Priority inversion with CKSyncEngine

[Jason-Abbott]

Priority inversion has been well discussed here:

• Xcode 14 beta: Thread running at QOS_CLASS_USER_INTERACTIVE waiting on a lower QoS thread running at QOS_CLASS_DEFAULT. Investigate ways to avoid priority inversions #1234
• https://forums.swift.org/t/xcode-14-beta-thread-performance-checker/57883

I am hoping for a quick sanity check before applying the advice there. The background is:

The thread checker is flagging an inversion at GRDB Pool.get() when CKSyncEngine.Event.sentRecordZoneChanges fires. If I log Task.currentPriority I see these events are basically all medium.

The CKSyncEngine delegate methods are synchronous so my GRDB calls within the event handlers are also synchronous. If I understand the previous discussion correctly, that is likely the source of the inversion.

Some of these events should be a lower priority but if I want to resolve the inversion, will I need to do something like wrapping the GRDB calls in Task {} so I can use the async methods?

[groue]

Hello @Jason-Abbott,

The thread checker is flagging an inversion at GRDB Pool.get() when CKSyncEngine.Event.sentRecordZoneChanges fires. If I log Task.currentPriority I see these events are basically all medium.

The quality of service of GRDB connections is controlled by Configuration (see Configuring the Quality of Service).

By default GRDB connections execute with the DispatchQoS.userInitiated quality of service. I don't quite know how the DispatchQoS cases are mapped to TaskPriority. I know even less how DispatchQueue quality of service at runtime informs Task.currentPriority. The priority inversion warnings that were previously discussed are Dispatch warnings, not Swift concurrency warnings.

• Please confirm your app does not override any of the quality of service settings in the Configuration of the DatabasePool.
• I wonder if Task.currentPriority is the correct probe.
• I would put a breakpoint on runtime issues (Xcode > Breakpoints Navigator > "+" > Runtime Issue Breakpoint), and inspect all threads and queues when the warning happens. See below:

The CKSyncEngine delegate methods are synchronous so my GRDB calls within the event handlers are also synchronous. If I understand the previous discussion correctly, that is likely the source of the inversion.

That would be my understanding as well. CKSyncEngine calls your app, which wants to perform a read and thus to acquire an available connection from Pool.get(). It has to wait due to some concurrent job. That concurrent job might be at a lower priority (hence the warning, in my current understanding).

That's why I'd put a breakpoint and look at all threads and queues (mandatory bt all in LLDB, optional screenshots). Something else is blocking the read, and we need to see what it is and why it has the low priority that is the reason for the runtime warning.

Some of these events should be a lower priority but if I want to resolve the inversion, will I need to do something like wrapping the GRDB calls in Task {} so I can use the async methods?

We probably need to figure out what's wrong, exactly, before modifying your code, or GRDB code.

[Jason-Abbott]

Hi @groue. Thank you for that clarification and those ideas. I’ve added the runtime issue breakpoints. I’ll comment again when I’ve gathered more actionable details.

[Jason-Abbott]

I captured the backtrace. There seem to be only a couple queries in play, to produce a CKRecord from a model within the CKSyncEngine delegate method nextRecordZoneChangeBatch. One reads the previously saved CKRecord data (if any) from a table and the other reads all the model column values.

Although these should be happening in the same await [DatabaseReader].read() call, it’s apparent I’m overlooking something.

I think I have enough information now to figure it out but in case something quickly catches your eye, here is that trace:

