Fixing BABE epochs to change between blocks (#3583)

* always fetch epoch from runtime

* node integration tests don't test light nodes

* give stand-in full node a FULL role

* rejig babe APIs

* introduce next-epoch-descriptor type

* overhaul srml-BABE epoch logic

* ensure VRF outputs end up in the right epoch-randomness

* rewrite `do_initialize` to remove unnecessary loop

* begin accounting for next epoch in epoch function

* slots passes header to epoch_data

* pass slot_number to SlotWorker::epoch_data

* begin extracting epoch-change logic into its own module

* aux methods for block weight

* aux methods for genesis configuration

* comment-out most, refactor header-check pipeline

* mostly flesh out verifier again

* reinstantiate babe BlockImport implementation

* reinstate import-queue instantiation

* reintroduce slot-worker implementation

* reinstate pretty much all the rest

* move fork-choice logic to BlockImport

* fix some, but not all errors

* patch test-runtime

* make is_descendent of slightly more generic

* get skeleton compiling when passing is_descendent_of

* make descendent-of-builder more succinct

* restore ordering of authority_index / slot_number

* start fiddling with tests

* fix warnings

* improve initialization architecture and handle genesis

* tests use correct block-import

* fix BABE tests

* fix some compiler errors

* fix node-cli compilation

* all crates compile

* bump runtime versions and fix some warnings

* tweak fork-tree search implementation

* do backtracking search in fork-tree

* node-cli integration tests now work

* fix broken assumption in test_connectivity

* babe tests fail for the right reasons.

* test genesis epoch logic for epoch_changes

* test that epochs can change between blocks

* First BABE SRML test

* Testing infrastructure for BABE

Also includes a trivial additional test.

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* A little more test progress

* More work on BABE testing

* Try to get the tests working

* Implement `UintAuthorityId`-based test mocks

* Fix compilation errors

* Adjust to upstream changes

* Block numbers are ignored in BABE epoch calculation

* authority_index() should ignore invalid authorities

* Fix compile error

* Add tests that session transitions happen

* Check if BABE produces logs

It currently does not.

* Fix test suite

This was really nasty, due to a type confusion that showed up as an
off-by-1 buffer error.

* Add additional tests

Most of these were derived from the current output, so they are only
useful to guard against regressions.

* Make the tests more readable

Also bump impl_version.

* Fix excessive line width

* Remove unused imports

* Update srml/babe/src/lib.rs

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

* try to fix imports

* Fix build errors in test suite

* tests did not pass

* Try to get at least one digest to be output

Currently, the code emits either no digests (if I don’t call
`Session::rotate_session()` or two digests (if I do), which is wrong.

* More tests

They still don’t work, but this should help debugging.

* fix silly error

* Don’t even try to compile a broken test

* remove broken check_epoch test and add one for genesis epoch

* Check that the length of the pre-digests is correct

* Bump `impl_version`

* use epoch_for_descendent_of even for genesis

* account for competing block 1s

* finish srml-babe docs

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

* address grumbles

substrate/core/consensus/slots/src/lib.rs

@@ -77,8 +77,9 @@ pub trait SimpleSlotWorker<B: BlockT> {
 	/// A handle to a `BlockImport`.
 	fn block_import(&self) -> Arc<Mutex<Self::BlockImport>>;
 
-	/// Returns the epoch data necessary for authoring.
-	fn epoch_data(&self, block: &B::Hash) -> Result<Self::EpochData, consensus_common::Error>;
+	/// Returns the epoch data necessary for authoring. For time-dependent epochs,
+	/// use the provided slot number as a canonical source of time.
+	fn epoch_data(&self, header: &B::Header, slot_number: u64) -> Result<Self::EpochData, consensus_common::Error>;
 
 	/// Returns the number of authorities given the epoch data.
 	fn authorities_len(&self, epoch_data: &Self::EpochData) -> usize;
@@ -120,7 +121,7 @@ pub trait SimpleSlotWorker<B: BlockT> {
 		let (timestamp, slot_number, slot_duration) =
 			(slot_info.timestamp, slot_info.number, slot_info.duration);
 
-		let epoch_data = match self.epoch_data(&chain_head.hash()) {
+		let epoch_data = match self.epoch_data(&chain_head, slot_number) {
 			Ok(epoch_data) => epoch_data,
 			Err(err) => {
 				warn!("Unable to fetch epoch data at block {:?}: {:?}", chain_head.hash(), err);
@@ -359,7 +360,8 @@ impl SlotData for u64 {
 }
 
 /// A slot duration. Create with `get_or_compute`.
-// The internal member should stay private here.
+// The internal member should stay private here to maintain invariants of
+// `get_or_compute`.
 #[derive(Clone, Copy, Debug, Encode, Decode, Hash, PartialOrd, Ord, PartialEq, Eq)]
 pub struct SlotDuration<T>(T);