Introduce inherent digests (#2466)

* Introduce inherent digests

* Implement inherent digests

* fix silly error

* Implementation of inherent digests in BABE

All tests pass. There are still limitations:

1. The runtime strips out inherent digests, so BABE must re-add them.
2. The test runtime checks that it can re-compute all digests.  It
   can’t, so I had to comment out that test.

* Fix compilation and seal import

Seals were not imported correctly: the pre-digest was imported twice,
instead of both it and the seal being imported.  Also, other parts of
the code did not compile due to incomplete refactoring.

* Remove bogus assertion

* Fix testsuite compilation

* Remove unused import

* Fix compiler diagnostics

* Add inherent digest parameters to block constructors

This enforces that inherent digests are added first.

* Fixup Cargo.lock

* Fix build errors

* Re-add an incorrectly removed import

* Bump primitive-types version

* Update Cargo.lock

* Refactoring

* Use inherent digests for AuRa

They do reach the runtime, but get stripped.  I have not figured out
where.

* Fix compilation errors

* Fix compilation errors due to incorrect types

* Fix whitespace

Suggested-by: Tomasz Drwiega <tomasz@parity.io>

* Add preamble

Suggested-by: Tomasz Drwiega <tomasz@parity.io>

* Fix silly compile error

* Refactor pre-digest finding code into a separate function

* Remove unwanted assertion

It is too likely to bring down the entire blockchain.

Suggested-by: Tomasz Drwiega <tomasz@parity.io>

* Use `find_pre_digest` after runtime, too

Also, use `Member` trait rather than rolling our own requirements.

Suggested-by: Tomasz Drwiega <tomasz@parity.io>

* Fix various warnings

mostly due to upgrading the dependency on `error_chain`.

* Pre-digests nearly complete

This nearly completes the implementation of pre-runtime digests.

* `Seal2` → `Seal` and fix test suite

* Try to fix the storage error

* Try to fix storage (again)

* Fix tests

* Hopefully finish pre-runtime digests

The key is to pass *only* the pre-runtime digests to the runtime.  The
others must be stripped out by `initialize_block`.

* Fix silly typo

* Fix another silly mistake

* Remove unnecessary filtering of BABE pre-digests

We no longer get duplicate BABE pre-digests, so if they appear, the
header should be rejected outright.

* Update Cargo.lock files

* Reformatting

* Fix silly typo in inherent digest code

Also, revert `error.rs` files that contained calls to the `error_chain!`
macro.

* Try to keep the runtime from stripping pre-digests

Currently runs into the “Storage root must match that calculated”
assertion.

* Don’t compute storage root until storage changes are done.

Also, fix a compilation error.

* Fix compile-time error

* Fix compilation errors

* Fix more compile errors

* Hopefully it compiles this time…

* Fix compilation and add docs

* Prevent BABE from adding duplicate pre-runtime digests

Found by comparing with the AuRa code.  I also did some refactoring.

* Respond to review and fix some warnings

* Delete some dead code introduced earlier

* More dead code goes away

* `ref mut` → `&mut`

* Respond to review and fix some warnings

* Fix compilation error

* Remove unneeded `HashT` type parameter

Suggested-by: Robert Habermeier <robert@parity.io>

* Remove spurious #[allow(deprecated)]

* Document inherent digest parameter to `build_block`

* Delete `Simple` trait

It wasn’t needed

* delete wrongly added files

* Fix trait bounds

* Digest serialization tests

I also did some reformatting and cleanup.

* Apply suggestions from code review

Reformatting

Co-Authored-By: André Silva <andre.beat@gmail.com>

* Swap two arguments to `propose` and `propose_with`

Also, remove some needless unsafe code.

* Remove bogus `#![allow(deprecated)]` annotations

With the removal of the deprecated `Seal` variant, these are not needed.

* Add a missing `#[allow(deprecated)]` in the AuRa tests

* Fix silly compile error

* Fix silly compiler error

RLS did not tell me that I hadn’t fixed `babe/lib.rs`, so I missed it.

* Fixes made automatically by Cargo

substrate/srml/executive/src/lib.rs

@@ -80,7 +80,7 @@ use rstd::result;
 use primitives::traits::{
 	self, Header, Zero, One, Checkable, Applyable, CheckEqual, OnFinalize,
 	OnInitialize, Digest, NumberFor, Block as BlockT, OffchainWorker,
-	ValidateUnsigned,
+	ValidateUnsigned, DigestItem,
 };
 use srml_support::{Dispatchable, traits::MakePayment};
 use parity_codec::{Codec, Encode};
@@ -152,11 +152,13 @@ where
 {
 	/// Start the execution of a particular block.
 	pub fn initialize_block(header: &System::Header) {
-		Self::initialize_block_impl(header.number(), header.parent_hash(), header.extrinsics_root());
+		let mut digests = System::Digest::default();
+		header.digest().logs().iter().for_each(|d| if d.as_pre_runtime().is_some() { digests.push(d.clone()) });
+		Self::initialize_block_impl(header.number(), header.parent_hash(), header.extrinsics_root(), &digests);
 	}
 
-	fn initialize_block_impl(block_number: &System::BlockNumber, parent_hash: &System::Hash, extrinsics_root: &System::Hash) {
-		<system::Module<System>>::initialize(block_number, parent_hash, extrinsics_root);
+	fn initialize_block_impl(block_number: &System::BlockNumber, parent_hash: &System::Hash, extrinsics_root: &System::Hash, digest: &System::Digest) {
+		<system::Module<System>>::initialize(block_number, parent_hash, extrinsics_root, digest);
 		<AllModules as OnInitialize<System::BlockNumber>>::on_initialize(*block_number);
 	}
 
@@ -262,7 +264,8 @@ where
 			Payment::make_payment(sender, encoded_len).map_err(|_| internal::ApplyError::CantPay)?;
 
 			// AUDIT: Under no circumstances may this function panic from here onwards.
-
+			// FIXME: ensure this at compile-time (such as by not defining a panic function, forcing
+			// a linker error unless the compiler can prove it cannot be called).
 			// increment nonce in storage
 			<system::Module<System>>::inc_account_nonce(sender);
 		}