[21.2.x] upgrade/migrations: max allowances upgrade

.changelog/4064.feature.md

@@ -0,0 +1,4 @@
+upgrade/migrations: add max allowances upgrade handler
+
+Adds `"consensus-staking-max-allowances-16"` upgrade handler which updates
+the consensus staking parameter `MaxAllowances` to `16`.

go/upgrade/migrations/consensus_max_allowances.go

@@ -0,0 +1,52 @@
+package migrations
+
+import (
+	"fmt"
+
+	abciAPI "github.com/oasisprotocol/oasis-core/go/consensus/tendermint/api"
+	stakingState "github.com/oasisprotocol/oasis-core/go/consensus/tendermint/apps/staking/state"
+)
+
+const (
+	// ConsensusMaxAllowances16Handler is the name of the upgrade that sets the
+	// staking max allowances consensus parameter to 16.
+	ConsensusMaxAllowances16Handler = "consensus-max-allowances-16"
+)
+
+var _ Handler = (*maxAllowances16Handler)(nil)
+
+type maxAllowances16Handler struct {
+}
+
+func (th *maxAllowances16Handler) StartupUpgrade(ctx *Context) error {
+	return nil
+}
+
+func (th *maxAllowances16Handler) ConsensusUpgrade(ctx *Context, privateCtx interface{}) error {
+	abciCtx := privateCtx.(*abciAPI.Context)
+	switch abciCtx.Mode() {
+	case abciAPI.ContextBeginBlock:
+		// Nothing to do during begin block.
+	case abciAPI.ContextEndBlock:
+		// Update a consensus parameter during EndBlock.
+		state := stakingState.NewMutableState(abciCtx.State())
+
+		params, err := state.ConsensusParameters(abciCtx)
+		if err != nil {
+			return fmt.Errorf("unable to load staking consensus parameters: %w", err)
+		}
+
+		params.MaxAllowances = 16
+
+		if err = state.SetConsensusParameters(abciCtx, params); err != nil {
+			return fmt.Errorf("failed to update staking consensus parameters: %w", err)
+		}
+	default:
+		return fmt.Errorf("upgrade handler called in unexpected context: %s", abciCtx.Mode())
+	}
+	return nil
+}
+
+func init() {
+	Register(ConsensusMaxAllowances16Handler, &maxAllowances16Handler{})
+}