bt all

thread #1, queue = 'com.apple.main-thread' frame #0: 0x000000019b615570 AppKit`___nonOverridableViewHitTest_block_invoke + 80 frame #1: 0x0000000196e491d4 CoreFoundation`__NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ + 24 frame #2: 0x0000000196e490b0 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 436 frame #3: 0x000000019b6156ac AppKit`___nonOverridableViewHitTest_block_invoke + 396 frame #4: 0x0000000196e491d4 CoreFoundation`__NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ + 24 frame #5: 0x0000000196e490b0 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 436 frame #6: 0x000000019b6156ac AppKit`___nonOverridableViewHitTest_block_invoke + 396 frame #7: 0x0000000196e491d4 CoreFoundation`__NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ + 24 frame #8: 0x0000000196e490b0 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 436 frame #9: 0x000000019b6156ac AppKit`___nonOverridableViewHitTest_block_invoke + 396 frame #10: 0x0000000196e491d4 CoreFoundation`__NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ + 24 frame #11: 0x0000000196e490b0 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 436 frame #12: 0x000000019b6156ac AppKit`___nonOverridableViewHitTest_block_invoke + 396 frame #13: 0x0000000196e491d4 CoreFoundation`__NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ + 24 frame #14: 0x0000000196e490b0 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 436 frame #15: 0x000000019b6156ac AppKit`___nonOverridableViewHitTest_block_invoke + 396 frame #16: 0x0000000196e491d4 CoreFoundation`__NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ + 24 frame #17: 0x0000000196e490b0 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 436 frame #18: 0x000000019b6156ac AppKit`___nonOverridableViewHitTest_block_invoke + 396 frame #19: 0x0000000196e491d4 CoreFoundation`__NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ + 24 frame #20: 0x0000000196e490b0 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 436 frame #21: 0x000000019b6156ac AppKit`___nonOverridableViewHitTest_block_invoke + 396 frame #22: 0x0000000196e491d4 CoreFoundation`__NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ + 24 frame #23: 0x0000000196e490b0 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 436 frame #24: 0x000000019b6156ac AppKit`___nonOverridableViewHitTest_block_invoke + 396 frame #25: 0x0000000196e491d4 CoreFoundation`__NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ + 24 frame #26: 0x0000000196e490b0 CoreFoundation`-[__NSArrayM enumerateObjectsWithOptions:usingBlock:] + 436 frame #27: 0x000000019b60e3e8 AppKit`-[_NSTrackingAreaAKManager _updateActiveTrackingAreasForWindowLocation:modifierFlags:] + 624 frame #28: 0x000000019b610d0c AppKit`-[_NSTrackingAreaAKManager routeEnterExitEvent:] + 28 frame #29: 0x000000019aea1b70 AppKit`-[NSWindow(NSEventRouting) _reallySendEvent:isDelayedEvent:] + 152 frame #30: 0x000000019aea1910 AppKit`-[NSWindow(NSEventRouting) sendEvent:] + 288 frame #31: 0x000000019b6089f0 AppKit`+[_NSTrackingAreaAKManager routeEnterExitEvent:] + 80 frame #32: 0x000000019b5c8edc AppKit`+[_NSTrackingAreaManager routeEnterExitEvent:] + 244 frame #33: 0x000000019b718ec0 AppKit`-[NSApplication(NSEventRouting) sendEvent:] + 368 frame #34: 0x000000019b31842c AppKit`-[NSApplication _handleEvent:] + 60 frame #35: 0x000000019ad6ec8c AppKit`-[NSApplication run] + 520 frame #36: 0x000000019ad4535c AppKit`NSApplicationMain + 880 frame #37: 0x00000001c70a3878 SwiftUI`merged generic specialization of function signature specialization of SwiftUI.runApp(__C.NSResponder & __C.NSApplicationDelegate) -> Swift.Never + 160 frame #38: 0x00000001c7501658 SwiftUI`SwiftUI.runApp<τ_0_0 where τ_0_0: SwiftUI.App>(τ_0_0) -> Swift.Never + 108 frame #39: 0x00000001c78625c0 SwiftUI`static SwiftUI.App.main() -> () + 224 frame #40: 0x0000000103ed445c ElidedApp (debug).debug.dylib`static TriageApp.main() at ElidedApp.swift:89:82 frame #42: 0x0000000103ed4be0 ElidedApp (debug).debug.dylib`main at ElidedApp.swift:0 frame #43: 0x00000001969cab98 dyld`start + 6076 thread #9, name = 'com.apple.NSEventThread' frame #0: 0x0000000196d28c34 libsystem_kernel.dylib`mach_msg2_trap + 8 frame #1: 0x0000000196d3b3a0 libsystem_kernel.dylib`mach_msg2_internal + 76 frame #2: 0x0000000196d31764 libsystem_kernel.dylib`mach_msg_overwrite + 484 frame #3: 0x0000000196d28fa8 libsystem_kernel.dylib`mach_msg + 24 frame #4: 0x0000000196e55e7c CoreFoundation`__CFRunLoopServiceMachPort + 160 frame #5: 0x0000000196e54798 CoreFoundation`__CFRunLoopRun + 1208 frame #6: 0x0000000196e53c58 CoreFoundation`CFRunLoopRunSpecific + 572 frame #7: 0x000000019ae9f7fc AppKit`_NSEventThread + 140 frame #8: 0x000000010107e558 libclang_rt.asan_osx_dynamic.dylib`asan_thread_start(void*) + 80 frame #9: 0x000000010099d08c libsystem_pthread.dylib`_pthread_start + 136 * thread #19, queue = 'RPAC issue generation workloop', stop reason = breakpoint 1.1 * frame #0: 0x0000000196ababb0 libsystem_trace.dylib`os_log_fault_default_callback frame #1: 0x0000000196ac5674 libsystem_trace.dylib`_os_log_invoke_callback + 200 frame #2: 0x0000000196ab57b4 libsystem_trace.dylib`_os_log_impl_flatten_and_send + 7684 frame #3: 0x0000000196ab3998 libsystem_trace.dylib`_os_log + 168 frame #4: 0x0000000196ab9ff4 libsystem_trace.dylib`_os_log_fault_impl + 28 frame #5: 0x0000000101bac528 libRPAC.dylib`culledOsLogFault + 1352 frame #6: 0x0000000101bad85c libRPAC.dylib`__generateCulledBacktrace_block_invoke_2 + 4436 frame #7: 0x0000000100bf52dc libdispatch.dylib`_dispatch_client_callout + 16 frame #8: 0x0000000100bdcd30 libdispatch.dylib`_dispatch_block_invoke_direct + 312 frame #9: 0x0000000101080510 libclang_rt.asan_osx_dynamic.dylib`__wrap_dispatch_async_block_invoke + 196 frame #10: 0x0000000100bd8514 libdispatch.dylib`_dispatch_call_block_and_release + 32 frame #11: 0x0000000100bf52dc libdispatch.dylib`_dispatch_client_callout + 16 frame #12: 0x0000000100be3e10 libdispatch.dylib`_dispatch_workloop_invoke + 2840 frame #13: 0x0000000100beffbc libdispatch.dylib`_dispatch_root_queue_drain_deferred_wlh + 664 frame #14: 0x0000000100bef414 libdispatch.dylib`_dispatch_workloop_worker_thread + 580 frame #15: 0x000000010099f7a4 libsystem_pthread.dylib`_pthread_wqthread + 292 thread #21, queue = 'actual.reader.1' frame #0: 0x00000001a88b19d8 libswiftCore.dylib`std::__1::enable_if const* const>::value, __swift::__runtime::llvm::hash_code>::type __swift::__runtime::llvm::hashing::detail::hash_combine_range_impl const* const>(swift::TargetMetadata const* const*, swift::TargetMetadata const* const*) + 36 frame #1: 0x00000001a88b179c libswiftCore.dylib`std::__1::pair*, unsigned int> swift::ConcurrentReadableHashMap, swift::Mutex>::find<(anonymous namespace)::TupleCacheEntry::Key>((anonymous namespace)::TupleCacheEntry::Key