I'm sure it's no different from the bankrupt pension systems we have here in the US.
The only way to fix it is to reduce benefits, delay benefits, or increase taxes. And I'm not so sure there's much room left for a tax increase in France's case.
This is an inherent problem with defined benefit plans, especially in the public sector (and it was also a big problem in the private sector, which is why most use a defined contribution plan instead). There's all kinds of incentives to underfund the pension now (to pay for other things), or overestimating the growth of the funds, figuring that by the time the problem manifests itself you'll have been out of office for years or dead of old age. That's why Illinois has $80 billion in unfunded pension obligations today. It's far too easy to kick the problem down the road for Someone Else to worry about.
Problem in France is it can't be kicked down the road any longer, and Someone Else is the guy in the mirror.