Calling std-based host function from a pallet

Question

Based on:

• Use std only rust library to extend substrate runtime interface？
• How to understand [runtime-interface] and `Externalities`?

I understand that I should be able to:

1. have a crate that is not wasm/no-std friendly, e.g. relies on std library
2. put its functionalities under a trait with #[sp_runtime_interface::runtime_interface] attribute
3. register corresponding host function in the node's executor
4. call the functionality from a pallet

However, when I'm trying to do so, I'm getting the classical error:

  error[E0152]: duplicate lang item in crate `std` (which `serde` depends on): `panic_impl`.
    |
    = note: the lang item is first defined in crate `sp_io` (which `frame_support` depends on)
    = note: first definition in `sp_io` loaded from /home/ec2-user/remote-builds/8351322047856898080/target/release/wbuild/aleph-runtime/target/wasm32-unknown-unknown/release/deps/libsp_io-adc573607c02c939.rmeta
    = note: second definition in `std` loaded from /home/ec2-user/.rustup/toolchains/nightly-2022-10-30-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libstd-013c81c18ec4954a.rlib

I'm pretty sure that I have set up all default-features and /std features correctly. The problem is gone when I remove the std-based dependencies from my crate, but that's the thing - I want to use them in my runtime.

Maybe the problem is in the way I should call the host function from the pallet. I have put my crate as a dependency and I'm calling it with <crate_name>::<snake_case_trait_name>::<trait_method_name>. Is this the correct way or should I use some magic with externalities etc. I couldn't find any example in the Substrate repo, apart from statement_store, which is actually is no-std/wasm compatible, so it doesn't help.

Answer 1

Substrate provide sp-std and sp-io for std calls in a no_std environment. By the error I can understand you are using std instead of sp-std or sp-io so it is giving you the error duplicate lang item in crate. I hope this helps.

Answer 2

If you have registered the HostFunctions in your host code, you should just include the sp_your_runtime_interface in your pallet to call the host function. Including your std-required crate in pallet will cause importing std.