Refactor: SelectView = BlockNo × TiebreakerView (#1591)

This PR does not change any behavior; most of the diff is purely
mechanical.

Previously, the `SelectView`s of all `ConsensusProtocol`s contained the
`BlockNo` as the most important criterion for comparing chains, with
only tiebreaking behavior (among chains of equal length) being different
across different protocols.

This PR makes this explicit: `SelectView` is now longer an associated
type family of `ConsensusProtocol`, but rather an ordinary data type
```haskell
data SelectView p = SelectView
  { svBlockNo :: !BlockNo
  , svTiebreakerView :: !(TiebreakerView p)
  }
```
where `TiebreakerView` is now an associated type family of
`ConsensusProtocol`.

This PR also removes the `WithBlockNo` type that the HFC was already
using to always attach a `BlockNo` to its `HardForkSelectView`. The code
can be simpler now that `SelectView`s always automatically contain an
explicit `BlockNo`.

Also see
#1542 (comment)

## Primary motivation

The main motivation for this is to enable weighted chain selection in
Peras (tweag/cardano-peras#62), which can now
be easily modeled by adding the weight of a chain to the block number
component, at least morally; we probably want to eventually have more
type safety here to make it hard to "forget" to account for the weight,
such as defining
```haskell
data WeightedSelectView p = WeightedSelectView
  { wsvWeight :: !PerasWeight
  , wsvTiebreakerView :: !(TiebreakerView p)
  }
```

If the only goal were to minimize the diff, then it would also be
possible to add a function `(BlockNo -> BlockNo) -> SelectView p ->
SelectView p`, but it seems more honest to "properly" extract out the
block number.

Author: amesgen

ouroboros-consensus-cardano/changelog.d/20250716_190309_alexander.esgen_tiebreaker_view.md

@@ -0,0 +1,5 @@
+### Patch
+
+- Adapted to changes related to `SelectView`.
+
+  Concretely, this changes the structure of `SelectView (BlockProtocol (CardanoBlock c))`, but it still contains the same data as before.

ouroboros-consensus-cardano/src/byron/Ouroboros/Consensus/Byron/Ledger/PBFT.hs

@@ -74,7 +74,7 @@ instance BlockSupportsProtocol ByronBlock where
    where
     epochSlots = byronEpochSlots cfg
 
-  selectView _ = mkPBftSelectView
+  tiebreakerView _ = mkPBftTiebreakerView
 
 toPBftLedgerView :: Delegation.Map -> PBftLedgerView PBftByronCrypto
 toPBftLedgerView = PBftLedgerView . Delegation.unMap

ouroboros-consensus-cardano/src/ouroboros-consensus-cardano/Ouroboros/Consensus/Cardano/CanHardFork.hs

@@ -82,6 +82,7 @@ import Ouroboros.Consensus.Protocol.PBFT.State (PBftState)
 import qualified Ouroboros.Consensus.Protocol.PBFT.State as PBftState
 import Ouroboros.Consensus.Protocol.Praos (Praos)
 import qualified Ouroboros.Consensus.Protocol.Praos as Praos
+import Ouroboros.Consensus.Protocol.Praos.Common (PraosTiebreakerView)
 import Ouroboros.Consensus.Protocol.TPraos
 import qualified Ouroboros.Consensus.Protocol.TPraos as TPraos
 import Ouroboros.Consensus.Shelley.HFEras ()
@@ -164,10 +165,10 @@ instance CardanoHardForkConstraints c => CanHardFork (CardanoEras c) where
       }
   hardForkChainSel =
     -- Byron <-> Shelley, ...
-    TCons (SOP.hpure CompareBlockNo)
+    TCons (SOP.hpure NoTiebreakerAcrossEras)
     -- Inter-Shelley-based
     $
-      Tails.hcpure (Proxy @(HasPraosSelectView c)) CompareSameSelectView
+      Tails.hcpure (Proxy @(HasPraosTiebreakerView c)) SameTiebreakerAcrossEras
   hardForkInjectTxs =
     PCons (ignoringBoth $ Pair2 cannotInjectTx cannotInjectValidatedTx)
       $ PCons
@@ -233,8 +234,8 @@ instance CardanoHardForkConstraints c => CanHardFork (CardanoEras c) where
     fromAlonzo x = fromConway $ ConwayMeasure x mempty
     fromConway x = x
 
-class SelectView (BlockProtocol blk) ~ PraosChainSelectView c => HasPraosSelectView c blk
-instance SelectView (BlockProtocol blk) ~ PraosChainSelectView c => HasPraosSelectView c blk
+class TiebreakerView (BlockProtocol blk) ~ PraosTiebreakerView c => HasPraosTiebreakerView c blk
+instance TiebreakerView (BlockProtocol blk) ~ PraosTiebreakerView c => HasPraosTiebreakerView c blk
 
 {-------------------------------------------------------------------------------
   Translation from Byron to Shelley

ouroboros-consensus-cardano/src/shelley/Ouroboros/Consensus/Shelley/Ledger/Block.hs

@@ -71,11 +71,8 @@ import Ouroboros.Consensus.HardFork.Combinator
   )
 import Ouroboros.Consensus.HeaderValidation
 import Ouroboros.Consensus.Protocol.Abstract
-  ( ChainDepState
-  , SelectView
-  )
 import Ouroboros.Consensus.Protocol.Praos.Common
-  ( PraosChainSelectView
+  ( PraosTiebreakerView
   )
 import Ouroboros.Consensus.Protocol.Signed (SignedHeader)
 import Ouroboros.Consensus.Shelley.Eras
@@ -118,7 +115,7 @@ class
   , Show (SL.TranslationContext era)
   , -- Currently the chain select view is identical
     -- Era and proto crypto must coincide
-    SelectView proto ~ PraosChainSelectView (ProtoCrypto proto)
+    TiebreakerView proto ~ PraosTiebreakerView (ProtoCrypto proto)
   , -- Need to be able to sign the protocol header
     SignedHeader (ShelleyProtocolHeader proto)
   , -- ChainDepState needs to be serialisable

ouroboros-consensus-cardano/src/shelley/Ouroboros/Consensus/Shelley/Ledger/Protocol.hs

