Add QMS for IAVL migration query multistore (#542)

## Describe your changes and provide context
This PR add an extra QMS used for serving online migration query
fallback. When the request height is lower than the migration height, we
will fall back to read from IAVL store, otherwise, it will use the
default CMS

## Testing performed to validate your change

baseapp/abci.go

@@ -13,20 +13,20 @@ import (
 	"time"
 
 	"github.com/armon/go-metrics"
-	"github.com/gogo/protobuf/proto"
-	abci "github.com/tendermint/tendermint/abci/types"
-	tmproto "github.com/tendermint/tendermint/proto/tendermint/types"
-	"google.golang.org/grpc/codes"
-	grpcstatus "google.golang.org/grpc/status"
-
 	"github.com/cosmos/cosmos-sdk/codec"
 	snapshottypes "github.com/cosmos/cosmos-sdk/snapshots/types"
+	"github.com/cosmos/cosmos-sdk/store/types"
 	"github.com/cosmos/cosmos-sdk/tasks"
 	"github.com/cosmos/cosmos-sdk/telemetry"
 	sdk "github.com/cosmos/cosmos-sdk/types"
 	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
 	"github.com/cosmos/cosmos-sdk/types/legacytm"
 	"github.com/cosmos/cosmos-sdk/utils"
+	"github.com/gogo/protobuf/proto"
+	abci "github.com/tendermint/tendermint/abci/types"
+	tmproto "github.com/tendermint/tendermint/proto/tendermint/types"
+	"google.golang.org/grpc/codes"
+	grpcstatus "google.golang.org/grpc/status"
 )
 
 // InitChain implements the ABCI interface. It runs the initialization logic
@@ -708,7 +708,8 @@ func checkNegativeHeight(height int64) error {
 // CreateQueryContext creates a new sdk.Context for a query, taking as args
 // the block height and whether the query needs a proof or not.
 func (app *BaseApp) CreateQueryContext(height int64, prove bool) (sdk.Context, error) {
-	if err := checkNegativeHeight(height); err != nil {
+	err := checkNegativeHeight(height)
+	if err != nil {
 		return sdk.Context{}, err
 	}
 
@@ -734,7 +735,13 @@ func (app *BaseApp) CreateQueryContext(height int64, prove bool) (sdk.Context, e
 			)
 	}
 
-	cacheMS, err := app.cms.CacheMultiStoreWithVersion(height)
+	var cacheMS types.CacheMultiStore
+	if height < app.migrationHeight && app.qms != nil {
+		cacheMS, err = app.qms.CacheMultiStoreWithVersion(height)
+	} else {
+		cacheMS, err = app.cms.CacheMultiStoreWithVersion(height)
+	}
+
 	if err != nil {
 		return sdk.Context{},
 			sdkerrors.Wrapf(
@@ -905,12 +912,25 @@ func handleQueryApp(app *BaseApp, path []string, req abci.RequestQuery) abci.Res
 }
 
 func handleQueryStore(app *BaseApp, path []string, req abci.RequestQuery) abci.ResponseQuery {
-	// "/store" prefix for store queries
-	queryable, ok := app.cms.(sdk.Queryable)
-	if !ok {
-		return sdkerrors.QueryResultWithDebug(sdkerrors.Wrap(sdkerrors.ErrUnknownRequest, "multistore doesn't support queries"), app.trace)
+
+	var (
+		queryable sdk.Queryable
+		ok        bool
+	)
+	// Check if online migration is enabled for fallback read
+	if req.Height < app.migrationHeight && app.qms != nil {
+		queryable, ok = app.qms.(sdk.Queryable)
+		if !ok {
+			return sdkerrors.QueryResultWithDebug(sdkerrors.Wrap(sdkerrors.ErrUnknownRequest, "multistore doesn't support queries"), app.trace)
+		}
+	} else {
+		queryable, ok = app.cms.(sdk.Queryable)
+		if !ok {
+			return sdkerrors.QueryResultWithDebug(sdkerrors.Wrap(sdkerrors.ErrUnknownRequest, "multistore doesn't support queries"), app.trace)
+		}
 	}
 
+	// "/store" prefix for store queries
 	req.Path = "/" + strings.Join(path[1:], "/")
 
 	if req.Height <= 1 && req.Prove {

baseapp/baseapp.go

@@ -179,9 +179,11 @@ type BaseApp struct { //nolint: maligned
 }
 
 type appStore struct {
-	db          dbm.DB               // common DB backend
-	cms         sdk.CommitMultiStore // Main (uncached) state
-	storeLoader StoreLoader          // function to handle store loading, may be overridden with SetStoreLoader()
+	db              dbm.DB               // common DB backend
+	cms             sdk.CommitMultiStore // Main (uncached) state
+	qms             sdk.MultiStore       // Query multistore used for migration only
+	migrationHeight int64
+	storeLoader     StoreLoader // function to handle store loading, may be overridden with SetStoreLoader()
 
 	// an inter-block write-through cache provided to the context during deliverState
 	interBlockCache sdk.MultiStorePersistentCache

baseapp/options.go

@@ -361,3 +361,13 @@ func (app *BaseApp) SetStreamingService(s StreamingService) {
 	// BaseApp will pass BeginBlock, DeliverTx, and EndBlock requests and responses to the streaming services to update their ABCI context
 	app.abciListeners = append(app.abciListeners, s)
 }
+
+// SetQueryMultiStore set a alternative MultiStore implementation to support online migration fallback read.
+func (app *BaseApp) SetQueryMultiStore(ms sdk.MultiStore) {
+	app.qms = ms
+}
+
+// SetMigrationHeight set the migration height for online migration so that query below this height will still be served from IAVL.
+func (app *BaseApp) SetMigrationHeight(height int64) {
+	app.migrationHeight = height
+}