Introduce scheduler and use it for the democracy dispatch queue (#5412)

* Initial draft of the logic

* Build and tests

* Make work with new initialize infratructure.

* Update frame/scheduler/src/lib.rs

Co-Authored-By: Marcio Diaz <marcio.diaz@gmail.com>

* Update frame/scheduler/src/lib.rs

Co-Authored-By: Marcio Diaz <marcio.diaz@gmail.com>

* Update frame/scheduler/src/lib.rs

Co-Authored-By: Marcio Diaz <marcio.diaz@gmail.com>

* Update frame/scheduler/src/lib.rs

Co-Authored-By: Marcio Diaz <marcio.diaz@gmail.com>

* Fix test

* Update frame/scheduler/src/lib.rs

Co-Authored-By: Marcio Diaz <marcio.diaz@gmail.com>

* Rejig interface to make it more useful for democracy.

* Try to get democraxy module to make use of scheduler.

* Make democracy use scheduler.

* Use actual max weight for enactent

* Remove TODO

* Fix runtime build

* Minor cleanup

* Fix scheduler.

* Fix benchmarks

* Fix

* Fix

* Fix

* More bench fixes

* Fix

* Fix.

* Add more bench constants.

* Fix cancel_queued bench.

* Fix test comment.

* Update frame/scheduler/src/lib.rs

Co-Authored-By: Marcio Diaz <marcio.diaz@gmail.com>

Co-authored-by: Marcio Diaz <marcio.diaz@gmail.com>

substrate/frame/support/src/traits.rs

@@ -19,7 +19,7 @@
 //! NOTE: If you're looking for `parameter_types`, it has moved in to the top-level module.
 
 use sp_std::{prelude::*, result, marker::PhantomData, ops::Div, fmt::Debug};
-use codec::{FullCodec, Codec, Encode, Decode};
+use codec::{FullCodec, Codec, Encode, Decode, EncodeLike};
 use sp_core::u32_trait::Value as U32;
 use sp_runtime::{
 	RuntimeDebug,
@@ -1144,6 +1144,84 @@ pub trait OffchainWorker<BlockNumber> {
 	fn offchain_worker(_n: BlockNumber) {}
 }
 
+pub mod schedule {
+	use super::*;
+
+	/// Information relating to the period of a scheduled task. First item is the length of the
+	/// period and the second is the number of times it should be executed in total before the task
+	/// is considered finished and removed.
+	pub type Period<BlockNumber> = (BlockNumber, u32);
+
+	/// Priority with which a call is scheduled. It's just a linear amount with lowest values meaning
+	/// higher priority.
+	pub type Priority = u8;
+
+	/// The highest priority. We invert the value so that normal sorting will place the highest
+	/// priority at the beginning of the list.
+	pub const HIGHEST_PRORITY: Priority = 0;
+	/// Anything of this value or lower will definitely be scheduled on the block that they ask for, even
+	/// if it breaches the `MaximumWeight` limitation.
+	pub const HARD_DEADLINE: Priority = 63;
+	/// The lowest priority. Most stuff should be around here.
+	pub const LOWEST_PRORITY: Priority = 255;
+
+	/// A type that can be used as a scheduler.
+	pub trait Anon<BlockNumber, Call> {
+		/// An address which can be used for removing a scheduled task.
+		type Address: Codec + Clone + Eq + EncodeLike + Debug;
+
+		/// Schedule a one-off dispatch to happen at the beginning of some block in the future.
+		///
+		/// This is not named.
+		///
+		/// Infallible.
+		fn schedule(
+			when: BlockNumber,
+			maybe_periodic: Option<Period<BlockNumber>>,
+			priority: Priority,
+			call: Call
+		) -> Self::Address;
+
+		/// Cancel a scheduled task. If periodic, then it will cancel all further instances of that,
+		/// also.
+		///
+		/// Will return an error if the `address` is invalid.
+		///
+		/// NOTE: This guaranteed to work only *before* the point that it is due to be executed.
+		/// If it ends up being delayed beyond the point of execution, then it cannot be cancelled.
+		///
+		/// NOTE2: This will not work to cancel periodic tasks after their initial execution. For
+		/// that, you must name the task explicitly using the `Named` trait.
+		fn cancel(address: Self::Address) -> Result<(), ()>;
+	}
+
+	/// A type that can be used as a scheduler.
+	pub trait Named<BlockNumber, Call> {
+		/// An address which can be used for removing a scheduled task.
+		type Address: Codec + Clone + Eq + EncodeLike + sp_std::fmt::Debug;
+
+		/// Schedule a one-off dispatch to happen at the beginning of some block in the future.
+		///
+		/// - `id`: The identity of the task. This must be unique and will return an error if not.
+		fn schedule_named(
+			id: impl Encode,
+			when: BlockNumber,
+			maybe_periodic: Option<Period<BlockNumber>>,
+			priority: Priority,
+			call: Call
+		) -> Result<Self::Address, ()>;
+
+		/// Cancel a scheduled, named task. If periodic, then it will cancel all further instances
+		/// of that, also.
+		///
+		/// Will return an error if the `id` is invalid.
+		///
+		/// NOTE: This guaranteed to work only *before* the point that it is due to be executed.
+		/// If it ends up being delayed beyond the point of execution, then it cannot be cancelled.
+		fn cancel_named(id: impl Encode) -> Result<(), ()>;
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;