Only create the readonlySemaphore on demand.

This will lower the amount of dispatch_semaphores created per Message when the
full object tree isn't walked in a way that would require them to be created.
Uses a dispatch_once_t for one time init of the dispatch_semaphore.

1 files changed, 12 insertions, 0 deletions

diff --git a/objectivec/GPBMessage_PackagePrivate.h b/objectivec/GPBMessage_PackagePrivate.h
index b7e24fc9..478db2cf 100644
--- a/objectivec/GPBMessage_PackagePrivate.h
+++ b/objectivec/GPBMessage_PackagePrivate.h
@@ -67,6 +67,10 @@ typedef struct GPBMessage_Storage *GPBMessage_StoragePtr;
   // 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_;
 }
 
@@ -103,6 +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,