FRAME: Create TransactionExtension as a replacement for SignedExtension (#2280)

Closes #2160

First part of [Extrinsic
Horizon](https://github.com/paritytech/polkadot-sdk/issues/2415)

Introduces a new trait `TransactionExtension` to replace
`SignedExtension`. Introduce the idea of transactions which obey the
runtime's extensions and have according Extension data (né Extra data)
yet do not have hard-coded signatures.

Deprecate the terminology of "Unsigned" when used for
transactions/extrinsics owing to there now being "proper" unsigned
transactions which obey the extension framework and "old-style" unsigned
which do not. Instead we have __*General*__ for the former and
__*Bare*__ for the latter. (Ultimately, the latter will be phased out as
a type of transaction, and Bare will only be used for Inherents.)

Types of extrinsic are now therefore:
- Bare (no hardcoded signature, no Extra data; used to be known as
"Unsigned")
- Bare transactions (deprecated): Gossiped, validated with
`ValidateUnsigned` (deprecated) and the `_bare_compat` bits of
`TransactionExtension` (deprecated).
  - Inherents: Not gossiped, validated with `ProvideInherent`.
- Extended (Extra data): Gossiped, validated via `TransactionExtension`.
  - Signed transactions (with a hardcoded signature).
  - General transactions (without a hardcoded signature).

`TransactionExtension` differs from `SignedExtension` because:
- A signature on the underlying transaction may validly not be present.
- It may alter the origin during validation.
- `pre_dispatch` is renamed to `prepare` and need not contain the checks
present in `validate`.
- `validate` and `prepare` is passed an `Origin` rather than a
`AccountId`.
- `validate` may pass arbitrary information into `prepare` via a new
user-specifiable type `Val`.
- `AdditionalSigned`/`additional_signed` is renamed to
`Implicit`/`implicit`. It is encoded *for the entire transaction* and
passed in to each extension as a new argument to `validate`. This
facilitates the ability of extensions to acts as underlying crypto.

There is a new `DispatchTransaction` trait which contains only default
function impls and is impl'ed for any `TransactionExtension` impler. It
provides several utility functions which reduce some of the tedium from
using `TransactionExtension` (indeed, none of its regular functions
should now need to be called directly).

Three transaction version discriminator ("versions") are now
permissible:
- 0b000000100: Bare (used to be called "Unsigned"): contains Signature
or Extra (extension data). After bare transactions are no longer
supported, this will strictly identify an Inherents only.
- 0b100000100: Old-school "Signed" Transaction: contains Signature and
Extra (extension data).
- 0b010000100: New-school "General" Transaction: contains Extra
(extension data), but no Signature.

For the New-school General Transaction, it becomes trivial for authors
to publish extensions to the mechanism for authorizing an Origin, e.g.
through new kinds of key-signing schemes, ZK proofs, pallet state,
mutations over pre-authenticated origins or any combination of the
above.

## Code Migration

### NOW: Getting it to build

Wrap your `SignedExtension`s in `AsTransactionExtension`. This should be
accompanied by renaming your aggregate type in line with the new
terminology. E.g. Before:

```rust
/// The SignedExtension to the basic transaction logic.
pub type SignedExtra = (
	/* snip */
	MySpecialSignedExtension,
);
/// Unchecked extrinsic type as expected by this runtime.
pub type UncheckedExtrinsic =
	generic::UncheckedExtrinsic<Address, RuntimeCall, Signature, SignedExtra>;
```

After:

```rust
/// The extension to the basic transaction logic.
pub type TxExtension = (
	/* snip */
	AsTransactionExtension<MySpecialSignedExtension>,
);
/// Unchecked extrinsic type as expected by this runtime.
pub type UncheckedExtrinsic =
	generic::UncheckedExtrinsic<Address, RuntimeCall, Signature, TxExtension>;
```

You'll also need to alter any transaction building logic to add a
`.into()` to make the conversion happen. E.g. Before:

```rust
fn construct_extrinsic(
		/* snip */
) -> UncheckedExtrinsic {
	let extra: SignedExtra = (
		/* snip */
		MySpecialSignedExtension::new(/* snip */),
	);
	let payload = SignedPayload::new(call.clone(), extra.clone()).unwrap();
	let signature = payload.using_encoded(|e| sender.sign(e));
	UncheckedExtrinsic::new_signed(
		/* snip */
		Signature::Sr25519(signature),
		extra,
	)
}
```

After:

```rust
fn construct_extrinsic(
		/* snip */
) -> UncheckedExtrinsic {
	let tx_ext: TxExtension = (
		/* snip */
		MySpecialSignedExtension::new(/* snip */).into(),
	);
	let payload = SignedPayload::new(call.clone(), tx_ext.clone()).unwrap();
	let signature = payload.using_encoded(|e| sender.sign(e));
	UncheckedExtrinsic::new_signed(
		/* snip */
		Signature::Sr25519(signature),
		tx_ext,
	)
}
```

### SOON: Migrating to `TransactionExtension`

Most `SignedExtension`s can be trivially converted to become a
`TransactionExtension`. There are a few things to know.

