From a parachain, how to get the current relay chain block number?

Great question!

You can include the following in your parachain's runtime:

use cumulus_pallet_parachain_system::RelaychainBlockNumberProvider; 

Then, in the parachain's runtime, define the following in your pallet's config:

type RelayChainBlockNumber = RelaychainBlockNumberProvider<Runtime>; 

In your pallet you will have to import BlockNumberProvider:

use sp_runtime::traits::BlockNumberProvider; 

Define it in your pallet's config:

type RelayChainBlockNumber: BlockNumberProvider<BlockNumber = Self::BlockNumber>; 

And, finally, you can get the relaychain's block number in your pallet:

let current_relaychain_block_number = T::RelayChainBlockNumber::current_block_number(); log::info!("The current relaychain block number is {:?}", current_relaychain_block_number); 

Here is an example:

• https://github.com/brunopgalvao/cumulus-001/commit/033bebb2eeaa7e29fac7d7dc25558cc80e6d9b2f