How are Solana transaction fees involving invoke calculated?

Question

In the code example taken from here we see the following invocation:

    invoke(
        &system_instruction::create_account(
            &user.key,
            &power.key,
            lamports_required,
            account_span as u64,
            program_id,
        ),
        &[
            user.clone(), power.clone(), system_program.clone()
        ]
    )?;

Question is, who signs for this transaction? My guess will be the original signer who signed for the transaction that called the program from which the cross program invocation is being made. Is this correct?

If so, how then is the transaction fee for this CPI calculated? And who pays for it?

Answer 1

looking at the code it looks like the new program account and the users wallet will be signers and the owner will be the program. So the keypair calling it will be able to change the account data I think. Paying for the rent exemption of the new account (lamports field) and the transaction fees will be also the caller of the transaction. In this case "user"

  public static TransactionInstruction CreateAccount(
  PublicKey fromAccount,
  PublicKey newAccountPublicKey,
  ulong lamports,
  ulong space,
  PublicKey programId)
{
  List<AccountMeta> accountMetaList = new List<AccountMeta>()
  {
    AccountMeta.Writable(fromAccount, true),
    AccountMeta.Writable(newAccountPublicKey, true)
  };
  return new TransactionInstruction()
  {
    ProgramId = SystemProgram.ProgramIdKey.KeyBytes,
    Keys = (IList<AccountMeta>) accountMetaList,
    Data = SystemProgramData.EncodeCreateAccountData(programId, lamports, space)
  };
}

internal static byte[] EncodeCreateAccountData(PublicKey owner, ulong lamports, ulong space)
{
  byte[] data = new byte[52];
  data.WriteU32(0U, 0);
  data.WriteU64(lamports, 4);
  data.WriteU64(space, 12);
  data.WritePubKey(owner, 20);
  return data;
}