Address review comments.

Author: bzbarsky-apple

src/darwin/Framework/CHIP/MTRUnfairLock.h

@@ -20,27 +20,18 @@
 
 #import <os/lock.h>
 
-class MTRAutoUnfairLock;
+#include <mutex>
 
-class MTRUnfairLock
+template <>
+class std::lock_guard<os_unfair_lock>
 {
 public:
-    MTRUnfairLock() { mOSLock = OS_UNFAIR_LOCK_INIT; }
+    explicit lock_guard(os_unfair_lock & lock) : mLock(lock) { os_unfair_lock_lock(&mLock); }
+    ~lock_guard() { os_unfair_lock_unlock(&mLock); }
 
-    void AssertOwner() { os_unfair_lock_assert_owner(&mOSLock); }
+    lock_guard(const lock_guard &)     = delete;
+    void operator=(const lock_guard &) = delete;
 
 private:
-    friend class MTRAutoUnfairLock;
-    os_unfair_lock mOSLock;
-};
-
-class MTRAutoUnfairLock
-{
-public:
-    MTRAutoUnfairLock(MTRUnfairLock & aLock) : mLock(aLock) { os_unfair_lock_lock(&mLock.mOSLock); }
-
-    ~MTRAutoUnfairLock() { os_unfair_lock_unlock(&mLock.mOSLock); }
-
-private:
-    MTRUnfairLock & mLock;
+    os_unfair_lock & mLock;
 };

src/darwin/Framework/CHIP/ServerEndpoint/MTRServerAttribute.mm

@@ -34,11 +34,11 @@
 
 MTR_DIRECT_MEMBERS
 @implementation MTRServerAttribute {
-    // _lock always protects access to _deviceController, _value, and _parentCluster.
-    // _serializedValue is protected when we are modifying it directly while we
-    // have no _deviceController.  Once we have one, _serializedValue is only
-    // modified on the Matter thread.
-    MTRUnfairLock _lock;
+    // _lock always protects access to _deviceController, _value, and
+    // _parentCluster.  _serializedValue is protected when we are modifying it
+    // directly while we have no _deviceController.  Once we have one,
+    // _serializedValue is only modified on the Matter thread.
+    os_unfair_lock _lock;
     MTRDeviceController * __weak _deviceController;
     NSDictionary<NSString *, id> * _value;
     app::ConcreteClusterPath _parentCluster;
@@ -75,6 +75,7 @@ - (nullable instancetype)initWithAttributeID:(NSNumber *)attributeID value:(NSDi
         return nil;
     }
 
+    _lock = OS_UNFAIR_LOCK_INIT;
     _attributeID = attributeID;
     _requiredReadPrivilege = requiredReadPrivilege;
     _writable = writable;
@@ -127,7 +128,7 @@ - (BOOL)setValue:(NSDictionary<NSString *, id> *)value
 
     // We serialized properly, so should be good to go on the value.  Lock
     // around our ivar accesses.
-    MTRAutoUnfairLock lock(_lock);
+    std::lock_guard lock(_lock);
 
     _value = [value copy];
 
@@ -151,13 +152,13 @@ - (BOOL)setValue:(NSDictionary<NSString *, id> *)value
 
 - (NSDictionary<NSString *, id> *)value
 {
-    MTRAutoUnfairLock lock(_lock);
+    std::lock_guard lock(_lock);
     return [_value copy];
 }
 
 - (BOOL)associateWithController:(nullable MTRDeviceController *)controller
 {
-    MTRAutoUnfairLock lock(_lock);
+    std::lock_guard lock(_lock);
 
     MTRDeviceController * existingController = _deviceController;
     if (existingController != nil) {
@@ -177,14 +178,14 @@ - (BOOL)associateWithController:(nullable MTRDeviceController *)controller
 
 - (void)invalidate
 {
-    MTRAutoUnfairLock lock(_lock);
+    std::lock_guard lock(_lock);
 
     _deviceController = nil;
 }
 
 - (BOOL)addToCluster:(const app::ConcreteClusterPath &)cluster
 {
-    MTRAutoUnfairLock lock(_lock);
+    std::lock_guard lock(_lock);
 
     if (_parentCluster.mClusterId != kInvalidClusterId) {
         MTR_LOG_ERROR("Cannot add attribute to cluster " ChipLogFormatMEI "; already added to cluster " ChipLogFormatMEI, ChipLogValueMEI(cluster.mClusterId), ChipLogValueMEI(_parentCluster.mClusterId));
@@ -197,13 +198,13 @@ - (BOOL)addToCluster:(const app::ConcreteClusterPath &)cluster
 
 - (void)updateParentCluster:(const app::ConcreteClusterPath &)cluster
 {
-    MTRAutoUnfairLock lock(_lock);
+    std::lock_guard lock(_lock);
     _parentCluster = cluster;
 }
 
 - (const chip::app::ConcreteClusterPath &)parentCluster
 {
-    MTRAutoUnfairLock lock(_lock);
+    std::lock_guard lock(_lock);
     return _parentCluster;
 }