@@ -11,8 +11,8 @@ module Ouroboros.Consensus.Shelley.Ledger.Protocol () where
 
 import qualified Cardano.Ledger.Shelley.API as SL
 import Ouroboros.Consensus.Block
+import Ouroboros.Consensus.Protocol.Praos.Common
 import Ouroboros.Consensus.Protocol.Signed
-import Ouroboros.Consensus.Protocol.TPraos
 import Ouroboros.Consensus.Shelley.Ledger.Block
 import Ouroboros.Consensus.Shelley.Ledger.Config (BlockConfig (..))
 import Ouroboros.Consensus.Shelley.Protocol.Abstract
@@ -30,13 +30,12 @@ import Ouroboros.Consensus.Shelley.Protocol.Abstract
 instance ShelleyCompatible proto era => BlockSupportsProtocol (ShelleyBlock proto era) where
   validateView _cfg = protocolHeaderView @proto . shelleyHeaderRaw
 
-  selectView _ hdr@(ShelleyHeader shdr _) =
-    PraosChainSelectView
-      { csvChainLength = blockNo hdr
-      , csvSlotNo = blockSlot hdr
-      , csvIssuer = hdrIssuer
-      , csvIssueNo = pHeaderIssueNo shdr
-      , csvTieBreakVRF = pTieBreakVRFValue shdr
+  tiebreakerView _ hdr@(ShelleyHeader shdr _) =
+    PraosTiebreakerView
+      { ptvSlotNo = blockSlot hdr
+      , ptvIssuer = hdrIssuer
+      , ptvIssueNo = pHeaderIssueNo shdr
+      , ptvTieBreakVRF = pTieBreakVRFValue shdr
       }
    where
     hdrIssuer :: SL.VKey 'SL.BlockIssuer

ouroboros-consensus-cardano/src/unstable-cardano-testlib/Test/ThreadNet/Infra/ShelleyBasedHardFork.hs

@@ -271,7 +271,7 @@ instance
         , translateTxOutWith = SL.upgradeTxOut
         }
 
-  hardForkChainSel = Tails.mk2 CompareSameSelectView
+  hardForkChainSel = Tails.mk2 SameTiebreakerAcrossEras
 
   hardForkInjectTxs =
     InPairs.mk2 $

ouroboros-consensus-cardano/test/cardano-test/Test/Consensus/Cardano/DiffusionPipelining.hs

@@ -22,6 +22,7 @@ import Ouroboros.Consensus.Byron.Ledger (ByronBlock)
 import Ouroboros.Consensus.Cardano (CardanoBlock)
 import Ouroboros.Consensus.HardFork.Combinator
 import Ouroboros.Consensus.HardFork.Combinator.AcrossEras
+import Ouroboros.Consensus.Protocol.Abstract
 import Ouroboros.Consensus.Protocol.PBFT
 import Ouroboros.Consensus.Shelley.Eras
 import Ouroboros.Consensus.Shelley.Ledger
@@ -79,7 +80,7 @@ instance GenTentativeHeaderViews ByronBlock where
     nubOrd . sort <$> listOf do
       bno <- arbitrary
       isEBB <- toIsEBB <$> arbitrary
-      pure $ PBftSelectView bno isEBB
+      pure $ SelectView bno (PBftTiebreakerView isEBB)
 
 instance ShelleyCompatible proto era => GenTentativeHeaderViews (ShelleyBlock proto era) where
   genTentativeHeaderViews _ = do

ouroboros-consensus-diffusion/test/consensus-test/Test/Consensus/HardFork/Combinator.hs

@@ -441,7 +441,7 @@ instance CanHardFork '[BlockA, BlockB] where
       , translateChainDepState = PCons chainDepState_AtoB PNil
       , crossEraForecast = PCons forecast_AtoB PNil
       }
-  hardForkChainSel = Tails.mk2 CompareBlockNo
+  hardForkChainSel = Tails.mk2 NoTiebreakerAcrossEras
   hardForkInjectTxs = InPairs.mk2 injectTx_AtoB
 
   hardForkInjTxMeasure = \case

ouroboros-consensus-protocol/changelog.d/20250716_190242_alexander.esgen_tiebreaker_view.md

@@ -0,0 +1,4 @@
+### Breaking
+
+- Removed `PraosChainSelectView`, use `SelectView (TPraos c)`/`SelectView (Praos
+  c)` instead.

ouroboros-consensus-protocol/src/ouroboros-consensus-protocol/Ouroboros/Consensus/Protocol/Praos.hs

@@ -389,7 +389,7 @@ instance PraosCrypto c => ConsensusProtocol (Praos c) where
   type ChainDepState (Praos c) = PraosState
   type IsLeader (Praos c) = PraosIsLeader c
   type CanBeLeader (Praos c) = PraosCanBeLeader c
-  type SelectView (Praos c) = PraosChainSelectView c
+  type TiebreakerView (Praos c) = PraosTiebreakerView c
   type LedgerView (Praos c) = Views.LedgerView
   type ValidationErr (Praos c) = PraosValidationErr c
   type ValidateView (Praos c) = PraosValidateView c