fetching the error message for a failed transaction

[sesameJar]

Hi community, I have been trying to fetch errors for failed transactions and it seems it's not as easy as it look on explorer pages. I have been checking different libraries how they do it. EthersJs appears to only mention there was a CALL_EXCEPTION on wait() in case the status on receipt is 0. I haven't been able to see how using EthersJs can retrieve an old failed transaction's error message(possibly having access to an archive node).

After searching a lot, it seems one solution is sending the exact transaction using eth_call to that exact block number. My question is what if there are two transactions in that block that have changed the state or failed and we are looking for the error message in the first failed transaction. For example:

tx1 => fails due to some later require statement
tz2 => pauses the contract
ethCall(tx1, same block) => ???? Is going to say contract is paused or the error message for tx1

Thanks.

[zemse]

That's true, for exact behaviour you might have to take support of non-standard rpc methods, e.g Geth's debug_tracetransaction or OpenEthereum's trace_replaytransaction.

[sesameJar]

So there's no standard specific thing such as state at a specific transaction, right? I suppose there's only state a certain block.

[zemse]

No your right that within a block after every transaction, there is a state change. However that state snapshot is not stored in the node for every tx, but state snapshot is stored after running all txs in a block, due to engineering reasons (disk space would blow up for archive nodes).

So imagine there's 100 txs in a block and you're interested in outcome of the 76th transaction. So with you use any of these methods, they start with state snapshot of the previous block, then run these 76 transactions over it and then you get the accurate execution result which is returned.