Remove the use of dispatch_once that is heap backed.

Apple recently updated the docs on dispatch_once to point out
that the storage for the dispatch_once_t must be static or global,
but not something that was ever used before as the implementation
doesn't use a memory barrier.  So we drop the use and create the
semaphore when needed and use an atomic swap deal with any
threading races.

1 files changed, 19 insertions, 0 deletions

diff --git a/objectivec/GPBMessage.m b/objectivec/GPBMessage.m
index 58a10fdb..78935b19 100644
--- a/objectivec/GPBMessage.m
+++ b/objectivec/GPBMessage.m
@@ -738,6 +738,25 @@ void GPBClearMessageAutocreator(GPBMessage *self) {
   self->autocreatorExtension_ = nil;
 }
 
+// Call this before using the readOnlySemaphore_. This ensures it is created only once.
+void GPBPrepareReadOnlySemaphore(GPBMessage *self) {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdirect-ivar-access"
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+
+  // Create the semaphore on demand (rather than init) as developers might not cause them
+  // to be needed, and the heap usage can add up.  The atomic swap is used to avoid needing
+  // another lock around creating it.
+  if (self->readOnlySemaphore_ == nil) {
+    dispatch_semaphore_t worker = dispatch_semaphore_create(1);
+    if (!OSAtomicCompareAndSwapPtrBarrier(NULL, worker, (void * volatile *)&(self->readOnlySemaphore_))) {
+      dispatch_release(worker);
+    }
+  }
+
+#pragma clang diagnostic pop
+}
+
 static GPBUnknownFieldSet *GetOrMakeUnknownFields(GPBMessage *self) {
   if (!self->unknownFields_) {
     self->unknownFields_ = [[GPBUnknownFieldSet alloc] init];