Ethereum mainnet Subgraph fails to sync

[devanoneth]

Subgraph fails when trying to process this tx: https://etherscan.io/tx/0x66975e3a403aded15b6181c511e9442941e24ccbbd69daf9f63e05d3411fccc5

Raw error:

Error: failed to process trigger: block #20362531 (0x6518…d30e), transaction 66975e3a403aded15b6181c511e9442941e24ccbbd69daf9f63e05d3411fccc5: Decoding function outputs for the call Function { name: "execTransaction", inputs: [Param { name: "to", kind: Address, internal_type: Some("address") }, Param { name: "value", kind: Uint(256), internal_type: Some("uint256") }, Param { name: "data", kind: Bytes, internal_type: Some("bytes") }, Param { name: "operation", kind: Uint(8), internal_type: Some("enum Enum.Operation") }, Param { name: "safeTxGas", kind: Uint(256), internal_type: Some("uint256") }, Param { name: "baseGas", kind: Uint(256), internal_type: Some("uint256") }, Param { name: "gasPrice", kind: Uint(256), internal_type: Some("uint256") }, Param { name: "gasToken", kind: Address, internal_type: Some("address") }, Param { name: "refundReceiver", kind: Address, internal_type: Some("address payable") }, Param { name: "signatures", kind: Bytes, internal_type: Some("bytes") }], outputs: [Param { name: "success", kind: Bool, internal_type: Some("bool") }], constant: None, state_mutability: Payable } failed, raw output: : Invalid name: please ensure the contract and method you're calling exist! failed to decode empty bytes. if you're using jsonrpc this is likely due to jsonrpc returning 0x in case contract or method don't exist

[devanoneth]

The transaction which we fail to index is an attempt at calling the execTransaction function on a comprimised Safe. At block 20362531, the Safe's singleton points to address(0) due to this tx which the exploiter made, effectively bricking the Safe.

We can see from the stack trace that the GnosisSafeProxy tries to delegate call execTransaction on the Safe's singleton, but fails because the singleton is address(0). Our indexer will still try to index this method call due to our execTransaction call handler, but will not be able to actually call any methods on the GnosisSafe contract instance because the methods we expect to work (i.e. nonce(), getTransactionHash(), etc) will fail because the singleton is address(0).

I suspect it's failing at a call to nonce() on the Safe, because that's the first call we make when indexing a transaction on a Safe. The improved_try_nonce function is supposed to handle the case that we get back 0x bytes, and I have written a test for it which passes, but in reality the Subgraph fails to process this transaction correctly.

Obviously an easy way to make our Subgraph sync would be to ignore this Safe or this transaction, but we should instead try to handle it gracefully, in case of future similar cases.

[devanoneth]

Having looked deeper into where the error really comes from, I think it might not be possible at all to try and handle this at the Subgraph layer. I'm not sure how exactly ethabi is used in the Graph's stack, but the error comes from here, and if it's used in the node as I suspect, we simply can't have any cases where a JSON-RPC call results in an empty 0x response, as the node will error before it even gets to the Subgraph code.

[devanoneth]

Seems related to this issue: graphprotocol/support#30 - we need a way to detect the empty bytes at the Subgraph layer, rather than it erroring out completely, so we could then mark the Safe as bricked and stop indexing it programmatically, rather than inserting some silly if address == brokenSafe code in the Subgraph.