Merge #17537: wallet: Cleanup and move opportunistic and superfluous TopUp()s

6e77a7b65c keypool: Add comment about TopUp and when to use it (Andrew Chow)
ea50e34b28 keypool: Move opportunistic TopUps from LegacyScriptPubKeyMan to CWallet and ReserveDestination (Andrew Chow)
bb2c8ce23c keypool: Remove  superfluous topup from CWallet::GetNewChangeDestination (Andrew Chow)

Pull request description:

  * The `TopUp()` in `CWallet::GetNewChangeDestination` is unnecessary as currently m_spk_man calls TopUp further down the call stack inside LegacyScriptPubKeyMan::ReserveKeyFromKeyPool (called by LegacyScriptPubKeyMan::GetReservedDestination). This also lets us prepare for future changes with multiple ScriptPubKeyMans in the wallet.
  * An opportunistic `TopUp()` is moved from `LegacyScriptPubKeyMan::GetNewDestination` to `CWallet::GetNewDestination`.
  * Another opportunistic `TopUp()` is moved from `LegacyScriptPubKeyMan::ReserveKeyFromKeyPool`

  Moving opportunistic TopUps ensures that ScriptPubKeyMans will always be topped up before requesting Destinations from them as we cannot  always rely on future ScriptPubKeyMan implementaions topping up internally.

  See also: https://github.com/bitcoin/bitcoin/pull/17373#discussion_r348598174

ACKs for top commit:
  instagibbs:
    utACK 6e77a7b65c only change is slight elaboration on comment
  ryanofsky:
    Code review ACK 6e77a7b65c. Only the comment changed since my previous review.

Tree-SHA512: bdfc8d303842c3fb7c3d40af7abfa6d9dac4ef71a24922bb92229674ee89bfe3113ebb46d3903ac48ef99f0a7d6eaac33282495844f2b31f91b8df55084c421f

src/wallet/scriptpubkeyman.cpp

@@ -14,7 +14,6 @@
 bool LegacyScriptPubKeyMan::GetNewDestination(const OutputType type, CTxDestination& dest, std::string& error)
 {
     error.clear();
-    TopUp();
 
     // Generate a new key that is added to wallet
     CPubKey new_key;
@@ -1153,8 +1152,6 @@ bool LegacyScriptPubKeyMan::ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& key
     {
         LOCK(cs_wallet);
 
-        TopUp();
-
         bool fReturningInternal = fRequestedInternal;
         fReturningInternal &= (IsHDEnabled() && m_storage.CanSupportFeature(FEATURE_HD_SPLIT)) || m_storage.IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS);
         bool use_split_keypool = set_pre_split_keypool.empty();

src/wallet/scriptpubkeyman.h

@@ -160,6 +160,10 @@ public:
     virtual void KeepDestination(int64_t index, const OutputType& type) {}
     virtual void ReturnDestination(int64_t index, bool internal, const CTxDestination& addr) {}
 
+    /** Fills internal address pool. Use within ScriptPubKeyMan implementations should be used sparingly and only
+      * when something from the address pool is removed, excluding GetNewDestination and GetReservedDestination.
+      * External wallet code is primarily responsible for topping up prior to fetching new addresses
+      */
     virtual bool TopUp(unsigned int size = 0) { return false; }
 
     //! Mark unused addresses as being used

src/wallet/wallet.cpp

@@ -3105,6 +3105,7 @@ bool CWallet::GetNewDestination(const OutputType type, const std::string label,
     bool result = false;
     auto spk_man = m_spk_man.get();
     if (spk_man) {
+        spk_man->TopUp();
         result = spk_man->GetNewDestination(type, dest, error);
     }
     if (result) {
@@ -3118,8 +3119,6 @@ bool CWallet::GetNewChangeDestination(const OutputType type, CTxDestination& des
 {
     error.clear();
 
-    m_spk_man->TopUp();
-
     ReserveDestination reservedest(this, type);
     if (!reservedest.GetReservedDestination(dest, true)) {
         error = "Error: Keypool ran out, please call keypoolrefill first";
@@ -3297,6 +3296,8 @@ bool ReserveDestination::GetReservedDestination(CTxDestination& dest, bool inter
 
     if (nIndex == -1)
     {
+        m_spk_man->TopUp();
+
         CKeyPool keypool;
         if (!m_spk_man->GetReservedDestination(type, internal, address, nIndex, keypool)) {
             return false;