How do I throw an error if there are other instructions in a transaction?

Question

I'm using anchor, and would like a handler call to error if there are other instructions within the same transaction. How can I accomplish this?

The use-case here is a liquidation call in a perp market, in an attempt to deter liquidation hunting.

i.e.

1. manipulate price
2. force liquidation
3. unwind + profit

all within the same transaction

Answer 1

You will have to use the solana instructions sysvar. You need to ensure that there is only one instruction and that that instruction is an invocation to your program, otherwise a user could create a program that calls your instruction and others in a single instruction.

This is usually not great to do though because it limits your program's composability. There are very few attacks that are stopped by running this check, the attack you listed is not one of them.