- Instead of a single trait like `SignedExtension`, you should now
implement two traits individually: `TransactionExtensionBase` and
`TransactionExtension`.
- Weights are now a thing and must be provided via the new function `fn
weight`.

#### `TransactionExtensionBase`

This trait takes care of anything which is not dependent on types
specific to your runtime, most notably `Call`.

- `AdditionalSigned`/`additional_signed` is renamed to
`Implicit`/`implicit`.
- Weight must be returned by implementing the `weight` function. If your
extension is associated with a pallet, you'll probably want to do this
via the pallet's existing benchmarking infrastructure.

#### `TransactionExtension`

Generally:
- `pre_dispatch` is now `prepare` and you *should not reexecute the
`validate` functionality in there*!
- You don't get an account ID any more; you get an origin instead. If
you need to presume an account ID, then you can use the trait function
`AsSystemOriginSigner::as_system_origin_signer`.
- You get an additional ticket, similar to `Pre`, called `Val`. This
defines data which is passed from `validate` into `prepare`. This is
important since you should not be duplicating logic from `validate` to
`prepare`, you need a way of passing your working from the former into
the latter. This is it.
- This trait takes two type parameters: `Call` and `Context`. `Call` is
the runtime call type which used to be an associated type; you can just
move it to become a type parameter for your trait impl. `Context` is not
currently used and you can safely implement over it as an unbounded
type.
- There's no `AccountId` associated type any more. Just remove it.

Regarding `validate`:
- You get three new parameters in `validate`; all can be ignored when
migrating from `SignedExtension`.
- `validate` returns a tuple on success; the second item in the tuple is
the new ticket type `Self::Val` which gets passed in to `prepare`. If
you use any information extracted during `validate` (off-chain and
on-chain, non-mutating) in `prepare` (on-chain, mutating) then you can
pass it through with this. For the tuple's last item, just return the
`origin` argument.

Regarding `prepare`:
- This is renamed from `pre_dispatch`, but there is one change:
- FUNCTIONALITY TO VALIDATE THE TRANSACTION NEED NOT BE DUPLICATED FROM
`validate`!!
- (This is different to `SignedExtension` which was required to run the
same checks in `pre_dispatch` as in `validate`.)

Regarding `post_dispatch`:
- Since there are no unsigned transactions handled by
`TransactionExtension`, `Pre` is always defined, so the first parameter
is `Self::Pre` rather than `Option<Self::Pre>`.

If you make use of `SignedExtension::validate_unsigned` or
`SignedExtension::pre_dispatch_unsigned`, then:
- Just use the regular versions of these functions instead.
- Have your logic execute in the case that the `origin` is `None`.
- Ensure your transaction creation logic creates a General Transaction
rather than a Bare Transaction; this means having to include all
`TransactionExtension`s' data.
- `ValidateUnsigned` can still be used (for now) if you need to be able
to construct transactions which contain none of the extension data,
however these will be phased out in stage 2 of the Transactions Horizon,
so you should consider moving to an extension-centric design.

## TODO

- [x] Introduce `CheckSignature` impl of `TransactionExtension` to
ensure it's possible to have crypto be done wholly in a
`TransactionExtension`.
- [x] Deprecate `SignedExtension` and move all uses in codebase to
`TransactionExtension`.
  - [x] `ChargeTransactionPayment`
  - [x] `DummyExtension`
  - [x] `ChargeAssetTxPayment` (asset-tx-payment)
  - [x] `ChargeAssetTxPayment` (asset-conversion-tx-payment)
  - [x] `CheckWeight`
  - [x] `CheckTxVersion`
  - [x] `CheckSpecVersion`
  - [x] `CheckNonce`
  - [x] `CheckNonZeroSender`
  - [x] `CheckMortality`
  - [x] `CheckGenesis`
  - [x] `CheckOnlySudoAccount`
  - [x] `WatchDummy`
  - [x] `PrevalidateAttests`
  - [x] `GenericSignedExtension`
  - [x] `SignedExtension` (chain-polkadot-bulletin)
  - [x] `RefundSignedExtensionAdapter`
- [x] Implement `fn weight` across the board.
- [ ] Go through all pre-existing extensions which assume an account
signer and explicitly handle the possibility of another kind of origin.
- [x] `CheckNonce` should probably succeed in the case of a non-account
origin.
- [x] `CheckNonZeroSender` should succeed in the case of a non-account
origin.
- [x] `ChargeTransactionPayment` and family should fail in the case of a
non-account origin.
  - [ ] 
- [x] Fix any broken tests.

