Offchain worker custom extension not working

[Congyuwang]

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• This is not a support question.

Description of bug

Offchain worker configuration allows passing custom extensions in node/src/service.rs(as in substrate-template repo):

polkadot-sdk/substrate/client/offchain/src/lib.rs

Line 140 in cfc6cc6

custom_extensions: Box<dyn Fn(Block::Hash) -> Vec<Box<dyn Extension>> + Send>,

which are later registered:

polkadot-sdk/substrate/client/offchain/src/lib.rs

Line 261 in cfc6cc6

let custom_extensions = (*self.custom_extensions)(hash);

polkadot-sdk/substrate/client/offchain/src/lib.rs

Line 286 in cfc6cc6

custom_extensions.into_iter().for_each(|ext| runtime.register_extension(ext));

However, since extensions are registered using TypeId, they would overwrite each other since they are all of the same type Box<dyn Extension>.

polkadot-sdk/substrate/primitives/externalities/src/lib.rs

Lines 336 to 338 in cfc6cc6

	fn register_extension<T: Extension>(&mut self, ext: T) -> Result<(), Error> {
	self.register_extension_with_type_id(TypeId::of::<T>(), Box::new(ext))
	}

Moreover, I wouldn't be able to retrieve them later in host function using self.extension::<MyExtension>() since the registered TypeId is a generic Box<dyn Extension>.type_id().

Calling self.extension::<MyExtension>().expect("should be registered") just gets panic.

Steps to reproduce

No response

[Congyuwang]

I think custom extensions should perhaps pass something like:

custom_extensions: Box<dyn Fn(Block::Hash) -> Extensions>

instead, and somehow merge the Extensions to the extensions of the environment.