"I was waiting at the airport when who should come along but Mr Pettigrew!" cambridge dictionary-come along
The part of the sentence ".....when who....." has caught my eye. This is the first time I have come across such a different structure.
Do people use it in daily life? For instance:
"I went shopping yesterday when who should I bump into but my primary school teacher."
This idiom is a little old. It isn't used very frequently, but you can still hear/read it. It is generally used with verbs such as see, come, arrive, etc. You can hear it in stories more often.
It means "I was waiting at the airport when Mr Pettigrew arrived", but in this way (the way in the Cambridge dictionary), the sentence has a strong connotation of surprise.
This is from Merriam-Webster
Supposing you are Turkish, it's very similar to saying "... bir de ne göreyim". Example: Deniz kenarında yürüyorduk ki bir de ne görelim! Sahilde oturan bir deniz kızı!
