1 files changed, 18 insertions, 1 deletions
diff --git a/objectivec/GPBMessage_PackagePrivate.h b/objectivec/GPBMessage_PackagePrivate.h
index d4c3501a..478db2cf 100644
--- a/objectivec/GPBMessage_PackagePrivate.h
+++ b/objectivec/GPBMessage_PackagePrivate.h
@@ -62,7 +62,16 @@ typedef struct GPBMessage_Storage *GPBMessage_StoragePtr;
   // 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.
-  OSSpinLock readOnlyMutex_;
+  // 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.
+  dispatch_once_t readOnlySemaphoreCreationOnce_;
+  dispatch_semaphore_t readOnlySemaphore_;
 }
 
 // Gets an extension value without autocreating the result if not found. (i.e.
@@ -98,6 +107,14 @@ typedef struct GPBMessage_Storage *GPBMessage_StoragePtr;
 
 CF_EXTERN_C_BEGIN
 
+
+// Call this before using the readOnlySemaphore_. This ensures it is created only once.
+NS_INLINE void GPBPrepareReadOnlySemaphore(GPBMessage *self) {
+  dispatch_once(&self->readOnlySemaphoreCreationOnce_, ^{
+    self->readOnlySemaphore_ = dispatch_semaphore_create(1);
+  });
+}
+
 // Returns a new instance that was automatically created by |autocreator| for
 // its field |field|.
 GPBMessage *GPBCreateMessageWithAutocreator(Class msgClass,