Merge #10952: [wallet] Remove vchDefaultKey and have better first run detection

e53615b Remove vchDefaultKey and have better first run detection (Andrew Chow)

Pull request description:

  Removes vchDefaultKey which was only used for first run detection. Improves wallet first run detection by checking to see if any keys were read from the database.

  This also fixes a (rather contrived) case where an encrypted non-HD wallet has corruption such that the default key is no longer valid and is loaded into a Core version that supports HD wallets. This causes a runtime exception since a new hd master key is generated as the software believes the wallet file is newly created but cannot add the generated key to the wallet since it is encrypted. I was only able to replicate this error by creating a non-hd wallet, encrypting it, then editing the wallet using `db_dump` and `db_load` before loading the wallet with hd enabled. This problem has been reported by [two](https://bitcointalk.org/index.php?topic=1993244.0) [users](https://bitcointalk.org/index.php?topic=1746976.msg17511261#msg17511261) so it is something that can happen, although that raises the question of "what corrupted the default key".

  ~P.S. I don't know what's up with the whitespace changes. I think my text editor is doing something stupid but I don't think those are important enough to attempt undoing them.~ Undid those

Tree-SHA512: 63b485f356566e8ffa033ad9b7101f7f6b56372b29ec2a43b947b0eeb1ada4c2cfe24740515d013aedd5f51aa1890dfbe499d2c5c062fc1b5d272324728a7d55

src/wallet/walletdb.cpp

@@ -130,11 +130,6 @@ bool CWalletDB::WriteOrderPosNext(int64_t nOrderPosNext)
     return WriteIC(std::string("orderposnext"), nOrderPosNext);
 }
 
-bool CWalletDB::WriteDefaultKey(const CPubKey& vchPubKey)
-{
-    return WriteIC(std::string("defaultkey"), vchPubKey);
-}
-
 bool CWalletDB::ReadPool(int64_t nPool, CKeyPool& keypool)
 {
     return batch.Read(std::make_pair(std::string("pool"), nPool), keypool);
@@ -452,7 +447,14 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
         }
         else if (strType == "defaultkey")
         {
-            ssValue >> pwallet->vchDefaultKey;
+            // We don't want or need the default key, but if there is one set,
+            // we want to make sure that it is valid so that we can detect corruption
+            CPubKey vchPubKey;
+            ssValue >> vchPubKey;
+            if (!vchPubKey.IsValid()) {
+                strErr = "Error reading wallet database: Default Key corrupt";
+                return false;
+            }
         }
         else if (strType == "pool")
         {
@@ -522,7 +524,6 @@ bool CWalletDB::IsKeyType(const std::string& strType)
 
 DBErrors CWalletDB::LoadWallet(CWallet* pwallet)
 {
-    pwallet->vchDefaultKey = CPubKey();
     CWalletScanState wss;
     bool fNoncriticalErrors = false;
     DBErrors result = DB_LOAD_OK;
@@ -565,7 +566,7 @@ DBErrors CWalletDB::LoadWallet(CWallet* pwallet)
             {
                 // losing keys is considered a catastrophic error, anything else
                 // we assume the user can live with:
-                if (IsKeyType(strType))
+                if (IsKeyType(strType) || strType == "defaultkey")
                     result = DB_CORRUPT;
                 else
                 {