When oil-rich Arab states started pouring investments into their economies instead of parking them abroad, commentators applauded. Policy makers are learning from the mistakes of past oil shocks, the pundits remarked, and they are, surprisingly, keen to develop new industries, to create jobs and to reduce the vulnerability of their economies when the next cycle of lower oil prices hits.

But then inflation soared, not only tormenting the people who were supposed to benefit from the boom but also the governments that had promised that oil revenues would, this time, trickle down.