Add candidate info to OccupiedCore (#2134)

* guide: add candidate information to OccupiedCore

* add descriptor and hash to occupied core type

* guide: add candidate hash to inclusion

* runtime: return candidate info in core state

* bitfield signing: stop querying runtime as much

* minimize going to runtime in availability distribution

* fix availability distribution tests

* guide: remove para ID from Occupied core

* get all crates compiling

polkadot/roadmap/implementers-guide/src/node/availability/bitfield-signing.md

@@ -24,6 +24,6 @@ If not running as a validator, do nothing.
 
 - Begin by waiting a fixed period of time so availability distribution has the chance to make candidates available.
 - Determine our validator index `i`, the set of backed candidates pending availability in `r`, and which bit of the bitfield each corresponds to.
-- Start with an empty bitfield. For each bit in the bitfield, if there is a candidate pending availability, query the [Availability Store](../utility/availability-store.md) for whether we have the availability chunk for our validator index.
+- Start with an empty bitfield. For each bit in the bitfield, if there is a candidate pending availability, query the [Availability Store](../utility/availability-store.md) for whether we have the availability chunk for our validator index. The `OccupiedCore` struct contains the candidate hash so the full candidate does not need to be fetched from runtime.
 - For all chunks we have, set the corresponding bit in the bitfield.
 - Sign the bitfield and dispatch a `BitfieldDistribution::DistributeBitfield` message.

polkadot/roadmap/implementers-guide/src/runtime-api/availability-cores.md

@@ -12,8 +12,7 @@ This is all the information that a validator needs about scheduling for the curr
 
 ```rust
 struct OccupiedCore {
-    /// The ID of the para occupying the core.
-    para_id: ParaId,
+    // NOTE: this has no ParaId as it can be deduced from the candidate descriptor.
     /// If this core is freed by availability, this is the assignment that is next up on this
     /// core, if any. None if there is nothing queued for this core.
     next_up_on_available: Option<ScheduledCore>,
@@ -31,6 +30,10 @@ struct OccupiedCore {
     availability: Bitfield,
     /// The group assigned to distribute availability pieces of this candidate.
     group_responsible: GroupIndex,
+    /// The hash of the candidate occupying the core.
+    candidate_hash: CandidateHash,
+    /// The descriptor of the candidate occupying the core.
+    candidate_descriptor: CandidateDescriptor,
 }
 
 struct ScheduledCore {

polkadot/roadmap/implementers-guide/src/runtime/inclusion.md

@@ -14,6 +14,7 @@ struct AvailabilityBitfield {
 
 struct CandidatePendingAvailability {
   core: CoreIndex, // availability core
+  hash: CandidateHash,
   descriptor: CandidateDescriptor,
   availability_votes: Bitfield, // one bit per validator.
   relay_parent_number: BlockNumber, // number of the relay-parent.