Address review comment.

src/protocols/secure_channel/CASESession.cpp

@@ -243,21 +243,32 @@ class CASESession::WorkHelper
         // Execute callback in background thread; data must be OK with this
         helper->mStatus = helper->mWorkCallback(helper->mData, cancel);
         VerifyOrReturn(!cancel && !helper->IsCancelled());
-        // Hold strong ptr while work is outstanding
+        // Hold strong ptr to ourselves while work is outstanding
         helper->mStrongPtr.swap(strongPtr);
         auto status = DeviceLayer::PlatformMgr().ScheduleWork(AfterWorkHandler, reinterpret_cast<intptr_t>(helper));
         if (status != CHIP_NO_ERROR)
         {
+            ChipLogError(SecureChannel, "Failed to Schedule the AfterWorkCallback on foreground thread: %" CHIP_ERROR_FORMAT,
+                         status.Format());
+
             // We failed to schedule after work callback, so setting mScheduleAfterWorkFailed flag to true
             // This can be checked from foreground thread and after work callback can be retried
             helper->mStatus = status;
-            ChipLogError(SecureChannel, "Failed to Schedule the AfterWorkCallback on foreground thread");
-            helper->mScheduleAfterWorkFailed.store(true);
 
-            // Release strong ptr since scheduling failed, because nothing guarantees that
-            // AfterWorkHandler will get called at this point to release the reference, and we
-            // don't want to leak.
-            helper->mStrongPtr.reset();
+            // Release strong ptr to self since scheduling failed, because nothing guarantees
+            // that AfterWorkHandler will get called at this point to release the reference,
+            // and we don't want to leak.  That said, we want to ensure that "helper" stays
+            // alive through the end of this function (so we can set mScheduleAfterWorkFailed
+            // on it), but also want to avoid racing on the single SharedPtr instance in
+            // helper->mStrongPtr.  That means we need to not touch helper->mStrongPtr after
+            // writing to mScheduleAfterWorkFailed.
+            //
+            // The simplest way to do this is to move the reference in helper->mStrongPtr to
+            // our stack, where it outlives all our accesses to "helper".
+            strongPtr.swap(helper->mStrongPtr);
+
+            // helper and any of its state should not be touched after storing mScheduleAfterWorkFailed.
+            helper->mScheduleAfterWorkFailed.store(true);
         }
     }
 
@@ -273,8 +284,10 @@ class CASESession::WorkHelper
         auto strongPtr(std::move(helper->mStrongPtr));
         if (!strongPtr)
         {
-            // This can happen if scheduling AfterWorkHandler failed.  Just grab a strong ref to
-            // handler directly, to fulfill our API contract.
+            // This can happen if scheduling AfterWorkHandler failed.  Just grab a strong ref
+            // to handler directly, to fulfill our API contract of holding a strong reference
+            // across the after-work callback.  At this point, we are guaranteed that the
+            // background thread is not touching the helper anymore.
             strongPtr = helper->mWeakPtr.lock();
         }
         if (auto * session = helper->mSession.load())
@@ -305,6 +318,9 @@ class CASESession::WorkHelper
 
     // If background thread fails to schedule AfterWorkCallback then this flag is set to true
     // and CASEServer then can check this one and run the AfterWorkCallback for us.
+    //
+    // When this happens, the write to this boolean _must_ be the last code that touches this
+    // object on the background thread.  After that, the Matter thread owns the object.
     std::atomic<bool> mScheduleAfterWorkFailed{ false };
 
 public: