unknown import: `ethereum::ethereum.call` has not been defined

Summary

This happens when a file data source handler uses contract calls directly or is in the same AssemblyScript file as other handlers which use contract calls. If the contract call is done by a different handler in the same file, this can be fixed by putting the file data source handler in its own file, which is best practice.

Potential Fix

Contract calls in file data source handlers could be enabled by somehow passing the ethereum.call host fn when instantiating an offchain data source, relevant code:
https://github.com/graphprotocol/graph-node/blob/92670f27d93ed08a6e83805ad644d9004d4d77ee/runtime/wasm/src/host.rs#L137-L141

从 runtime/wasm/src/host.rs 约第 137-141 行开始，追踪 offchain 数据源是如何实例化的。确认缺失的 ethereum.call host 函数如何影响文件数据源处理程序，然后确定是否可以在实例化时传入该函数。当合约调用在所述处理程序场景中不再产生 unknown import 错误时，即表示完成。