roothash: Store past runtime state and I/O roots in consensus state

.changelog/5359.breaking.md

@@ -0,0 +1,6 @@
+roothash: Store past runtime state and I/O roots in consensus state
+
+A new roothash consensus parameter was added (`MaxPastRootsStored`),
+which enables storing runtime state and I/O roots for the past
+`MaxPastRootsStored` rounds in the consensus state.
+This enables easier cross-runtime communication.

go/consensus/cometbft/apps/roothash/liveness_test.go

@@ -61,6 +61,8 @@ func TestLivenessProcessing(t *testing.T) {
 
 	// Initialize roothash state.
 	roothashState := roothashState.NewMutableState(ctx.State())
+	err = roothashState.SetConsensusParameters(ctx, &roothash.ConsensusParameters{})
+	require.NoError(err, "SetConsensusParameters")
 	blk := block.NewGenesisBlock(runtime.ID, 0)
 	rtState := &roothash.RuntimeState{
 		Runtime:            &runtime,

go/consensus/cometbft/apps/roothash/messages.go

@@ -131,6 +131,12 @@ func (app *rootHashApplication) changeParameters(ctx *api.Context, msg interface
 	if err != nil {
 		return nil, fmt.Errorf("roothash: failed to load consensus parameters: %w", err)
 	}
+	var needToDeletePastRoots bool
+	if changes.MaxPastRootsStored != nil && *changes.MaxPastRootsStored < params.MaxPastRootsStored {
+		// If we've reduced the number of past roots stored, we need to delete
+		// the excess when applying the new parameters.
+		needToDeletePastRoots = true
+	}
 	if err = changes.SanityCheck(); err != nil {
 		return nil, fmt.Errorf("roothash: failed to validate consensus parameter changes: %w", err)
 	}
@@ -143,6 +149,13 @@ func (app *rootHashApplication) changeParameters(ctx *api.Context, msg interface
 
 	// Apply changes.
 	if apply {
+		if needToDeletePastRoots {
+			err = state.ShrinkPastRoots(ctx, params.MaxPastRootsStored)
+			if err != nil {
+				return nil, fmt.Errorf("roothash: failed to shrink past stored roots: %w", err)
+			}
+		}
+
 		if err = state.SetConsensusParameters(ctx, params); err != nil {
 			return nil, fmt.Errorf("roothash: failed to update consensus parameters: %w", err)
 		}