docs: add section on creating a testnets from mainnet exports

docs/build/building-apps/05-app-testnet.md

@@ -0,0 +1,235 @@
+---
+sidebar_position: 1
+---
+
+# Application Testnets
+
+Building an application is complicated and requires a lot of testing. The Cosmos SDK provides a way to test your application in a real-world environment: a testnet. 
+
+We allow developers to take the state from their mainnet and run tests against the state. This is useful for testing upgrade migrations, or for testing the application in a real-world environment.
+
+## Testnet Setup
+
+We will be breaking down the steps to create a testnet from mainnet state. 
+
+```go 
+  // InitMerlinAppForTestnet is broken down into two sections:
+  // Required Changes: Changes that, if not made, will cause the testnet to halt or panic
+  // Optional Changes: Changes to customize the testnet to one's liking (lower vote times, fund accounts, etc)
+  func InitMerlinAppForTestnet(app *MerlinApp, newValAddr bytes.HexBytes, newValPubKey crypto.PubKey, newOperatorAddress, upgradeToTrigger string) *MerlinApp {
+  ...
+  }
+```
+
+### Required Changes
+
+#### Staking
+
+When creating a testnet the important part is migrate the validator set from many validators to one or a few. This allows developers to spin up the chain without needing to replace validator keys. 
+
+```go
+	ctx := app.BaseApp.NewUncachedContext(true, tmproto.Header{})
+	pubkey := &ed25519.PubKey{Key: newValPubKey.Bytes()}
+	pubkeyAny, err := types.NewAnyWithValue(pubkey)
+	if err != nil {
+		tmos.Exit(err.Error())
+	}
+
+	// STAKING
+	//
+
+	// Create Validator struct for our new validator.
+	_, bz, err := bech32.DecodeAndConvert(newOperatorAddress)
+	if err != nil {
+		tmos.Exit(err.Error())
+	}
+	bech32Addr, err := bech32.ConvertAndEncode("simvaloper", bz)
+	if err != nil {
+		tmos.Exit(err.Error())
+	}
+	newVal := stakingtypes.Validator{
+		OperatorAddress: bech32Addr,
+		ConsensusPubkey: pubkeyAny,
+		Jailed:          false,
+		Status:          stakingtypes.Bonded,
+		Tokens:          sdk.NewInt(900000000000000),
+		DelegatorShares: sdk.MustNewDecFromStr("10000000"),
+		Description: stakingtypes.Description{
+			Moniker: "Testnet Validator",
+		},
+		Commission: stakingtypes.Commission{
+			CommissionRates: stakingtypes.CommissionRates{
+				Rate:          sdk.MustNewDecFromStr("0.05"),
+				MaxRate:       sdk.MustNewDecFromStr("0.1"),
+				MaxChangeRate: sdk.MustNewDecFromStr("0.05"),
+			},
+		},
+		MinSelfDelegation: sdk.OneInt(),
+	}
+
+	// Remove all validators from power store
+	stakingKey := app.GetKey(stakingtypes.ModuleName)
+	stakingStore := ctx.KVStore(stakingKey)
+	iterator := app.StakingKeeper.ValidatorsPowerStoreIterator(ctx)
+	for ; iterator.Valid(); iterator.Next() {
+		stakingStore.Delete(iterator.Key())
+	}
+	iterator.Close()
+
+	// Remove all valdiators from last validators store
+	iterator = app.StakingKeeper.LastValidatorsIterator(ctx)
+	for ; iterator.Valid(); iterator.Next() {
+		stakingStore.Delete(iterator.Key())
+	}
+	iterator.Close()
+
+	// Add our validator to power and last validators store
+	app.StakingKeeper.SetValidator(ctx, newVal)
+	err = app.StakingKeeper.SetValidatorByConsAddr(ctx, newVal)
+	if err != nil {
+		tmos.Exit(err.Error())
+	}
+	app.StakingKeeper.SetValidatorByPowerIndex(ctx, newVal)
+	app.StakingKeeper.SetLastValidatorPower(ctx, newVal.GetOperator(), 0)
+	if err := app.StakingKeeper.Hooks().AfterValidatorCreated(ctx, newVal.GetOperator()); err != nil {
+		panic(err)
+	}
+```
+
+#### Distribution
+
+Since the validator set has changed, we need to update the distribution records for the new validator.
+
+
+```go
+	// Initialize records for this validator across all distribution stores
+	app.DistrKeeper.SetValidatorHistoricalRewards(ctx, newVal.GetOperator(), 0, distrtypes.NewValidatorHistoricalRewards(sdk.DecCoins{}, 1))
+	app.DistrKeeper.SetValidatorCurrentRewards(ctx, newVal.GetOperator(), distrtypes.NewValidatorCurrentRewards(sdk.DecCoins{}, 1))
+	app.DistrKeeper.SetValidatorAccumulatedCommission(ctx, newVal.GetOperator(), distrtypes.InitialValidatorAccumulatedCommission())
+	app.DistrKeeper.SetValidatorOutstandingRewards(ctx, newVal.GetOperator(), distrtypes.ValidatorOutstandingRewards{Rewards: sdk.DecCoins{}})
+```
+
+#### Slashing
+
+We also need to set the validator signing info for the new validator.
+
+```go
+  // SLASHING
+	//
+
+	// Set validator signing info for our new validator.
