Merge #15777: [docs] Add doxygen comments for keypool classes

f1a77b0c51 [docs] Add doxygen comment for CReserveKey (John Newbery)
37796b2dd4 [docs] Add doxygen comment for CKeyPool (John Newbery)
ef2d515af3 [wallet] move-only: move CReserveKey to be next to CKeyPool (John Newbery)

Pull request description:

  Docs/move-only

  Adds doxygen comments for the CKeyPool and CReserveKey objects. The way these work is pretty confusing and it's easy to overlook details (eg https://github.com/bitcoin/bitcoin/pull/15557#discussion_r271956393).

  These are on the verbose side, but I think too much commenting is better than not enough. Happy to take feedback on what's an appropriate level.

ACKs for commit f1a77b:
  jonatack:
    Thanks, John. Re-ACK f1a77b0c51, doc-only changes with respect to previous review.
  jb55:
    ACK f1a77b0c51

Tree-SHA512: 8bc97c7029cd2e8d9bfd2d2144eeff73474c71eda5a9d10817e1578ca0b70da677252037d83143faaff1808e2193408a21a8a89d36049eac77fd313990f0b67b

src/wallet/wallet.h

@@ -141,14 +141,61 @@ enum WalletFlags : uint64_t {
 
 static constexpr uint64_t g_known_wallet_flags = WALLET_FLAG_DISABLE_PRIVATE_KEYS | WALLET_FLAG_BLANK_WALLET | WALLET_FLAG_KEY_ORIGIN_METADATA;
 
-/** A key pool entry */
+/** A key from a CWallet's keypool
+ *
+ * The wallet holds one (for pre HD-split wallets) or several keypools. These
+ * are sets of keys that have not yet been used to provide addresses or receive
+ * change.
+ *
+ * The Bitcoin Core wallet was originally a collection of unrelated private
+ * keys with their associated addresses. If a non-HD wallet generated a
+ * key/address, gave that address out and then restored a backup from before
+ * that key's generation, then any funds sent to that address would be
+ * lost definitively.
+ *
+ * The keypool was implemented to avoid this scenario (commit: 10384941). The
+ * wallet would generate a set of keys (100 by default). When a new public key
+ * was required, either to give out as an address or to use in a change output,
+ * it would be drawn from the keypool. The keypool would then be topped up to
+ * maintain 100 keys. This ensured that as long as the wallet hadn't used more
+ * than 100 keys since the previous backup, all funds would be safe, since a
+ * restored wallet would be able to scan for all owned addresses.
+ *
+ * A keypool also allowed encrypted wallets to give out addresses without
+ * having to be decrypted to generate a new private key.
+ *
+ * With the introduction of HD wallets (commit: f1902510), the keypool
+ * essentially became an address look-ahead pool. Restoring old backups can no
+ * longer definitively lose funds as long as the addresses used were from the
+ * wallet's HD seed (since all private keys can be rederived from the seed).
+ * However, if many addresses were used since the backup, then the wallet may
+ * not know how far ahead in the HD chain to look for its addresses. The
+ * keypool is used to implement a 'gap limit'. The keypool maintains a set of
+ * keys (by default 1000) ahead of the last used key and scans for the
+ * addresses of those keys.  This avoids the risk of not seeing transactions
+ * involving the wallet's addresses, or of re-using the same address.
+ *
+ * The HD-split wallet feature added a second keypool (commit: 02592f4c). There
+ * is an external keypool (for addresses to hand out) and an internal keypool
+ * (for change addresses).
+ *
+ * Keypool keys are stored in the wallet/keystore's keymap. The keypool data is
+ * stored as sets of indexes in the wallet (setInternalKeyPool,
+ * setExternalKeyPool and set_pre_split_keypool), and a map from the key to the
+ * index (m_pool_key_to_index). The CKeyPool object is used to
+ * serialize/deserialize the pool data to/from the database.
+ */
 class CKeyPool
 {
 public:
+    //! The time at which the key was generated. Set in AddKeypoolPubKeyWithDB
     int64_t nTime;
+    //! The public key
     CPubKey vchPubKey;
-    bool fInternal; // for change outputs
+    //! Whether this keypool entry is in the internal keypool (for change outputs)
-    bool m_pre_split; // For keys generated before keypool split upgrade
+    bool fInternal;
+    //! Whether this key was generated for a keypool before the wallet was upgraded to HD-split
+    bool m_pre_split;
 
     CKeyPool();
     CKeyPool(const CPubKey& vchPubKeyIn, bool internalIn);
@@ -187,6 +234,57 @@ public:
     }
 };
 
+/** A wrapper to reserve a key from a wallet keypool
+ *
+ * CReserveKey is used to reserve a key from the keypool. It is passed around
+ * during the CreateTransaction/CommitTransaction procedure.
+ *
+ * Instantiating a CReserveKey does not reserve a keypool key. To do so,
+ * GetReservedKey() needs to be called on the object. Once a key has been
+ * reserved, call KeepKey() on the CReserveKey object to make sure it is not
+ * returned to the keypool. Call ReturnKey() to return the key to the keypool
+ * so it can be re-used (for example, if the key was used in a new transaction
+ * and that transaction was not completed and needed to be aborted).
+ *
+ * If a key is reserved and KeepKey() is not called, then the key will be
+ * returned to the keypool when the CReserveObject goes out of scope.
+ */
+class CReserveKey
+{
+protected:
+    //! The wallet to reserve the keypool key from
+    CWallet* pwallet;
+    //! The index of the key in the keypool
+    int64_t nIndex{-1};
+    //! The public key
+    CPubKey vchPubKey;
+    //! Whether this is from the internal (change output) keypool
+    bool fInternal{false};
+
+public:
+    //! Construct a CReserveKey object. This does NOT reserve a key from the keypool yet
+    explicit CReserveKey(CWallet* pwalletIn)
+    {
+        pwallet = pwalletIn;
+    }
+
+    CReserveKey(const CReserveKey&) = delete;
+    CReserveKey& operator=(const CReserveKey&) = delete;
+
+    //! Destructor. If a key has been reserved and not KeepKey'ed, it will be returned to the keypool
+    ~CReserveKey()
+    {
+        ReturnKey();
+    }
+
+    //! Reserve a key from the keypool
+    bool GetReservedKey(CPubKey &pubkey, bool internal = false);
+    //! Return a key to the keypool
+    void ReturnKey();
+    //! Keep the key. Do not return it to the keypool when this object goes out of scope
+    void KeepKey();
+};
+
 /** Address book data */
 class CAddressBookData
 {
@@ -1201,34 +1299,6 @@ public:
  */
 void MaybeResendWalletTxs();
 
-/** A key allocated from the key pool. */
-class CReserveKey
-{
-protected:
-    CWallet* pwallet;
-    int64_t nIndex{-1};
-    CPubKey vchPubKey;
-    bool fInternal{false};
-
-public:
-    explicit CReserveKey(CWallet* pwalletIn)
-    {
-        pwallet = pwalletIn;
-    }
-
-    CReserveKey(const CReserveKey&) = delete;
-    CReserveKey& operator=(const CReserveKey&) = delete;
-
-    ~CReserveKey()
-    {
-        ReturnKey();
-    }
-
-    void ReturnKey();
-    bool GetReservedKey(CPubKey &pubkey, bool internal = false);
-    void KeepKey();
-};
-
 /** RAII object to check and reserve a wallet rescan */
 class WalletRescanReserver
 {