Work around strange error with atomic and swift under Xcode 8.3.3.

Haven't been able to make a repo case, but this should "fix" the problem
by avoid it completely.

- Move readOnlySemaphore_ into the .m file so it isn't exposed in any
  header.
- Move GPBGetObjectIvarWithField() also to go with the new limited
  visibility on the readOnlySemaphore_.

1 files changed, 0 insertions, 14 deletions

diff --git a/objectivec/GPBMessage_PackagePrivate.h b/objectivec/GPBMessage_PackagePrivate.h
index 366e5bbf..ca10983b 100644
--- a/objectivec/GPBMessage_PackagePrivate.h
+++ b/objectivec/GPBMessage_PackagePrivate.h
@@ -61,20 +61,6 @@ typedef struct GPBMessage_Storage *GPBMessage_StoragePtr;
   // GPBMessage_Storage with _has_storage__ as the first field.
   // Kept public because static functions need to access it.
   GPBMessage_StoragePtr messageStorage_;
-
-  // A lock to provide mutual exclusion from internal data that can be modified
-  // by *read* operations such as getters (autocreation of message fields and
-  // message extensions, not setting of values). Used to guarantee thread safety
-  // for concurrent reads on the message.
-  // NOTE: OSSpinLock may seem like a good fit here but Apple engineers have
-  // pointed out that they are vulnerable to live locking on iOS in cases of
-  // priority inversion:
-  //   http://mjtsai.com/blog/2015/12/16/osspinlock-is-unsafe/
-  //   https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20151214/000372.html
-  // Use of readOnlySemaphore_ must be prefaced by a call to
-  // GPBPrepareReadOnlySemaphore to ensure it has been created. This allows
-  // readOnlySemaphore_ to be only created when actually needed.
-  _Atomic(dispatch_semaphore_t) readOnlySemaphore_;
 }
 
 // Gets an extension value without autocreating the result if not found. (i.e.