utils::phased_barrier holds a `lw_shared_ptr<gate>` that is
typically `enter()`ed in `phased_barrier::start()`,
and left when the operation is destroyed in `~operation`.
Currently, the operation move-assign implementation is the
default one that just moves the lw_shared gate ptr from the
other operation into this one, without calling `_gate->leave()` first.
This change first destroys *this when move-assigned (if not self)
to call _gate->leave() if engaged, before reassigning the
gate with the other operation::_gate.
A unit test that reproduces the issue before this change
and passes with the fix was added to serialized_action_test.
Fixes#8613
Test: unit(dev), serialized_action_test(debug)
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Message-Id: <20210510120703.1520328-1-bhalevy@scylladb.com>
9ba960a388