DestroyContext, nullptr);
     Fn = other.Fn;
-    Context = Fn(Context, Command::CopyContext, nullptr);
+    Context = Fn(other.Context, Command::CopyContext, nullptr);
 
     return *this;
   }
@@ -169,6 +169,8 @@ template <typename T> class TypeLookupErrorOr {
   TaggedUnion<T, TypeLookupError> Value;
 
 public:
+  TypeLookupErrorOr() : Value(TypeLookupError("freshly constructed error")) {}
+
   TypeLookupErrorOr(const T &t) : Value(t) {
     if (!t)
       Value = TypeLookupError("unknown error");

stdlib/public/runtime/Metadata.cpp

@@ -5006,7 +5006,7 @@ swift_getAssociatedTypeWitnessSlowImpl(
     Demangle::makeSymbolicMangledNameStringRef(mangledNameBase);
 
   // Demangle the associated type.
-  TypeLookupErrorOr<TypeInfo> result = TypeInfo();
+  TypeLookupErrorOr<TypeInfo> result;
   if (inProtocolContext) {
     // The protocol's Self is the only generic parameter that can occur in the
     // type.

stdlib/public/runtime/ProtocolConformance.cpp

@@ -22,6 +22,7 @@
 #include "swift/Runtime/HeapObject.h"
 #include "swift/Runtime/Metadata.h"
 #include "swift/Basic/Unreachable.h"
+#include "llvm/ADT/DenseMap.h"
 #include "CompatibilityOverride.h"
 #include "ImageInspection.h"
 #include "Private.h"
@@ -470,6 +471,7 @@ swift_conformsToProtocolImpl(const Metadata *const type,
     return found.second;
 
   // Scan conformance records.
+  llvm::SmallDenseMap<const Metadata *, const WitnessTable *> foundWitnesses;
   auto processSection = [&](const ConformanceSection &section) {
     // Eagerly pull records for nondependent witnesses into our cache.
     auto processDescriptor = [&](const ProtocolConformanceDescriptor &descriptor) {
@@ -484,6 +486,7 @@ swift_conformsToProtocolImpl(const Metadata *const type,
       if (auto *matchingType = candidate.getMatchingType(type)) {
         auto witness = descriptor.getWitnessTable(matchingType);
         C.cacheResult(matchingType, protocol, witness, /*always cache*/ 0);
+        foundWitnesses.insert({matchingType, witness});
       }
     };
 
@@ -505,14 +508,23 @@ swift_conformsToProtocolImpl(const Metadata *const type,
       processSection(section);
   }
 
-  // Try the search again to look for the most specific cached conformance.
-  found = searchInConformanceCache(type, protocol);
+  // Find the most specific conformance that was scanned.
+  const WitnessTable *foundWitness = nullptr;
+  const Metadata *searchType = type;
+  while (!foundWitness && searchType) {
+    foundWitness = foundWitnesses.lookup(searchType);
 
-  // If it's not authoritative, then add an authoritative entry for this type.
-  if (!found.first)
-    C.cacheResult(type, protocol, found.second, snapshot.count());
+    // If there's no entry here, move up to the superclass (if any).
+    if (!foundWitness)
+      searchType = _swift_class_getSuperclass(searchType);
+  }
+
+  // If it's for a superclass or if we didn't find anything, then add an
+  // authoritative entry for this type.
+  if (searchType != type)
+    C.cacheResult(type, protocol, foundWitness, snapshot.count());
 
-  return found.second;
+  return foundWitness;
 }
 
 const ContextDescriptor *