---------

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substrate/primitives/runtime/src/generic/checked_extrinsic.rs

@@ -18,81 +18,115 @@
 //! Generic implementation of an extrinsic that has passed the verification
 //! stage.
 
+use codec::Encode;
+
 use crate::{
 	traits::{
-		self, DispatchInfoOf, Dispatchable, MaybeDisplay, Member, PostDispatchInfoOf,
-		SignedExtension, ValidateUnsigned,
+		self, transaction_extension::TransactionExtension, DispatchInfoOf, DispatchTransaction,
+		Dispatchable, MaybeDisplay, Member, PostDispatchInfoOf, ValidateUnsigned,
 	},
 	transaction_validity::{TransactionSource, TransactionValidity},
 };
 
+/// The kind of extrinsic this is, including any fields required of that kind. This is basically
+/// the full extrinsic except the `Call`.
+#[derive(PartialEq, Eq, Clone, sp_core::RuntimeDebug)]
+pub enum ExtrinsicFormat<AccountId, Extension> {
+	/// Extrinsic is bare; it must pass either the bare forms of `TransactionExtension` or
+	/// `ValidateUnsigned`, both deprecated, or alternatively a `ProvideInherent`.
+	Bare,
+	/// Extrinsic has a default `Origin` of `Signed(AccountId)` and must pass all
+	/// `TransactionExtension`s regular checks and includes all extension data.
+	Signed(AccountId, Extension),
+	/// Extrinsic has a default `Origin` of `None` and must pass all `TransactionExtension`s.
+	/// regular checks and includes all extension data.
+	General(Extension),
+}
+
+// TODO: Rename ValidateUnsigned to ValidateInherent
+// TODO: Consider changing ValidateInherent API to avoid need for duplicating validate
+//   code into pre_dispatch (rename that to `prepare`).
+// TODO: New extrinsic type corresponding to `ExtrinsicFormat::General`, which is
+//   unsigned but includes extension data.
+// TODO: Move usage of `signed` to `format`:
+// - Inherent instead of None.
+// - Signed(id, extension) instead of Some((id, extra)).
+// - Introduce General(extension) for one without a signature.
+
 /// Definition of something that the external world might want to say; its existence implies that it
 /// has been checked and is good, particularly with regards to the signature.
 ///
 /// This is typically passed into [`traits::Applyable::apply`], which should execute
 /// [`CheckedExtrinsic::function`], alongside all other bits and bobs.
 #[derive(PartialEq, Eq, Clone, sp_core::RuntimeDebug)]
-pub struct CheckedExtrinsic<AccountId, Call, Extra> {
+pub struct CheckedExtrinsic<AccountId, Call, Extension> {
 	/// Who this purports to be from and the number of extrinsics have come before
 	/// from the same signer, if anyone (note this is not a signature).
-	pub signed: Option<(AccountId, Extra)>,
+	pub format: ExtrinsicFormat<AccountId, Extension>,
 
 	/// The function that should be called.
 	pub function: Call,
 }
 
-impl<AccountId, Call, Extra, RuntimeOrigin> traits::Applyable
-	for CheckedExtrinsic<AccountId, Call, Extra>
+impl<AccountId, Call, Extension, RuntimeOrigin> traits::Applyable
+	for CheckedExtrinsic<AccountId, Call, Extension>
 where
 	AccountId: Member + MaybeDisplay,
-	Call: Member + Dispatchable<RuntimeOrigin = RuntimeOrigin>,
-	Extra: SignedExtension<AccountId = AccountId, Call = Call>,
+	Call: Member + Dispatchable<RuntimeOrigin = RuntimeOrigin> + Encode,
+	Extension: TransactionExtension<Call, ()>,
 	RuntimeOrigin: From<Option<AccountId>>,
 {
 	type Call = Call;
 
