From 17315930f54cad7ae50e559222d93d5d8c0bb015 Mon Sep 17 00:00:00 2001
From: Boris Zbarsky <bzbarsky@apple.com>
Date: Thu, 25 May 2023 22:30:01 -0400
Subject: [PATCH] Fix ordering issue with subscribeToAttributesWithEndpointID
 over XPC. (#26850)

We had an extra queue hop for reports compared with the subscriptionEstablished
notification, so the API consumer could get subscriptionEstablished before
getting all the priming reports.

The fix is to just ensure that the code flows the same way, in terms of queue
dispatch, for both priming reports and subscriptionEstablished.
---
 .../CHIP/MTRDeviceControllerXPCConnection.h    |  3 ++-
 .../CHIP/MTRDeviceControllerXPCConnection.mm   |  7 +++++++
 src/darwin/Framework/CHIP/MTRDeviceOverXPC.mm  | 18 ++++++++++--------
 3 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/src/darwin/Framework/CHIP/MTRDeviceControllerXPCConnection.h b/src/darwin/Framework/CHIP/MTRDeviceControllerXPCConnection.h
index 3c6a82c1235ec0..8660a4fbb07a10 100644
--- a/src/darwin/Framework/CHIP/MTRDeviceControllerXPCConnection.h
+++ b/src/darwin/Framework/CHIP/MTRDeviceControllerXPCConnection.h
@@ -43,7 +43,7 @@ typedef void (^MTRGetProxyHandleHandler)(dispatch_queue_t queue, MTRDeviceContro
 @interface MTRDeviceControllerXPCConnection<MTRDeviceControllerClientProtocol> : NSObject
 
 /**
- * This method is just for test purpsoe.
+ * This method is just for test purpose.
  */
 + (MTRDeviceControllerXPCConnection *)connectionWithWorkQueue:(dispatch_queue_t)workQueue
                                                  connectBlock:(MTRXPCConnectBlock)connectBlock;
@@ -55,6 +55,7 @@ typedef void (^MTRGetProxyHandleHandler)(dispatch_queue_t queue, MTRDeviceContro
 - (void)deregisterReportHandlersWithController:(id<NSCopying>)controller
                                         nodeID:(NSNumber *)nodeID
                                     completion:(dispatch_block_t)completion;
+- (void)callSubscriptionEstablishedHandler:(dispatch_block_t)handler;
 
 @end
 
diff --git a/src/darwin/Framework/CHIP/MTRDeviceControllerXPCConnection.mm b/src/darwin/Framework/CHIP/MTRDeviceControllerXPCConnection.mm
index 9620f4ba757339..e44308244cc00f 100644
--- a/src/darwin/Framework/CHIP/MTRDeviceControllerXPCConnection.mm
+++ b/src/darwin/Framework/CHIP/MTRDeviceControllerXPCConnection.mm
@@ -203,4 +203,11 @@ - (void)handleReportWithController:(id)controller
     });
 }
 
+- (void)callSubscriptionEstablishedHandler:(dispatch_block_t)handler
+{
+    // Call the handler from our _workQueue, so that we guarantee the same
+    // number of queue hops as for handleReportWithController.
+    dispatch_async(_workQueue, handler);
+}
+
 @end
diff --git a/src/darwin/Framework/CHIP/MTRDeviceOverXPC.mm b/src/darwin/Framework/CHIP/MTRDeviceOverXPC.mm
index 17cd039278d294..cdc2dacb20c37c 100644
--- a/src/darwin/Framework/CHIP/MTRDeviceOverXPC.mm
+++ b/src/darwin/Framework/CHIP/MTRDeviceOverXPC.mm
@@ -272,14 +272,16 @@ - (void)subscribeToAttributesWithEndpointID:(NSNumber * _Nullable)endpointID
                                            maxInterval:params.maxInterval
                                                 params:[MTRDeviceController encodeXPCSubscribeParams:params]
                                     establishedHandler:^{
-                                        dispatch_async(queue, ^{
-                                            MTR_LOG_DEBUG("Subscription established");
-                                            subscriptionEstablishedHandler();
-                                            // The following captures the proxy handle in the closure so that the handle
-                                            // won't be released prior to block call.
-                                            __auto_type handleRetainer = handle;
-                                            (void) handleRetainer;
-                                        });
+                                        [self.xpcConnection callSubscriptionEstablishedHandler:^{
+                                            dispatch_async(queue, ^{
+                                                MTR_LOG_DEBUG("Subscription established");
+                                                subscriptionEstablishedHandler();
+                                                // The following captures the proxy handle in the closure so that the handle
+                                                // won't be released prior to block call.
+                                                __auto_type handleRetainer = handle;
+                                                (void) handleRetainer;
+                                            });
+                                        }];
                                     }];
     };