Add set_account to solana-program-test

[ruuda]

Problem

It is extremely tedious, and sometimes impossible, to recreate certain states of the bank using solana_program_test. When it is tedious to write tests for common Solana attack vectors, that results in those tests not being written at all.

Summary of Changes

Add a “god mode” to ProgramTestContext: a method set_account that allows you to arbitrarily change accounts. This allows us to jump to states of the bank that would otherwise be tedious or impossible to reach.

Details

For writing tests, it is often desirable to create a certain situation that would be tedious, or even impossible to create by sending individual transactions.

For example, a common attack vector on Solana is to create a copy of some existing account that stores configuration data, but manipulate the data stored there, and then to call a program and pass in the manipulated account, instead of the real account.

If one wants to test this, one option is to write a program that you can call to write arbitrary data into a new account account (and possibly change its owner), then include that program in the test context, and send a transaction to call it. This is extremely tedious, and developers are not going to bother doing it. I myself would rather fork solana-program-test to add this set_account method, than to write that program. Having a “god mode” method to just write an account, lowers the barrier to writing comprehensive tests.

A second reason for introducing this method, is defense in depth. There may be states of the bank that are not reachable yet by only sending transactions, but that you might want to test against either way. For example, right now, the balance of a stake account cannot decrease without going through the stake program. But what if Solana were to introduce slashing in the future, and you want to ensure your program is robust against decreases in stake account balance? Right now there is no way to test this, but by introducing this “god mode” to write accounts, the situation becomes testable.

[jstarry]

Very useful, thanks! Now we just need something like this for solana-test-validator!

[mergify]

automerge label removed due to a CI failure