Wallet: Refactor ReserveKeyFromKeyPool for safety

ReserveKeyFromKeyPool's previous behaviour is to set nIndex to -1 if the keypool is
empty, OR throw an exception for technical failures. Instead, we now return false
if the keypool is empty, true if the operation succeeded.

This is to make failure more easily detectable by calling code.

Author: Empact

src/wallet/wallet.cpp

@@ -3427,7 +3427,7 @@ bool CWallet::TopUpKeyPool(unsigned int kpSize)
     return true;
 }
 
-void CWallet::ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool, bool fRequestedInternal)
+bool CWallet::ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool, bool fRequestedInternal)
 {
     nIndex = -1;
     keypool.vchPubKey = CPubKey();
@@ -3438,11 +3438,13 @@ void CWallet::ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool, bool fRe
             TopUpKeyPool();
 
         bool fReturningInternal = IsHDEnabled() && CanSupportFeature(FEATURE_HD_SPLIT) && fRequestedInternal;
-        std::set<int64_t>& setKeyPool = set_pre_split_keypool.empty() ? (fReturningInternal ? setInternalKeyPool : setExternalKeyPool) : set_pre_split_keypool;
+        bool use_split_keypool = set_pre_split_keypool.empty();
+        std::set<int64_t>& setKeyPool = use_split_keypool ? (fReturningInternal ? setInternalKeyPool : setExternalKeyPool) : set_pre_split_keypool;
 
         // Get the oldest key
-        if(setKeyPool.empty())
-            return;
+        if (setKeyPool.empty()) {
+            return false;
+        }
 
         WalletBatch batch(*database);
 
@@ -3456,14 +3458,17 @@ void CWallet::ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool, bool fRe
             throw std::runtime_error(std::string(__func__) + ": unknown key in key pool");
         }
         // If the key was pre-split keypool, we don't care about what type it is
-        if (set_pre_split_keypool.size() == 0 && keypool.fInternal != fReturningInternal) {
+        if (use_split_keypool && keypool.fInternal != fReturningInternal) {
             throw std::runtime_error(std::string(__func__) + ": keypool entry misclassified");
         }
+        if (!keypool.vchPubKey.IsValid()) {
+            throw std::runtime_error(std::string(__func__) + ": keypool entry invalid");
+        }
 
-        assert(keypool.vchPubKey.IsValid());
         m_pool_key_to_index.erase(keypool.vchPubKey.GetID());
         LogPrintf("keypool reserve %d\n", nIndex);
     }
+    return true;
 }
 
 void CWallet::KeepKey(int64_t nIndex)
@@ -3496,10 +3501,8 @@ bool CWallet::GetKeyFromPool(CPubKey& result, bool internal)
     CKeyPool keypool;
     {
         LOCK(cs_wallet);
-        int64_t nIndex = 0;
-        ReserveKeyFromKeyPool(nIndex, keypool, internal);
-        if (nIndex == -1)
-        {
+        int64_t nIndex;
+        if (!ReserveKeyFromKeyPool(nIndex, keypool, internal)) {
             if (IsLocked()) return false;
             WalletBatch batch(*database);
             result = GenerateNewKey(batch, internal);
@@ -3701,12 +3704,10 @@ bool CReserveKey::GetReservedKey(CPubKey& pubkey, bool internal)
     if (nIndex == -1)
     {
         CKeyPool keypool;
-        pwallet->ReserveKeyFromKeyPool(nIndex, keypool, internal);
-        if (nIndex != -1)
-            vchPubKey = keypool.vchPubKey;
-        else {
+        if (!pwallet->ReserveKeyFromKeyPool(nIndex, keypool, internal)) {
             return false;
         }
+        vchPubKey = keypool.vchPubKey;
         fInternal = keypool.fInternal;
     }
     assert(vchPubKey.IsValid());

src/wallet/wallet.h

@@ -999,7 +999,22 @@ class CWallet final : public CCryptoKeyStore, public CValidationInterface
     bool NewKeyPool();
     size_t KeypoolCountExternalKeys() EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
     bool TopUpKeyPool(unsigned int kpSize = 0);
-    void ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool, bool fRequestedInternal);
+
+    /**
+     * Reserves a key from the keypool and sets nIndex to its index
+     *
+     * @param[out] nIndex the index of the key in keypool
+     * @param[out] keypool the keypool the key was drawn from, which could be the
+     *     the pre-split pool if present, or the internal or external pool
+     * @param fRequestedInternal true if the caller would like the key drawn
+     *     from the internal keypool, false if external is preferred
+     *
+     * @return true if succeeded, false if failed due to empty keypool
+     * @throws std::runtime_error if keypool read failed, key was invalid,
+     *     was not found in the wallet, or was misclassified in the internal
+     *     or external keypool
+     */
+    bool ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool, bool fRequestedInternal);
     void KeepKey(int64_t nIndex);
     void ReturnKey(int64_t nIndex, bool fInternal, const CPubKey& pubkey);
     bool GetKeyFromPool(CPubKey &key, bool internal = false);