const&, swift::ConcurrentReadableHashMap, swift::Mutex>::IndexStorage, unsigned long, swift::HashMapElementWrapper<(anonymous namespace)::TupleCacheEntry>*) + 72 frame #2: 0x00000001a8897500 libswiftCore.dylib`std::__1::pair<(anonymous namespace)::TupleCacheEntry*, swift::MetadataResponse> swift::LockingConcurrentMap<(anonymous namespace)::TupleCacheEntry, (anonymous namespace)::TupleCacheStorage>::getOrInsert<(anonymous namespace)::TupleCacheEntry::Key, swift::MetadataRequest&, swift::TargetValueWitnessTable const*&>((anonymous namespace)::TupleCacheEntry::Key, swift::MetadataRequest&, swift::TargetValueWitnessTable const*&) + 336 frame #3: 0x00000001a88971ac libswiftCore.dylib`swift_getTupleTypeMetadata + 96 frame #4: 0x00000001a8898334 libswiftCore.dylib`swift_getTupleTypeMetadata2 + 32 frame #5: 0x0000000105727740 ElidedKit`static CKRecord.fetch(id=(single = 09A65E5A-B025-482F-BF00-685FA47FC30C, compound = (Foundation.UUID, ElidedKit.Nothing) @ 0x000000016f89ccc0), storage=ElidedKit.TableAndKeys @ 0x000000012e567c20, db=0x000061500000f300) at CKRecord+SQL.swift:107:63 frame #6: 0x000000010573d530 ElidedKit`static CloudSyncable.fetchAsCloudRecord(id=(single = 09A65E5A-B025-482F-BF00-685FA47FC30C, compound = (Foundation.UUID, ElidedKit.Nothing) @ 0x000000016f89ccc0), db=0x000061500000f300) at CloudSyncable.swift:120:39 frame #8: 0x000000010573c9dc ElidedKit`static CloudSyncable.fetchAsCloudRecord(unparsedID=1 value, db=0x000061500000f300) at CloudSyncable.swift:92:13 frame #9: 0x000000010576dd20 ElidedKit`closure #1 in closure #2 in CloudSyncDelegate.nextRecordZoneChangeBatch($0=0x000061500000f300, model=ElidedKit.Node, unparsedID=1 value) at CloudSyncDelegate+outgoing.swift:39:35 frame #11: 0x00000001075f1acc GRDB`closure #1 in closure #1 in closure #1 in closure #1 in DatabasePool.read(db=0x000061500000f300, value=0x0000000105781fc4 ElidedKit`partial apply forwarder for closure #1 @Sendable (GRDB.Database) throws -> Swift.Optional<__C.CKRecord> in closure #2 @Sendable (__C.CKRecordID) async -> Swift.Optional<__C.CKRecord> in ElidedKit.CloudSyncDelegate.nextRecordZoneChangeBatch(_: CloudKit.CKSyncEngine.SendChangesContext, syncEngine: CloudKit.CKSyncEngine) async -> Swift.Optional at ) at DatabasePool.swift:378:44 frame #13: 0x00000001075e0238 GRDB`closure #2 in Mutex.inDatabase(work=0x0000000107603efc GRDB`partial apply forwarder for closure #1 () throws -> A in closure #1 @Sendable (GRDB.Database) -> () in closure #1 @Sendable (Swift.Result<(element: GRDB.SerializedDatabase, release: @Sendable (GRDB.PoolCompletion) -> ()), Swift.Error>) -> () in closure #1 (Swift.UnsafeContinuation) -> () in GRDB.DatabasePool.read(@Sendable (GRDB.Database) throws -> A) async throws -> A at ) at SerializedDatabase.swift:396:17 frame #15: 0x00000001078dd560 GRDB`throwingFirstError(execute=0x00000001075e0324 GRDB`partial apply forwarder for closure #2 () throws -> A1 in GRDB.Mutex.inDatabase(_: GRDB.Database, execute: () throws -> A1) throws -> A1 at , finally=0x00000001075e0638 GRDB`partial apply forwarder for closure #3 () throws -> () in GRDB.Mutex.inDatabase(_: GRDB.Database, execute: () throws -> A1) throws -> A1 at ) at Utils.swift:107:22 frame #16: 0x00000001075dbf98 GRDB`Mutex.inDatabase(db=0x000061500000f300, work=0x0000000107603efc GRDB`partial apply forwarder for closure #1 () throws -> A in closure #1 @Sendable (GRDB.Database) -> () in closure #1 @Sendable (Swift.Result<(element: GRDB.SerializedDatabase, release: @Sendable (GRDB.PoolCompletion) -> ()), Swift.Error>) -> () in closure #1 (Swift.UnsafeContinuation) -> () in GRDB.DatabasePool.read(@Sendable (GRDB.Database) throws -> A) async throws -> A at ) at SerializedDatabase.swift:395:20 frame #17: 0x00000001075f14c0 GRDB`closure #1 in closure #1 in closure #1 in DatabasePool.read(db=0x000061500000f300, releaseReader=0x0000000107603e6c GRDB`partial apply forwarder for reabstraction thunk helper from @escaping @callee_guaranteed @Sendable (@in_guaranteed GRDB.PoolCompletion) -> (@out ()) to @escaping @callee_guaranteed @Sendable (@unowned GRDB.PoolCompletion) -> () with unmangled suffix ".81" at , dbAccess=0x000060300058e4c0, value=0x0000000105781fc4 ElidedKit`partial apply forwarder for closure #1 @Sendable (GRDB.Database) throws -> Swift.Optional<__C.CKRecord> in closure #2 @Sendable (__C.CKRecordID) async -> Swift.Optional<__C.CKRecord> in ElidedKit.CloudSyncDelegate.nextRecordZoneChangeBatch(_: CloudKit.CKSyncEngine.SendChangesContext, syncEngine: CloudKit.CKSyncEngine) async -> Swift.Optional at , continuation=Swift.UnsafeContinuation, Swift.Error> @ 0x000000012e4ff0c0) at DatabasePool.swift:374:55 frame #18: 0x00000001075d90f4 GRDB`closure #1 in SerializedDatabase.async(block=0x0000000107603ed0 GRDB`partial apply forwarder for closure #1 @Sendable (GRDB.Database) -> () in closure #1 @Sendable (Swift.Result<(element: GRDB.SerializedDatabase, release: @Sendable (GRDB.PoolCompletion) -> ()), Swift.Error>) -> () in closure #1 (Swift.UnsafeContinuation) -> () in GRDB.DatabasePool.read(@Sendable (GRDB.Database) throws -> A) async throws -> A at ) at SerializedDatabase.swift:228:13 frame #20: 0x0000000101080510 libclang_rt.asan_osx_dynamic.dylib`__wrap_dispatch_async_block_invoke + 196 frame #21: 0x0000000100bd8514 libdispatch.dylib`_dispatch_call_block_and_release + 32 frame #22: 0x0000000100bf52dc libdispatch.dylib`_dispatch_client_callout + 16 frame #23: 0x0000000100be1594 libdispatch.dylib`_dispatch_lane_serial_drain + 828 frame #24: 0x0000000100be2438 libdispatch.dylib`_dispatch_lane_invoke + 440 frame #25: 0x0000000100beffbc libdispatch.dylib`_dispatch_root_queue_drain_deferred_wlh + 664 frame #26: 0x0000000100bef414 libdispatch.dylib`_dispatch_workloop_worker_thread + 580 frame #27: 0x000000010099f7a4 libsystem_pthread.dylib`_pthread_wqthread + 292 thread #22, queue = 'com.apple.cloudkit.operation-50B732E4BB736853.callback' frame #0: 0x0000000196d2aa8c libsystem_kernel.dylib`__ulock_wake + 8 frame #1: 0x0000000196da2420 libsystem_platform.dylib`_os_unfair_lock_unlock_slow + 56 frame #2: 0x000000019697a0cc libobjc.A.dylib`lookUpImpOrForward + 612 frame #3: 0x0000000196979b84 libobjc.A.dylib`_objc_msgSend_uncached + 68 frame #4: 0x00000001a092b4f4 CloudKit`___lldb_unnamed_symbol13651 + 204 frame #5: 0x00000001a07dd148 CloudKit`___lldb_unnamed_symbol5181 + 712 frame #6: 0x00000001a090c4f0 CloudKit`___lldb_unnamed_symbol13048 + 876 frame #7: 0x00000001a07dbcfc CloudKit`___lldb_unnamed_symbol5160 + 612 frame #8: 0x00000001a09ca350 CloudKit`___lldb_unnamed_symbol16879 + 180 frame #9: 0x0000000196e364d4 CoreFoundation`__invoking___ + 148 frame #10: 0x0000000196e36364 CoreFoundation`-[NSInvocation invoke] + 424 frame #11: 0x0000000196e6a678 CoreFoundation`-[NSInvocation invokeWithTarget:] + 64 frame #12: 0x00000001a092bd40 CloudKit`___lldb_unnamed_symbol13654 + 276 frame #13: 0x0000000100bf52dc libdispatch.dylib`_dispatch_client_callout + 16 frame #14: 0x0000000100bdcd30 libdispatch.dylib`_dispatch_block_invoke_direct + 312 frame #15: 0x00000001a094ea28 CloudKit`ck_call_or_dispatch_async_if_not_key + 84 frame #16: 0x00000001a092b950 CloudKit`___lldb_unnamed_symbol13652 + 964 frame #17: 0x0000000196e34bbc CoreFoundation`___forwarding___ + 956 frame #18: 0x0000000196e34740 CoreFoundation`_CF_forwarding_prep_0 + 96 frame #19: 0x000000019842bd70 Foundation`__NSXPCCONNECTION_IS_CALLING_OUT_TO_EXPORTED_OBJECT_S2__ + 16 frame #20: 0x0000000198d9b698 Foundation`-[NSXPCConnection _decodeAndInvokeMessageWithEvent:reply:flags:] + 1664 frame #21: 0x0000000198d9ccd8 Foundation`message_handler_message + 88 frame #22: 0x00000001983e8a94 Foundation`message_handler + 152 frame #23: 0x0000000196a72830 libxpc.dylib`_xpc_connection_call_event_handler + 112 frame #24: 0x0000000196a7112c libxpc.dylib`_xpc_connection_mach_event + 1196 frame #25: 0x0000000100bf5324 libdispatch.dylib`_dispatch_client_callout4 + 16 frame #26: 0x0000000100bf7fec libdispatch.dylib`_dispatch_mach_msg_invoke + 608 frame #27: 0x0000000100be13a8 libdispatch.dylib`_dispatch_lane_serial_drain + 336 frame #28: 0x0000000100bf9738 libdispatch.dylib`_dispatch_mach_invoke + 516 frame #29: 0x0000000100be13a8 libdispatch.dylib`_dispatch_lane_serial_drain + 336 frame #30: 0x0000000100be246c libdispatch.dylib`_dispatch_lane_invoke + 492 frame #31: 0x0000000100beffbc libdispatch.dylib`_dispatch_root_queue_drain_deferred_wlh + 664 frame #32: 0x0000000100bef414 libdispatch.dylib`_dispatch_workloop_worker_thread + 580 frame #33: 0x000000010099f7a4 libsystem_pthread.dylib`_pthread_wqthread + 292 thread #23, stop reason = thread-creation frame #0: 0x00000001009a6f40 libsystem_pthread.dylib`start_wqthread thread #24, stop reason = thread-creation frame #0: 0x00000001009a6f40 libsystem_pthread.dylib`start_wqthread thread #25 frame #0: 0x000000010099f684 libsystem_pthread.dylib`_pthread_wqthread + 4

[groue]

This bt all is surprising in the context of this issue.

A single thread (21) is busy doing GRDB stuff (more precisely, doing ElidedKit stuff during a regular, unblocked, GRDB read). It is not busy in Pool.get as in the initial report. I don't see any thread blocked waiting for anything, as I would have expected for a priority inversion runtime warning.

[Jason-Abbott]

This code path isn’t overly complex so old-school debugging won’t be too hard: comment things out, see what changes.

Just in case it matters, this is all kicked off by a TransactionObserver implementation which relies on the SQLite pre-update hook for databaseWillChange(). I use the initialValue / finalValue in the event to determine which changes are of interest without further queries.

Then in databaseDidCommit, I convert those to CKSyncEngine.PendingRecordZoneChanges (and databaseDidRollback(), of course, clears the queued changes) and send them to CKSyncEngine.

Those operations are all complete by the time CKSyncEngine decides to send a batch of changes and invokes the code that triggers the inversion warning. In spite of that warning, this all seems to be working exactly as expected between actual, separate devices.

I’ll comment again when I’ve solved it or have something clearer to act on.

Thank you, as always!