Data visitor #95
Data visitor #95
Conversation
| import "package:ffi/ffi.dart" show allocate, free; | ||
|
|
||
| int _lastId = 0; | ||
| final _callbacks = <int, bool Function(Pointer<Uint8> dataPtr, int length)>{}; |
There was a problem hiding this comment.
What do we need that for? E.g. this is one of the visitor C APIs:
obx_err obx_query_visit(OBX_query* query, obx_data_visitor* visitor, void* user_data, uint64_t offset, uint64_t limit);
I assume you want to pass "id" as user_data? Is this more "convenient" that passing in a lambda-like thingy as visitor? The latter would already be thread-safe, which the current map-based is not, I think?
There was a problem hiding this comment.
Lambdas can't be passed as a C callback, FFI requires static functions. I've tried that (lambda) approach but had to resolve to this trampoline one instead (basically the same thing we do in Go)
There was a problem hiding this comment.
Sure, please make sure to document that in the code so that this explanation is not lost
There was a problem hiding this comment.
The latter would already be thread-safe, which the current map-based is not, I think?
I think this was not addressed yet. Does that require additional thread synchronization?
There was a problem hiding this comment.
There is no concurrent memory access in Dart. All code in a single isolate executes on a single event-loop and isolates don't share memory.
There was a problem hiding this comment.
how so? I don't see a problem with the code in this PR. You mean something unrelated, e.g. transactions?
There was a problem hiding this comment.
To my limited understanding, I imagine this to happen:
- Let's assume two isolates, both starting with IDs at 1 (produce colliding IDs, because isolated...)
- Callbacks from core happen on a different thread
- (Open question: not sure about how a "foreign" thread is mapped to an isolate. So, which isolate gets the callback?)
- Now, I don't see how we ensured that one isolate gets only its callback in using its ID.
E.g. Isolate 2 registers a callback for ID 1, but isolate 1 gets to consume it because it also has a pending callback for ID 1.
There was a problem hiding this comment.
Callbacks from core happen on a different thread
They don't with objectbox-c visitors - all calls are synchronous, executed on the same thread. This approach wouldn't work for other kinds of callbacks, where you register the callback and it may happen in the future (e.g. some kind of event listeners). Not sure how that could be handled with dart, if at all.
There was a problem hiding this comment.
Looks like it's an open issue with some workarounds available dart-lang/sdk#37022
There was a problem hiding this comment.
Right, that one is strictly synchronous and if FFI guarantees synchronous callbacks to execute in the same isolate (I assume that is defined somewhere?), we should be fine.
|
👍 From my brief look it's good to merge after those two tiny things are addressed. |
Fixes #94