Utility for increasing budget during execution of PyTeal applications

[algoidurovic]

This PR implements a PyTeal utility for increasing the opcode budget during execution of an app call using inner app call transactions.

Two modes are provided:

1. Explicit: the app to call for increasing budget is provided by the user.
2. OnCall: the app to call is created during each request to increase the budget, then deleted afterwards.

Two budget increase methods are provided:

1. ensure_budget(budget): increase the current opcode budget to at least the budget requested by the user.
2. maximize_budget(fee): increase the current opcode budget as much as possible without spending more than the given fee. This can be used when the contract developer doesn't want to spend more than the fee provided to increase the budget without needing to do in depth opcode cost analysis on their program. If the fee isn't sufficient for executing the program then a failure may be desired. It can also allow the app caller to specify the maximum fee they want to spend for increasing opcode budget. This behavior can fairly easily be extended to support a global fee limit for budget increase requests. (Inspired by the algofi implementation of opup).

Testing
Unit tests for type checking enforcement and sandbox tests for e2e functionality. The PyTeal programs I used for testing in sandbox are located in examples/application/opup.py

[tzaffi]

Two budget increase methods are provided:

1. ensure_budget(budget): increase the current opcode budget to at least the budget requested by the user.
2. maximize_budget(fee): increase the current opcode budget as much as possible without spending more than the given fee.

I think method 1 is essential, but I'm not as sure about method 2. On the face of it, maximize_budget() gives you an unknown amount of extra budget, so you could run out unexpectedly or get much more than you really need. One could argue that method 1 suffers from the opposite problem - what if fees increase and the app can no longer afford to op-up? But that seems a little easier to ameliorate dynamically, if another method is added fee_for_budget(budget) which would let the user know how much they'll use up and adapt accordingly.

[algoidurovic]

Two budget increase methods are provided:

1. ensure_budget(budget): increase the current opcode budget to at least the budget requested by the user.
2. maximize_budget(fee): increase the current opcode budget as much as possible without spending more than the given fee.

I think method 1 is essential, but I'm not as sure about method 2. On the face of it, maximize_budget() gives you an unknown amount of extra budget, so you could run out unexpectedly or get much more than you really need. One could argue that method 1 suffers from the opposite problem - what if fees increase and the app can no longer afford to op-up? But that seems a little easier to ameliorate dynamically, if another method is added fee_for_budget(budget) which would let the user know how much they'll use up and adapt accordingly.

I updated the PR description to further elaborate on the use cases for maximize_budget(). I do think it's useful to have a fee_for_budget() method available as well.

[jasonpaulos]

Except for my one minor comment, this looks good to me, thank you for incorporating my feedback.

The one comment is about shrinking the on call program code. I edited the TEAL by hand, and even though it's a simple change I'd still recommend you briefly test it in sandbox again just to make sure there are no unintended consequences.

[michaeldiamant]

@algoidurovic Thanks for your effort here - I'm excited to see the feature used by the community! ☕

I left an optional remark in #274 (comment). I'd appreciate considering it before merging though it need not hold us up.

[jasonpaulos]

The docs look great, I just have some suggestions, only one of which is important (the 700 vs ~700 comment).

[jasonpaulos]

Looks great, I just have one comment that I missed before

[jasonpaulos]

This line shouldn't be necessary

[ahangsu]

just one question about ensure_budget

[ahangsu]

minor proposal, do you think MIN_TXN_FEE should be available in a larger scope, say it lives in pyteal/config.py? I guess some folks would want this available not just in opup.

[jasonpaulos]

Global.min_txn_fee() is available to PyTeal, perhaps this code should use that instead?

[ahangsu]

^ sounds pretty reasonable to me