commit d6590d653415c0bfacf97e7f768dd3c994cb8d26
author Thomas Van Lenten <thomasvl@google.com> Thu Dec 17 14:35:44 2015 -0500
committer Thomas Van Lenten <thomasvl@google.com> Thu Dec 17 16:05:50 2015 -0500
tree 8c0f6f98eb867d520e95d30d32a6d0e4fe55906c
parent afbc89a263d57bb2e36ce5f07b979c739bb2e8cd

Drop all use of OSSpinLock

Apple engineers have pointed out that OSSpinLocks 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 a dispatch_semaphore_t within the extension registry.
- Use a dispatch_semaphore_t for protecting autocreation within messages.
- Drop the custom/internal GPBString class since we don't have really good
  numbers to judge the locking replacements and it isn't required. We can
  always bring it back with real data in the future.

objectivec/GPBMessage.m

diff --git a/objectivec/GPBMessage.m b/objectivec/GPBMessage.m
index d9080c3..208cfe4 100644
--- a/objectivec/GPBMessage.m
+++ b/objectivec/GPBMessage.m
@@ -568,13 +568,13 @@
   id array = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
   if (!array) {
     // Check again after getting the lock.
-    OSSpinLockLock(&self->readOnlyMutex_);
+    dispatch_semaphore_wait(self->readOnlySemaphore_, DISPATCH_TIME_FOREVER);
     array = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
     if (!array) {
       array = CreateArrayForField(field, self);
       GPBSetAutocreatedRetainedObjectIvarWithField(self, field, array);
     }
-    OSSpinLockUnlock(&self->readOnlyMutex_);
+    dispatch_semaphore_signal(self->readOnlySemaphore_);
   }
   return array;
 }
@@ -598,13 +598,13 @@
   id dict = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
   if (!dict) {
     // Check again after getting the lock.
-    OSSpinLockLock(&self->readOnlyMutex_);
+    dispatch_semaphore_wait(self->readOnlySemaphore_, DISPATCH_TIME_FOREVER);
     dict = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
     if (!dict) {
       dict = CreateMapForField(field, self);
       GPBSetAutocreatedRetainedObjectIvarWithField(self, field, dict);
     }
-    OSSpinLockUnlock(&self->readOnlyMutex_);
+    dispatch_semaphore_signal(self->readOnlySemaphore_);
   }
   return dict;
 }
@@ -810,7 +810,7 @@
     messageStorage_ = (GPBMessage_StoragePtr)(
         ((uint8_t *)self) + class_getInstanceSize([self class]));
 
-    readOnlyMutex_ = OS_SPINLOCK_INIT;
+    readOnlySemaphore_ = dispatch_semaphore_create(1);
   }
 
   return self;
@@ -1723,7 +1723,7 @@
   }
 
   // Check for an autocreated value.
-  OSSpinLockLock(&readOnlyMutex_);
+  dispatch_semaphore_wait(readOnlySemaphore_, DISPATCH_TIME_FOREVER);
   value = [autocreatedExtensionMap_ objectForKey:extension];
   if (!value) {
     // Auto create the message extensions to match normal fields.
@@ -1740,7 +1740,7 @@
     [value release];
   }
 
-  OSSpinLockUnlock(&readOnlyMutex_);
+  dispatch_semaphore_signal(readOnlySemaphore_);
   return value;
 }