Is it possible to simulate two subsequent contract calls at together?

[ysfaran]

Issue

With ethers you can simulate a call to a smart contracts write function using callStatic:

try {
  await erc20Contract.callStatic.approve(...args);
  console.log("Simulation successful");
} catch {
  console.log("Simulation failed");
}

I want to do this so I don't need to spend gas fees on a transaction that will fail anyway and also to save some time (simulation is much faster than waiting for a failing transaction).

In my case I need to send two transactions, where the latter transaction is depended on the other:

 await erc20Contract.approve(...args0); // first transaction
 await routerContract.swapExactTokensForTokens(...args1); // second transaction, depends on first to be successful

So if erc20Contract.approve would fail, also routerContract.swapExactTokensForTokens would fail 100%.

The issue now is that erc20Contract.approve almost never fails while routerContract.swapExactTokensForTokens fails very often. But since erc20Contract.approve is a "pre-condition" for routerContract.swapExactTokensForTokens I currently always need to execute erc20Contract.approve (without callStatic) and then simulate routerContract.callStaticSwapExactTokensForTokens. This is unnecessary (in manner of gas fees and time) because I wouldn't need to execute erc20Contract.approve if I would just know that routerContract.callStatic.swapExactTokensForTokens will fail anyway.

Question

Is it possible to simulate multiple state-changing contract calls which are dependent on each other?

Following is an example of what could be a solution in my head, but is obviously not part of the current API of ethers:

try {
  await callStaticContext(async () => {
    await erc20Contract.callStatic.approve(...args0);
    // should be evaluated as if `approve` worked
    await routerContract.callStatic.swapExactTokensForTokens(...args1);
  })
  console.log("Simulation successful");
} catch {
  console.log("Simulation failed");
}

Are there maybe alternative approaches?

Any help is greatly appreciated.

[ricmoo]

This may be something you could use the forking features of hardhat to accomplish, but I haven't done any of that myself. Perhaps someone with more experience in this area can help?

You might also consider trying out https://tenderly.co, as I think they also support this sort of thing?

[ysfaran]

Thanks for the hints.

So there is no out-of-the-box solution without another third party unless I want to fork mainnet myself (which i don't 😄)?

I hope etherum, other eth based blockchains and all other upcoming blockchains will focus more on testability in the future. As a dev you always feel kind of unsafe implementing something and running code on the mainnet. Being able to simulate a transaction-chain in a less complex manner would be huge. IMO that would make any blockchain outstanding. But that's another topic I guess.. 😃

[ricmoo]

But the third-party solutions are part of the blockchain solution, and as our-of-the-box as it comes.

There is no need to include this functionality in something like Geth; it should focus on doing what it does best. And let the ecosystem provide the tools like Tenderly and Ganache with forking. You can instantly fork mainnet using these tools, run your tests and discard the network when you are done. :)