Runtime: Bridging an Error to NSError requires only checking its domain.

The lazy population of the NSError fields is ordered such that the domain is written into the object last with acq/rel ordering, so another thread only needs to check the domain to see whether the initialization has already happened. (The initialization itself is idempotent, so we can optimistically perform the initialization and discard the results if we race.) Checking the user info is redundant, and is also wrong for real NSError objects, since [NSError errorWithDomain:d code:c userInfo:nil] will in fact plant nil in the userInfo field of the object, leading us to attempt to bridge an already-native NSError.

Author: jckarter

stdlib/public/runtime/ErrorObject.mm

@@ -424,14 +424,22 @@ typedef SWIFT_CC(swift)
 static id _swift_bridgeErrorToNSError_(SwiftError *errorObject) {
   auto ns = reinterpret_cast<NSError *>(errorObject);
 
-  // If we already have a domain and userInfo set, then we've already
-  // initialized.
-  // FIXME: This might be overly strict; can we look only at the domain?
-  if (errorObject->domain.load(std::memory_order_acquire) &&
-      errorObject->userInfo.load(std::memory_order_acquire))
+  // If we already have a domain set, then we've already initialized.
+  // If this is a real NSError, then Cocoa and Core Foundation's initializers
+  // guarantee that the domain is never nil, so if this test fails, we can
+  // assume we're working with a bridged error. (Note that Cocoa and CF
+  // **will** allow the userInfo storage to be initialized to nil.)
+  //
+  // If this is a bridged error, then the domain, code, and user info are
+  // lazily computed, and the domain will be nil if they haven't been computed
+  // yet. The initialization is ordered in such a way that all other lazy
+  // initialization of the object happens-before the domain initialization so
+  // that the domain can be used alone as a flag for the initialization of the
+  // object.
+  if (errorObject->domain.load(std::memory_order_acquire))
     return ns;
 
-  // Otherwise, calculate the domain and code (TODO: and user info), and
+  // Otherwise, calculate the domain, code, and user info, and
   // initialize the NSError.
   auto value = SwiftError::getIndirectValue(&errorObject);
   auto type = errorObject->getType();
@@ -463,8 +471,8 @@ static id _swift_bridgeErrorToNSError_(SwiftError *errorObject) {
   // We also need to cmpxchg in the domain; if somebody beat us to it,
   // we need to release.
   //
-  // Storing the domain must be the LAST THING we do, since it's
-  // the signal that the NSError has been initialized.
+  // Storing the domain must be the **LAST THING** we do, since it's
+  // also the flag that the NSError has been initialized.
   CFStringRef expectedDomain = nullptr;
   if (!errorObject->domain.compare_exchange_strong(expectedDomain,
                                                    (CFStringRef)domain,