From 2779f4dba35c2cd631a894cf34d05205e349150c Mon Sep 17 00:00:00 2001
From: Jae Kwon <jae@tendermint.com>
Date: Wed, 7 Nov 2018 00:03:02 -0800
Subject: [PATCH] Add general merkle absence proof (also for empty substores)
 (#2685)

---
 PENDING.md                    |  1 +
 store/iavlstore.go            | 17 +++++++--
 store/multistoreproof_test.go | 66 +++++++++++++++++++++++++++++++++++
 store/rootmultistore.go       |  4 +++
 4 files changed, 86 insertions(+), 2 deletions(-)

diff --git a/PENDING.md b/PENDING.md
index 232d28f631..9796c00881 100644
--- a/PENDING.md
+++ b/PENDING.md
@@ -52,6 +52,7 @@ IMPROVEMENTS
  - \#2660 [x/mock/simulation] Staking transactions get tested far more frequently
  - \#2610 [x/stake] Block redelegation to and from the same validator
  - \#2652 [x/auth] Add benchmark for get and set account
+ - \#2685 [x/store] Add general merkle absence proof (also for empty substores)
 
 * Tendermint
 
diff --git a/store/iavlstore.go b/store/iavlstore.go
index bb588f029b..fccde38e27 100644
--- a/store/iavlstore.go
+++ b/store/iavlstore.go
@@ -225,8 +225,21 @@ func (st *iavlStore) Query(req abci.RequestQuery) (res abci.ResponseQuery) {
 				res.Log = err.Error()
 				break
 			}
-			res.Value = value
-			res.Proof = &merkle.Proof{Ops: []merkle.ProofOp{iavl.NewIAVLValueOp(key, proof).ProofOp()}}
+			if proof == nil {
+				// Proof == nil implies that the store is empty.
+				if value != nil {
+					panic("unexpected value for an empty proof")
+				}
+			}
+			if value != nil {
+				// value was found
+				res.Value = value
+				res.Proof = &merkle.Proof{Ops: []merkle.ProofOp{iavl.NewIAVLValueOp(key, proof).ProofOp()}}
+			} else {
+				// value wasn't found
+				res.Value = nil
+				res.Proof = &merkle.Proof{Ops: []merkle.ProofOp{iavl.NewIAVLAbsenceOp(key, proof).ProofOp()}}
+			}
 		} else {
 			_, res.Value = tree.GetVersioned(key, res.Height)
 		}
diff --git a/store/multistoreproof_test.go b/store/multistoreproof_test.go
index 19feef1600..db3b65cad5 100644
--- a/store/multistoreproof_test.go
+++ b/store/multistoreproof_test.go
@@ -106,3 +106,69 @@ func TestVerifyMultiStoreQueryProof(t *testing.T) {
 	err = prt.VerifyValue(res.Proof, cid.Hash, "/iavlStoreKey/MYKEY", []byte(nil))
 	require.NotNil(t, err)
 }
+
+func TestVerifyMultiStoreQueryProofEmptyStore(t *testing.T) {
+	// Create main tree for testing.
+	db := dbm.NewMemDB()
+	store := NewCommitMultiStore(db)
+	iavlStoreKey := sdk.NewKVStoreKey("iavlStoreKey")
+
+	store.MountStoreWithDB(iavlStoreKey, sdk.StoreTypeIAVL, nil)
+	store.LoadVersion(0)
+	cid := store.Commit() // Commit with empty iavl store.
+
+	// Get Proof
+	res := store.Query(abci.RequestQuery{
+		Path:  "/iavlStoreKey/key", // required path to get key/value+proof
+		Data:  []byte("MYKEY"),
+		Prove: true,
+	})
+	require.NotNil(t, res.Proof)
+
+	// Verify proof.
+	prt := DefaultProofRuntime()
+	err := prt.VerifyAbsence(res.Proof, cid.Hash, "/iavlStoreKey/MYKEY")
+	require.Nil(t, err)
+
+	// Verify (bad) proof.
+	prt = DefaultProofRuntime()
+	err = prt.VerifyValue(res.Proof, cid.Hash, "/iavlStoreKey/MYKEY", []byte("MYVALUE"))
+	require.NotNil(t, err)
+}
+
+func TestVerifyMultiStoreQueryProofAbsence(t *testing.T) {
+	// Create main tree for testing.
+	db := dbm.NewMemDB()
+	store := NewCommitMultiStore(db)
+	iavlStoreKey := sdk.NewKVStoreKey("iavlStoreKey")
+
+	store.MountStoreWithDB(iavlStoreKey, sdk.StoreTypeIAVL, nil)
+	store.LoadVersion(0)
+
+	iavlStore := store.GetCommitStore(iavlStoreKey).(*iavlStore)
+	iavlStore.Set([]byte("MYKEY"), []byte("MYVALUE"))
+	cid := store.Commit() // Commit with empty iavl store.
+
+	// Get Proof
+	res := store.Query(abci.RequestQuery{
+		Path:  "/iavlStoreKey/key", // required path to get key/value+proof
+		Data:  []byte("MYABSENTKEY"),
+		Prove: true,
+	})
+	require.NotNil(t, res.Proof)
+
+	// Verify proof.
+	prt := DefaultProofRuntime()
+	err := prt.VerifyAbsence(res.Proof, cid.Hash, "/iavlStoreKey/MYABSENTKEY")
+	require.Nil(t, err)
+
+	// Verify (bad) proof.
+	prt = DefaultProofRuntime()
+	err = prt.VerifyAbsence(res.Proof, cid.Hash, "/MYABSENTKEY")
+	require.NotNil(t, err)
+
+	// Verify (bad) proof.
+	prt = DefaultProofRuntime()
+	err = prt.VerifyValue(res.Proof, cid.Hash, "/iavlStoreKey/MYABSENTKEY", []byte(""))
+	require.NotNil(t, err)
+}
diff --git a/store/rootmultistore.go b/store/rootmultistore.go
index 6c491bda13..cd2d0135f1 100644
--- a/store/rootmultistore.go
+++ b/store/rootmultistore.go
@@ -295,6 +295,10 @@ func (rs *rootMultiStore) Query(req abci.RequestQuery) abci.ResponseQuery {
 		return res
 	}
 
+	if res.Proof == nil || len(res.Proof.Ops) == 0 {
+		return sdk.ErrInternal("substore proof was nil/empty when it should never be").QueryResult()
+	}
+
 	commitInfo, errMsg := getCommitInfo(rs.db, res.Height)
 	if errMsg != nil {
 		return sdk.ErrInternal(errMsg.Error()).QueryResult()