Qiskit / qiskit / 10394560985

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github

Committed by web-flow

Commit Message

Use `OnceCell` to cache Python operations (#12942)

We were previously using `RefCell` to handle the interior mutability for
the Python-operation caches.  This works fine, but has two problems:

- `RefCell<Py<PyAny>>` is two pointers wide
- we had to do heavier dynamic runtime checks for the borrow status on
  every access, including simple gets.

This commit moves the caching to instead use `OnceCell`.  The internal
tracking for a `OnceCell<T>` is just whether an internal `Option<T>` is
in the `Some` variant, and since `Option<Py<PyAny>>` will use the
null-pointer optimisation for space, this means that
`OnceCell<Py<PyAny>>` is only a single pointer wide.  Since it's not
permissible to take out a mutable reference to the interior, there is
also no dynamic runtime checking---just the null-pointer check that the
cell is initialised---so access is faster as well.

We can still clear the cache if we hold a mutable reference to the
`OnceCell`, and since every method that can invalidate the cache also
necessarily changes Rust-space, they certainly require mutable
references to the cell, making it safe.

There is a risk here, though, in `OnceCell::get_or_init`.  This function
is not re-entrant; one cannot attempt the initialisation while another
thread is initialising it.  Typically this is not an issue, since the
type isn't `Sync`, however PyO3 allows types that are only `Send` and
_not_ `Sync` to be put in `pyclass`es, which Python can then share
between threads.  This is usually safe due to the GIL mediating access
to Python-space objects, so there are no data races.  However, if the
initialiser passed to `get_or_init` yields control at any point to the
Python interpreter (such as by calling a method on a Python object),
this gives it a chance to block the thread and allow another Python
thread to run.  The other thread could then attempt to enter the same
cache initialiser, which is a violation of its contract.

PyO3's `GILOnceCell` can protect against ... (continued)

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