[docs] Add doxygen comment for CKeyPool

Author: jnewbery

src/wallet/wallet.h

@@ -135,14 +135,61 @@ 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
-    bool m_pre_split; // For keys generated before keypool split upgrade
+    //! Whether this keypool entry is in the internal keypool (for change outputs)
+    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);