Trying to get OneSessionHandler to trigger in custom pallet

Question

I am trying to split our validators of a substrate chain into signing subgroups for key signing purposes. The OneSessionHandler gives me the perfect information I need to do this. On the end of each session I get to know have the validator set changed, what it changed from to what it changed to

It seems like the event only gets propagated to pallets that are contained in the key type. I do not need another key type however I want the event to get propagated to my custom pallet.

Is there a way to get this event triggered in my custom pallet or maybe am I thinking about this wrong and there is a better way to get this information on validator changes?

Edit: Additional info

When I add the pallet types manually and add in my custom pallet straight to the config of pallet session

type SessionHandler = (Grandpa, Babe, ImOnline, AuthorityDiscovery, CustomPallet);

I get a runtime panic

022-10-12 11:55:44 Substrate Node    
2022-10-12 11:55:44 ✌️  version 3.0.0-monthly-2021-10-9ec3350-x86_64-macos    
2022-10-12 11:55:44 ❤️  by Substrate DevHub <https://github. Fm/substrate-developer-hub>, 2017-2022    
2022-10-12 11:55:44 📋 Chain specification: Development    
2022-10-12 11:55:44 🏷  Node name: foregoing-seat-4750    
2022-10-12 11:55:44 👤 Role: AUTHORITY    
2022-10-12 11:55:44 💾 Database: RocksDb at /var/folders/gr/dhyt1nqs0_v9rcrz70wgf5lr0000gn/T/substrate30JPcz/chains/dev/db/full    
2022-10-12 11:55:44 ⛓  Native runtime: node-267 (substrate-node-1.tx2.au10)    

====================

Version: 3.0.0-monthly-2021-10-9ec3350-x86_64-macos

   0: backtrace::capture::Backtrace::new
   1: sp_panic_handler::set::{{closure}}
   2: std::panicking::rust_panic_with_hook
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/panicking.rs:702:17
   3: std::panicking::begin_panic_handler::{{closure}}
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/panicking.rs:586:13
   4: std::sys_common::backtrace::__rust_end_short_backtrace
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/sys_common/backtrace.rs:138:18
   5: rust_begin_unwind
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/panicking.rs:584:5
   6: core::panicking::panic_fmt
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/core/src/panicking.rs:142:14
   7: std::thread::local::LocalKey<T>::with
   8: sp_state_machine::basic::BasicExternalities::execute_with_storage
   9: <entropy_runtime::GenesisConfig as sp_runtime::BuildStorage>::assimilate_storage
  10: sp_runtime::BuildStorage::build_storage
  11: <sc_chain_spec::chain_spec::ChainSpec<G,E> as sp_runtime::BuildStorage>::build_storage
  12: sc_service::client::client::Client<B,E,Block,RA>::new
  13: sc_service::builder::new_full_parts
  14: entropy::service::new_partial
  15: entropy::service::new_full_base
  16: entropy::service::new_full
  17: std::thread::local::LocalKey<T>::with
  18: tokio::park::thread::CachedParkThread::block_on
  19: tokio::runtime::thread_pool::ThreadPool::block_on
  20: sc_cli::runner::Runner<C>::run_node_until_exit
  21: entropy::command::run
  22: entropy::main
  23: std::sys_common::backtrace::__rust_begin_short_backtrace
  24: std::rt::lang_start::{{closure}}
  25: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/core/src/ops/function.rs:280:13
      std::panicking::try::do_call
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/panicking.rs:492:40
      std::panicking::try
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/panicking.rs:456:19
      std::panic::catch_unwind
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/panic.rs:137:14
      std::rt::lang_start_internal::{{closure}}
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/rt.rs:128:48
      std::panicking::try::do_call
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/panicking.rs:492:40
      std::panicking::try
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/panicking.rs:456:19
      std::panic::catch_unwind
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/panic.rs:137:14
      std::rt::lang_start_internal
             at /rustc/29e4a9ee0253cd39e552a77f51f11f9a5f1c41e6/library/std/src/rt.rs:128:20
  26: _main


Thread 'main' panicked at 'Number of keys in session handler and session keys does not match', /Users/jesseabramowitz/.cargo/git/checkouts/substrate-7e08433d4c370a21/c89d524/frame/session/src/lib.rs:433

This is a bug. Please report it at:

    https://github.com/paritytech/substrate/issues/new

Answer 1

You should use the SessionManager:

struct SessionManager<I>(sp_std::marker::PhantomData<I>);

impl<I: pallet_session::SessionManager<ValidatorId>, ValidatorId> pallet_session::SessionManager<ValidatorId> for SessionManager<I> {
    fn new_session(new_index: SessionIndex) -> Option<Vec<ValidatorId>> {
        let new_session = I::new_session(new_index);

        if let Some(validators) = &new_session {
            // Note the validators
        }

        new_session
    }

    fn new_session_genesis(new_index: SessionIndex) -> Option<Vec<ValidatorId>> {
        I::new_session_genesis(new_index)
    }

    fn end_session(end_index: SessionIndex) { I::end_session(end_index); }
    fn start_session(start_index: SessionIndex) { I::start_session(start_index); }
}

Then you use this custom SessionManager and pass the original session manager I.