Return app_hash in InitChain (cosmos#7208)

* Return app_hash in InitChain

* Hash empty string

baseapp/abci.go

@@ -1,6 +1,7 @@
 package baseapp
 
 import (
+	"crypto/sha256"
 	"errors"
 	"fmt"
 	"os"
@@ -81,9 +82,25 @@ func (app *BaseApp) InitChain(req abci.RequestInitChain) (res abci.ResponseInitC
 		}
 	}
 
+	// In the case of a new chain, AppHash will be the hash of an empty string.
+	// During an upgrade, it'll be the hash of the last committed block.
+	var appHash []byte
+	if !app.LastCommitID().IsZero() {
+		appHash = app.LastCommitID().Hash
+	} else {
+		// $ echo -n '' | sha256sum
+		// e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
+		emptyHash := sha256.Sum256([]byte{})
+		appHash = emptyHash[:]
+	}
+
 	// NOTE: We don't commit, but BeginBlock for block `initial_height` starts from this
 	// deliverState.
-	return res
+	return abci.ResponseInitChain{
+		ConsensusParams: res.ConsensusParams,
+		Validators:      res.Validators,
+		AppHash:         appHash,
+	}
 }
 
 // Info implements the ABCI interface.

baseapp/baseapp_test.go

@@ -564,7 +564,16 @@ func TestInitChainer(t *testing.T) {
 	require.Nil(t, err)
 	require.Equal(t, int64(0), app.LastBlockHeight())
 
-	app.InitChain(abci.RequestInitChain{AppStateBytes: []byte("{}"), ChainId: "test-chain-id"}) // must have valid JSON genesis file, even if empty
+	initChainRes := app.InitChain(abci.RequestInitChain{AppStateBytes: []byte("{}"), ChainId: "test-chain-id"}) // must have valid JSON genesis file, even if empty
+
+	// The AppHash returned by a new chain is the sha256 hash of "".
+	// $ echo -n '' | sha256sum
+	// e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
+	require.Equal(
+		t,
+		[]byte{0xe3, 0xb0, 0xc4, 0x42, 0x98, 0xfc, 0x1c, 0x14, 0x9a, 0xfb, 0xf4, 0xc8, 0x99, 0x6f, 0xb9, 0x24, 0x27, 0xae, 0x41, 0xe4, 0x64, 0x9b, 0x93, 0x4c, 0xa4, 0x95, 0x99, 0x1b, 0x78, 0x52, 0xb8, 0x55},
+		initChainRes.AppHash,
+	)
 
 	// assert that chainID is set correctly in InitChain
 	chainID := app.deliverState.ctx.ChainID()