Why is my view function running out of gas when reading from mapping/HashMap?

Question

I have the following vyper code:

address_to_addresses: public(HashMap[address, address[2])

Vyper auto generates a view function called address_to_addresses(arg0: address, arg1: uint256) -> address that allows you to read the value of the public variable.

The abi for this function looks like this:

"name": "address_to_addresses",
"outputs": [
  {
    "type": "address",
    "name": ""
  }
],
"inputs": [
  {
    "type": "address",
    "name": "arg0"
  },
  {
    "type": "uint256",
    "name": "arg1"
  }
],
"stateMutability": "view",
"constant": true

But when I execute the following using ethersJS:

myContract.address_to_addresses(<someAddress>, 0)

I get a transaction revert error that says I have run out of gas:

Error: VM Exception while processing transaction: out of gas

How is it possible that I can be running out of gas when I am calling a view function? How can I call this function without "running out of gas"?

Answer 1

It turns out this was a small issue with EthersJS.

The estimated gas is not input correctly. By manually defining the gas limit, it works:

myContract.address_to_addresses(<someAddress>, 0, { gasLimit: 60000 });

It's unclear why this is. If I set the gas limit to the actual cost of the function as defined in the abi (1654) then it fails - I have to set it to something quite large for it to work. So perhaps this is not an issue with EthersJS but an issue with truffle's abi generation being incorrect.

Since no gas is actually used in this transaction, again, i'm not sure why this is even needed.