Replace vAnchorCache with a cache size counter

The anchor is obtained from the returned witnesses; since all witnesses are to
the same point (the latest blockchain tip), they all have the same root.

src/wallet/gtest/test_wallet.cpp

@@ -402,7 +402,8 @@ TEST(wallet_tests, cached_witnesses_chain_tip) {
         witnesses.clear();
         wallet.GetNoteWitnesses(notes, witnesses, anchor3);
         EXPECT_FALSE((bool) witnesses[0]);
-        EXPECT_EQ(anchor1, anchor3);
+        // Should not equal first anchor because none of these notes had witnesses
+        EXPECT_NE(anchor1, anchor3);
 
         // Re-incrementing with the same block should give the same result
         uint256 anchor4;

src/wallet/wallet.cpp

@@ -649,12 +649,11 @@ void CWallet::IncrementNoteWitnesses(const CBlockIndex* pindex,
                 }
             }
         }
-        vAnchorCache.push_front(tree.root());
-        if (vAnchorCache.size() > WITNESS_CACHE_SIZE) {
-            vAnchorCache.pop_back();
+        if (nWitnessCacheSize < WITNESS_CACHE_SIZE) {
+            nWitnessCacheSize += 1;
         }
         if (fFileBacked) {
-            CWalletDB(strWalletFile).WriteAnchorCache(vAnchorCache);
+            CWalletDB(strWalletFile).WriteWitnessCacheSize(nWitnessCacheSize);
         }
     }
 }
@@ -671,12 +670,11 @@ void CWallet::DecrementNoteWitnesses()
                 }
             }
         }
-        if (vAnchorCache.size() > 0) {
-            vAnchorCache.pop_front();
-        }
-        // TODO: If vAnchorCache is empty, we need to regenerate the caches (#1302)
+        nWitnessCacheSize -= 1;
+        // TODO: If nWitnessCache is zero, we need to regenerate the caches (#1302)
+        assert(nWitnessCacheSize > 0);
         if (fFileBacked) {
-            CWalletDB(strWalletFile).WriteAnchorCache(vAnchorCache);
+            CWalletDB(strWalletFile).WriteWitnessCacheSize(nWitnessCacheSize);
         }
     }
 }
@@ -1070,18 +1068,24 @@ void CWallet::GetNoteWitnesses(std::vector<JSOutPoint> notes,
     {
         LOCK(cs_wallet);
         witnesses.resize(notes.size());
+        boost::optional<uint256> rt;
         int i = 0;
         for (JSOutPoint note : notes) {
             if (mapWallet.count(note.hash) &&
                     mapWallet[note.hash].mapNoteData.count(note) &&
                     mapWallet[note.hash].mapNoteData[note].witnesses.size() > 0) {
                 witnesses[i] = mapWallet[note.hash].mapNoteData[note].witnesses.front();
+                if (!rt) {
+                    rt = witnesses[i]->root();
+                } else {
+                    assert(*rt == witnesses[i]->root());
+                }
             }
             i++;
         }
-        // vAnchorCache should only be empty here before the genesis block (so, never)
-        if (vAnchorCache.size() > 0) {
-            final_anchor = vAnchorCache.front();
+        // All returned witnesses have the same anchor
+        if (rt) {
+            final_anchor = *rt;
         }
     }
 }

src/wallet/wallet.h

@@ -580,10 +580,11 @@ private:
 
 public:
     /*
-     * Cached anchors corresponding to the cached incremental witnesses for the
-     * notes in our wallet.
+     * Size of the incremental witness cache for the notes in our wallet.
+     * This will always be greater than or equal to the size of the largest
+     * incremental witness cache in any transaction in mapWallet.
      */
-    std::list<uint256> vAnchorCache;
+    int64_t nWitnessCacheSize;
 
 protected:
     void IncrementNoteWitnesses(const CBlockIndex* pindex,

src/wallet/walletdb.cpp

@@ -162,10 +162,10 @@ bool CWalletDB::WriteDefaultKey(const CPubKey& vchPubKey)
     return Write(std::string("defaultkey"), vchPubKey);
 }
 
-bool CWalletDB::WriteAnchorCache(const std::list<uint256>& vAnchorCache)
+bool CWalletDB::WriteWitnessCacheSize(int64_t nWitnessCacheSize)
 {
     nWalletDBUpdated++;
-    return Write(std::string("anchorcache"), vAnchorCache);
+    return Write(std::string("witnesscachesize"), nWitnessCacheSize);
 }
 
 bool CWalletDB::ReadPool(int64_t nPool, CKeyPool& keypool)
@@ -637,9 +637,9 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
                 return false;
             }
         }
-        else if (strType == "anchorcache")
+        else if (strType == "witnesscachesize")
         {
-            ssValue >> pwallet->vAnchorCache;
+            ssValue >> pwallet->nWitnessCacheSize;
         }
     } catch (...)
     {

src/wallet/walletdb.h

@@ -106,7 +106,7 @@ public:
 
     bool WriteDefaultKey(const CPubKey& vchPubKey);
 
-    bool WriteAnchorCache(const std::list<uint256>& vAnchorCache);
+    bool WriteWitnessCacheSize(int64_t nWitnessCacheSize);
 
     bool ReadPool(int64_t nPool, CKeyPool& keypool);
     bool WritePool(int64_t nPool, const CKeyPool& keypool);