Qiskit / qiskit / 18887901267

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Commit Message

Fix reuse of `ConsolidateBlocks` instances (#15258) (#15266)

* Fix reuse of `ConsolidateBlocks` instances

The `_qubit_map` state added to `ConsolidateBlocks` in #15083 (cb103d95)
was attempting to get round an awkward internal `PassManager` use to
pass the mapping into recursive calls.  Unfortunately, the state was not
cleared on exit to the pass, so re-use of the pass object would be
invalid, such as by calling the `PassManager.run` method on the result
of `generate_preset_pass_manager` more than once on circuits with
different sizes.

Rather than trying to plaster over the poor use of state, this commit
instead moves the runtime logic to be in the `PropertySet`, so re-use
troubles are limited to single executions.  It's still technically
possible for the `PropertySet` to become polluted if `ConsolidateBlocks`
throws an internal exception that is forcibly suppressed such that the
pipeline continues, but this is completely non-standard, would require
significant effort to achieve, and most likely there are large numbers
of transpiler passes that are not completely exception-safe anyway.

* Add test of `PassManager.run` over a list

On some platforms this will be multiprocessing, but it doesn't hurt to
try it for the platforms that aren't.

* Document use of property-set key

(cherry picked from commit 4019fd9d9)

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

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