Keep Python opaque data alive for the lifetime of the request

[Gopalakrishnan Nallasamy (GopalakrishnanN)]

Request.SetOpaqueData() stores the PyObject* unowned by design, and the Python binding passed opaque_data.ptr() straight through without holding a reference. Anything that is not independently kept alive by the caller is therefore freed as soon as set_opaque_data() returns, and get_opaque_data() hands back a dangling pointer.

The natural way to write it is a use-after-free:

req.set_opaque_data(Sink(prompt))     # temporary, freed immediately
...
sink = ready.get_opaque_data()        # dangling
sink.text += ...                      # general protection fault in libpython

That faults inside libpython rather than raising, so it looks like a runtime bug rather than an ownership mistake. The shipped continuous-batching example only avoids it because it passes a long-lived RequestPool.

Add pybind11::keep_alive<1, 2> so the object lives as long as the request that references it.

[Copilot]

Pull request overview

This PR fixes a Python binding lifetime hazard in ONNX Runtime GenAI’s request “opaque data” feature by ensuring Python objects passed to Request.set_opaque_data() are kept alive for as long as the Request instance that references them, preventing use-after-free crashes when retrieving the pointer later.

Changes:

• Adds a pybind11::keep_alive<1, 2> call policy to Request.set_opaque_data() so the Python object’s lifetime is tied to the Request.
• Adds an in-code comment explaining the unowned PyObject* storage in the core and why the binding must enforce lifetime.

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