/**
 * This type decorates a Client object with a stack of active CurOp objects.
 *
 * It encapsulates the nesting logic for curops attached to a Client, along with
 * the notion that there is always a root CurOp attached to a Client.
 *
 * The stack itself is represented in the _parent pointers of the CurOp class.
 */
class CurOp::CurOpStack {
...
private:
    OperationContext* _opCtx = nullptr;

    // Top of the stack of CurOps for a Client.
    CurOp* _top = nullptr;

    // The bottom-most CurOp for a client.
    const CurOp _base;
};

void CurOpStack::push(OperationContext* opCtx, CurOp* curOp) {
    invariant(opCtx);
    if (_opCtx) {
        invariant(_opCtx == opCtx);
    } else {
        _opCtx = opCtx;
    }
    stdx::lock_guard<Client> lk(*_opCtx->getClient());
    push_nolock(curOp);
}

class CurOpClientObserver final : public ServiceContext::ClientObserver {
    void onCreateClient(Client*) {}
    void onDestroyClient(Client*) {}
    void onCreateOperationContext(OperationContext* opCtx) {
        _curopStack(opCtx).setOpCtx(opCtx);
    }
    void onDestroyOperationContext(OperationContext*) {}
};

ServiceContext::ConstructorActionRegisterer registerCurOpClientObserver{"CurOpClientObserver", [](ServiceContext* service) {
    service->registerClientObserver(std::make_unique<CurOpClientObserver>());
}};