+	newConsAddr := sdk.ConsAddress(newValAddr.Bytes())
+	newValidatorSigningInfo := slashingtypes.ValidatorSigningInfo{
+		Address:     newConsAddr.String(),
+		StartHeight: app.LastBlockHeight() - 1,
+		Tombstoned:  false,
+	}
+	app.SlashingKeeper.SetValidatorSigningInfo(ctx, newConsAddr, newValidatorSigningInfo)
+```
+
+#### Bank
+
+It is useful to create new accounts for your testing purposes. This avoids the need to have the same key as you may have on mainnet. 
+
+```go
+  // BANK
+	//
+
+	defaultCoins := sdk.NewCoins(sdk.NewInt64Coin("ustake", 1000000000000))
+
+	localMerlinAccounts := []sdk.AccAddress{
+		sdk.MustAccAddressFromBech32("cosmos12smx2wdlyttvyzvzg54y2vnqwq2qjateuf7thj"),
+		sdk.MustAccAddressFromBech32("cosmos1cyyzpxplxdzkeea7kwsydadg87357qnahakaks"),
+		sdk.MustAccAddressFromBech32("cosmos18s5lynnmx37hq4wlrw9gdn68sg2uxp5rgk26vv"),
+		sdk.MustAccAddressFromBech32("cosmos1qwexv7c6sm95lwhzn9027vyu2ccneaqad4w8ka"),
+		sdk.MustAccAddressFromBech32("cosmos14hcxlnwlqtq75ttaxf674vk6mafspg8xwgnn53"),
+		sdk.MustAccAddressFromBech32("cosmos12rr534cer5c0vj53eq4y32lcwguyy7nndt0u2t"),
+		sdk.MustAccAddressFromBech32("cosmos1nt33cjd5auzh36syym6azgc8tve0jlvklnq7jq"),
+		sdk.MustAccAddressFromBech32("cosmos10qfrpash5g2vk3hppvu45x0g860czur8ff5yx0"),
+		sdk.MustAccAddressFromBech32("cosmos1f4tvsdukfwh6s9swrc24gkuz23tp8pd3e9r5fa"),
+		sdk.MustAccAddressFromBech32("cosmos1myv43sqgnj5sm4zl98ftl45af9cfzk7nhjxjqh"),
+		sdk.MustAccAddressFromBech32("cosmos14gs9zqh8m49yy9kscjqu9h72exyf295afg6kgk"),
+		sdk.MustAccAddressFromBech32("cosmos1jllfytsz4dryxhz5tl7u73v29exsf80vz52ucc")}
+
+  // Fund localMerlin accounts
+	for _, account := range localMerlinAccounts {
+		err := app.BankKeeper.MintCoins(ctx, minttypes.ModuleName, defaultCoins)
+		if err != nil {
+			tmos.Exit(err.Error())
+		}
+		err = app.BankKeeper.SendCoinsFromModuleToAccount(ctx, minttypes.ModuleName, account, defaultCoins)
+		if err != nil {
+			tmos.Exit(err.Error())
+		}
+	}
+```
+
+#### Upgrade
+
+If you would like to schedule an upgrade the below can be used. 
+
+```go
+	// UPGRADE
+	//
+
+	if upgradeToTrigger != "" {
+		upgradePlan := upgradetypes.Plan{
+			Name:   upgradeToTrigger,
+			Height: app.LastBlockHeight(),
+		}
+		err = app.UpgradeKeeper.ScheduleUpgrade(ctx, upgradePlan)
+		if err != nil {
+			panic(err)
+		}
+	}
+```
+
+### Optional Changes
+
+If you have custom modules that rely on specific state from the above modules and/or you would like to test your custom module, you will need to update the state of your custom module to reflect your needs
+
+## Running the Testnet
+
+Before we can run the testnet we must plug everything together. 
+
+in `root.go`, in the `initRootCmd` function we add:
+
+```diff
+  server.AddCommands(rootCmd, simapp.DefaultNodeHome, newApp, createMerlinAppAndExport, addModuleInitFlags)
+	++ server.AddTestnetCreatorCommand(rootCmd, simapp.DefaultNodeHome, newTestnetApp, addModuleInitFlags)
+```
+
+Next we will add a newTestnetApp helper function:
+
+```diff
+// newTestnetApp starts by running the normal newApp method. From there, the app interface returned is modified in order
+// for a testnet to be created from the provided app.
+func newTestnetApp(logger log.Logger, db cometbftdb.DB, traceStore io.Writer, appOpts servertypes.AppOptions) servertypes.Application {
+	// Create an app and type cast to an MerlinApp
+	app := newApp(logger, db, traceStore, appOpts)
+	simApp, ok := app.(*simapp.SimApp)
+	if !ok {
+		panic("app created from newApp is not of type simApp")
+	}
+
+	newValAddr, ok := appOpts.Get(server.KeyNewValAddr).(bytes.HexBytes)
+	if !ok {
+		panic("newValAddr is not of type bytes.HexBytes")
+	}
+	newValPubKey, ok := appOpts.Get(server.KeyUserPubKey).(crypto.PubKey)
+	if !ok {
+		panic("newValPubKey is not of type crypto.PubKey")
+	}
+	newOperatorAddress, ok := appOpts.Get(server.KeyNewOpAddr).(string)
+	if !ok {
+		panic("newOperatorAddress is not of type string")
+	}
+	upgradeToTrigger, ok := appOpts.Get(server.KeyTriggerTestnetUpgrade).(string)
+	if !ok {
+		panic("upgradeToTrigger is not of type string")
+	}
+
+	// Make modifications to the normal MerlinApp required to run the network locally
+	return meriln.InitMerlinAppForTestnet(simApp, newValAddr, newValPubKey, newOperatorAddress, upgradeToTrigger)
+}
+```