Over the past few days, many countries have decided to ground the 737 MAX. Among these, several have also decided to deny passage to the affected models through that country's airspace. How does this affect flights which were already in the air at the time of the announcement, typically? I can see several options:
The flight returns to its origin, fuel permitting
The flight lands at the closest viable airport in the destination country if fuel is insufficient for a return to origin (minimizing the amount of time in the destination country's airspace)
The flight proceeds to its destination airport as filed, even if this means spending several more hours in the destination country's airspace than the second option
The flight proceeds to the closest viable airport regardless of whether that airport is in some third country being overflown (although I can foresee this causing immigration/visa/customs issues for the passengers)
I'm assuming that (1) is commonly used, given the flight paths on internet flight trackers yesterday, but which of the other options are rooted in reality?