Test runtime-assertable invariants every block (#2807)

* Initial pass
* Minor cleanup

PENDING.md

@@ -34,6 +34,7 @@ FEATURES
   * [app] \#2791 Support export at a specific height, with `gaiad export --height=HEIGHT`.
   * [x/gov] [#2479](https://github.com/cosmos/cosmos-sdk/issues/2479) Implemented querier
   for getting governance parameters.
+  * [app] \#2663 - Runtime-assertable invariants
   * [app] \#2791 Support export at a specific height, with `gaiad export --height=HEIGHT`.
 
 * SDK

cmd/gaia/app/app.go

@@ -210,6 +210,8 @@ func (app *GaiaApp) EndBlocker(ctx sdk.Context, req abci.RequestEndBlock) abci.R
 	tags := gov.EndBlocker(ctx, app.govKeeper)
 	validatorUpdates := stake.EndBlocker(ctx, app.stakeKeeper)
 
+	app.assertRuntimeInvariants()
+
 	return abci.ResponseEndBlock{
 		ValidatorUpdates: validatorUpdates,
 		Tags:             tags,

cmd/gaia/app/invariants.go

@@ -0,0 +1,34 @@
+package app
+
+import (
+	"fmt"
+	"time"
+
+	banksim "github.com/cosmos/cosmos-sdk/x/bank/simulation"
+	distrsim "github.com/cosmos/cosmos-sdk/x/distribution/simulation"
+	"github.com/cosmos/cosmos-sdk/x/mock/simulation"
+	stakesim "github.com/cosmos/cosmos-sdk/x/stake/simulation"
+)
+
+func (app *GaiaApp) runtimeInvariants() []simulation.Invariant {
+	return []simulation.Invariant{
+		banksim.NonnegativeBalanceInvariant(app.accountKeeper),
+		distrsim.ValAccumInvariants(app.distrKeeper, app.stakeKeeper),
+		stakesim.SupplyInvariants(app.bankKeeper, app.stakeKeeper,
+			app.feeCollectionKeeper, app.distrKeeper, app.accountKeeper),
+		stakesim.PositivePowerInvariant(app.stakeKeeper),
+	}
+}
+
+func (app *GaiaApp) assertRuntimeInvariants() {
+	invariants := app.runtimeInvariants()
+	start := time.Now()
+	for _, inv := range invariants {
+		if err := inv(app.BaseApp); err != nil {
+			panic(fmt.Errorf("invariant broken: %s", err))
+		}
+	}
+	end := time.Now()
+	diff := end.Sub(start)
+	app.BaseApp.Logger.With("module", "invariants").Info("Asserted all invariants", "duration", diff)
+}