[UR][OpenCL] Break reference cycle between in-order queue and last event

[uditagarwal97]

Problem

ur_queue_handle_t_::storeLastEvent() retained the ur_event_handle_t_ it stores (queue.hpp:71), and ur_event_handle_t_'s constructor retains its Queue (event.hpp:31). For an in-order queue that is an unbreakable cycle:

• queue -> LastEvent (strong, via urEventRetain)
• event -> Queue (strong, via urQueueRetain)

The queue's reference count never reaches zero, so ~ur_queue_handle_t_ never runs and its urContextRelease / urDeviceRelease never fire. The queue, its context, its device and the event are all leaked for the lifetime of the process.

Proposed Fix

Store the cl_event instead of the UR event, retained with clRetainEvent and released in the destructor. The queue only ever needed that event to answer UR_QUEUE_INFO_EMPTY, and a cl_event holds no reference back to the UR queue, so the cycle cannot form.

The UR_QUEUE_INFO_EMPTY query now calls clGetEventInfo(CL_EVENT_COMMAND_EXECUTION_STATUS) directly, which is the identical call the neighbouring branch for a queue with no stored event already made.

How it was found

An ASan+UBSan build of the runtime running sycl/test-e2e/Basic on opencl:cpu. LeakSanitizer reported it as four indirect leaks with no direct leak at all, which is the signature of a reference cycle (no chunk is unreferenced, so nothing qualifies as the root):

Indirect leak of 72 byte(s) in 1 object(s) allocated from:
    #2 ur::opencl::urQueueCreate(...) queue.cpp:165
Indirect leak of 56 byte(s) in 1 object(s) allocated from:
    #2 ur::opencl::urContextCreate(...) context.cpp:84
Indirect leak of 40 byte(s) in 1 object(s) allocated from:
    #2 ur::opencl::createUREvent(...) event.hpp:74
Indirect leak of 8 byte(s) in 1 object(s)
SUMMARY: AddressSanitizer: 176 byte(s) leaked in 4 allocation(s).

The proposed fix resolves all the leaks.

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Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com

[Copilot]

🟡 Changes recommended

The ownership fix lacks a lifecycle regression test capable of detecting the original reference cycle.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Breaks the OpenCL in-order queue/event reference cycle while preserving empty-queue tracking.

Changes:

• Stores and manages a retained native cl_event.
• Queries native event completion status directly.
• Passes native events into queue tracking.

File summaries

File	Description
queue.hpp	Replaces strong UR-event ownership with native-event ownership.
queue.cpp	Queries native event execution status.
event.hpp	Records the native event after enqueue.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Balanced

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