Hidden gotchas in the Mumbai to Amoy migration

We’ve been using the Mumbai Testnet to support our developer community, who are building onchain mobile apps. We have GSN and all of our gasless transaction infrastructure, RallyTransact, set up there. As part of the Goerli deprecation, Mumbai is on its way out and being replaced by Amoy. More info on that here.

GSN was not deployed to Amoy yet so we took it upon ourselves to do so and get the first relayer set up. When we started testing we noticed the following error on every attempt to relay a transaction:

insufficient funds for gas * price + value: address 0x0000000000000000000000000000000000000000 have 2170300023505000000 want 900000000000000000000 (supplied gas 600000000)

We initially assumed it was a misconfiguration from our GSN deploy or one of our clients. It’s easy to miss a setting. Oddly everything worked on our integration test infra. Our integration tests use a hardhat node forking off of the testnet networks before every run. So something is different between sending transactions to hardhat vs sending transactions to Amoy.

The above error message is specific to the Polygon bor client. The same error message in geth is “insufficient balance to pay for gas” after googling around for “0x0000000000000000000000000000000000000000” and “insufficient balance to pay for gas” we found this issue in the geth repo. Aha! So sending an eth_call with a gas price set has different behavior in different node clients! Now we had something to look for.

Digging in to the GSN relayer codebase we uncovered this line:

await this.relayHubInstance.calculateCharge(
  maxPossibleGasFactorReserve,
  relayTransactionRequest.relayRequest.relayData,
  { gasPrice: relayTransactionRequest.relayRequest.relayData.maxFeePerGas }
)

Getting rid of that pesky gasPrice was all it took and caused the same behavior across the board and we were up and running on Amoy.

We are happy to support our developer community in building onchain mobile apps by getting RallyTransact up on Amoy but we wish a few things had been easier in this migration:

It seems there are different versions of bor running on Amoy vs Polygon PoS mainnet. This makes the utility of a testnet a bit diminished as we can’t fully trust apples to apples comparisons between the two networks.

I really did miss etherscan functionality when trying to work with the Oklink explorer. Once again, differences between mainnet and testnets makes things harder for the dev community.

About RallyProtocol

RallyProtocol is an all-in-one web3 mobile toolkit that enables developers to create frictionless, end-to-end onchain experiences for native mobile apps. Through instant embedded wallet creation and gasless transactions, our mission is to give developers the power to create user-friendly mobile UX with fewer taps and zero web3 touchpoints.

Want to learn more? Dive more into our developer documentation here.