wallet: background sync with just the view key

- When background syncing, the wallet wipes the spend key
from memory and processes all new transactions. The wallet saves
all receives, spends, and "plausible" spends of receives the
wallet does not know key images for.
- When background sync disabled, the wallet processes all
background synced txs and then clears the background sync cache.
- Adding "plausible" spends to the background sync cache ensures
that the wallet does not need to query the daemon to see if any
received outputs were spent while background sync was enabled.
This would harm privacy especially for users of 3rd party daemons.
- To enable the feature in the CLI wallet, the user can set
background-sync to reuse-wallet-password or
custom-background-password and the wallet automatically syncs in
the background when the wallet locks, then processes all
background synced txs when the wallet is unlocked.
- The custom-background-password option enables the user to
open a distinct background wallet that only has a view key saved
and can be opened/closed/synced separately from the main wallet.
When the main wallet opens, it processes the background wallet's
cache.
- To enable the feature in the RPC wallet, there is a new
`/setup_background_sync` endpoint.
- HW, multsig and view-only wallets cannot background sync.

src/cryptonote_basic/account.cpp

@@ -152,6 +152,17 @@ DISABLE_VS_WARNINGS(4244 4345)
     m_keys.m_multisig_keys.clear();
   }
   //-----------------------------------------------------------------
+  void account_base::set_spend_key(const crypto::secret_key& spend_secret_key)
+  {
+    // make sure derived spend public key matches saved public spend key
+    crypto::public_key spend_public_key;
+    crypto::secret_key_to_public_key(spend_secret_key, spend_public_key);
+    CHECK_AND_ASSERT_THROW_MES(m_keys.m_account_address.m_spend_public_key == spend_public_key,
+        "Unexpected derived public spend key");
+
+    m_keys.m_spend_secret_key = spend_secret_key;
+  }
+  //-----------------------------------------------------------------
   crypto::secret_key account_base::generate(const crypto::secret_key& recovery_key, bool recover, bool two_random)
   {
     crypto::secret_key first = generate_keys(m_keys.m_account_address.m_spend_public_key, m_keys.m_spend_secret_key, recovery_key, recover);

src/cryptonote_basic/account.h

@@ -95,6 +95,7 @@ namespace cryptonote
     bool store(const std::string& file_path);
 
     void forget_spend_key();
+    void set_spend_key(const crypto::secret_key& spend_secret_key);
     const std::vector<crypto::secret_key> &get_multisig_keys() const { return m_keys.m_multisig_keys; }
 
     void encrypt_keys(const crypto::chacha_key &key) { m_keys.encrypt(key); }

src/cryptonote_config.h

@@ -241,6 +241,8 @@ namespace config
   const unsigned char HASH_KEY_ENCRYPTED_PAYMENT_ID = 0x8d;
   const unsigned char HASH_KEY_WALLET = 0x8c;
   const unsigned char HASH_KEY_WALLET_CACHE = 0x8d;
+  const unsigned char HASH_KEY_BACKGROUND_CACHE = 0x8e;
+  const unsigned char HASH_KEY_BACKGROUND_KEYS_FILE = 0x8f;
   const unsigned char HASH_KEY_RPC_PAYMENT_NONCE = 0x58;
   const unsigned char HASH_KEY_MEMORY = 'k';
   const unsigned char HASH_KEY_MULTISIG[] = {'M', 'u', 'l', 't' , 'i', 's', 'i', 'g', 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };

src/simplewallet/simplewallet.h

@@ -147,6 +147,7 @@ namespace cryptonote
     bool set_ignore_outputs_above(const std::vector<std::string> &args = std::vector<std::string>());
     bool set_ignore_outputs_below(const std::vector<std::string> &args = std::vector<std::string>());
     bool set_track_uses(const std::vector<std::string> &args = std::vector<std::string>());
+    bool setup_background_sync(const std::vector<std::string> &args = std::vector<std::string>());
     bool set_show_wallet_name_when_locked(const std::vector<std::string> &args = std::vector<std::string>());
     bool set_inactivity_lock_timeout(const std::vector<std::string> &args = std::vector<std::string>());
     bool set_setup_background_mining(const std::vector<std::string> &args = std::vector<std::string>());