Qiskit / qiskit / 30016770968

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Commit Message

Fix deadlock in Optimize1qGatesDecomposition (#16592) (#16645)

* Fix deadlock in Optimize1qGatesDecomposition

This commit fixes an oversight in the new multithreaded
Optimize1qGatesDecomposition pass introduced in #15667 that could cause
a deadlock with the Python GIL when using custom 1q gates.
If there are custom gates defined in Python then we need Python
to get the matrix of that custom gate. However, when running in a
multithreaded context Python only allows one thread to interact with the
interpreter at a time via the GIL. When operating in parallel the pass
was not releasing the GIL from the parent thread running the pass. This
meant that the children threads would wait to acquire the GIL to call
Python but never be able to get the lock because the parent thread that
spawned it never released the GIL.

This commit fixes that issue by adding a dedicate entrypoint for calling
the pass via Python and having that entrypoint release the GIL prior to
launching an parallel threads. It then reacquires the GIL before
modifying the output DAG which might need Python for working with the
custom gates. This mirrors the pattern used in the other parallel passes
that work in a similar way such as `UnitarySynthesis` and
`TwoQubitPeepholeOptimization`.

Fixes #16591

* Fix clippy warning

(cherry picked from commit 2440bbb99)

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

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27 existing lines in 5 files now uncovered.

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