-	fn validate<U: ValidateUnsigned<Call = Self::Call>>(
+	fn validate<I: ValidateUnsigned<Call = Self::Call>>(
 		&self,
-		// TODO [#5006;ToDr] should source be passed to `SignedExtension`s?
-		// Perhaps a change for 2.0 to avoid breaking too much APIs?
 		source: TransactionSource,
 		info: &DispatchInfoOf<Self::Call>,
 		len: usize,
 	) -> TransactionValidity {
-		if let Some((ref id, ref extra)) = self.signed {
-			Extra::validate(extra, id, &self.function, info, len)
-		} else {
-			let valid = Extra::validate_unsigned(&self.function, info, len)?;
-			let unsigned_validation = U::validate_unsigned(source, &self.function)?;
-			Ok(valid.combine_with(unsigned_validation))
+		match self.format {
+			ExtrinsicFormat::Bare => {
+				let inherent_validation = I::validate_unsigned(source, &self.function)?;
+				#[allow(deprecated)]
+				let legacy_validation = Extension::validate_bare_compat(&self.function, info, len)?;
+				Ok(legacy_validation.combine_with(inherent_validation))
+			},
+			ExtrinsicFormat::Signed(ref signer, ref extension) => {
+				let origin = Some(signer.clone()).into();
+				extension.validate_only(origin, &self.function, info, len).map(|x| x.0)
+			},
+			ExtrinsicFormat::General(ref extension) =>
+				extension.validate_only(None.into(), &self.function, info, len).map(|x| x.0),
 		}
 	}
 
-	fn apply<U: ValidateUnsigned<Call = Self::Call>>(
+	fn apply<I: ValidateUnsigned<Call = Self::Call>>(
 		self,
 		info: &DispatchInfoOf<Self::Call>,
 		len: usize,
 	) -> crate::ApplyExtrinsicResultWithInfo<PostDispatchInfoOf<Self::Call>> {
-		let (maybe_who, maybe_pre) = if let Some((id, extra)) = self.signed {
-			let pre = Extra::pre_dispatch(extra, &id, &self.function, info, len)?;
-			(Some(id), Some(pre))
-		} else {
-			Extra::pre_dispatch_unsigned(&self.function, info, len)?;
-			U::pre_dispatch(&self.function)?;
-			(None, None)
-		};
-		let res = self.function.dispatch(RuntimeOrigin::from(maybe_who));
-		let post_info = match res {
-			Ok(info) => info,
-			Err(err) => err.post_info,
-		};
-		Extra::post_dispatch(
-			maybe_pre,
-			info,
-			&post_info,
-			len,
-			&res.map(|_| ()).map_err(|e| e.error),
-		)?;
-		Ok(res)
+		match self.format {
+			ExtrinsicFormat::Bare => {
+				I::pre_dispatch(&self.function)?;
+				// TODO: Remove below once `pre_dispatch_unsigned` is removed from `LegacyExtension`
+				//   or `LegacyExtension` is removed.
+				#[allow(deprecated)]
+				Extension::validate_bare_compat(&self.function, info, len)?;
+				#[allow(deprecated)]
+				Extension::pre_dispatch_bare_compat(&self.function, info, len)?;
+				let res = self.function.dispatch(None.into());
+				let post_info = res.unwrap_or_else(|err| err.post_info);
+				let pd_res = res.map(|_| ()).map_err(|e| e.error);
+				// TODO: Remove below once `pre_dispatch_unsigned` is removed from `LegacyExtension`
+				//   or `LegacyExtension` is removed.
+				#[allow(deprecated)]
+				Extension::post_dispatch_bare_compat(info, &post_info, len, &pd_res)?;
+				Ok(res)
+			},
+			ExtrinsicFormat::Signed(signer, extension) =>
+				extension.dispatch_transaction(Some(signer).into(), self.function, info, len),
+			ExtrinsicFormat::General(extension) =>
+				extension.dispatch_transaction(None.into(), self.function, info, len),
+		}
 	}
 }

substrate/primitives/runtime/src/generic/mod.rs

@@ -29,9 +29,9 @@ mod unchecked_extrinsic;
 
 pub use self::{
 	block::{Block, BlockId, SignedBlock},
-	checked_extrinsic::CheckedExtrinsic,
+	checked_extrinsic::{CheckedExtrinsic, ExtrinsicFormat},
 	digest::{Digest, DigestItem, DigestItemRef, OpaqueDigestItemId},
 	era::{Era, Phase},
 	header::Header,
-	unchecked_extrinsic::{SignedPayload, UncheckedExtrinsic},
+	unchecked_extrinsic::{Preamble, SignedPayload, UncheckedExtrinsic},
 };