accounts: implement UpdateBalance endpoint

With all underlying functionality in place to support updating an
existing off-chain account’s balance by a specified amount, this commit
implements the `UpdateBalance` endpoint to utilize that functionality.

Author: ViktorT-11

accounts/rpcserver.go

@@ -137,7 +137,47 @@ func (s *RPCServer) UpdateAccount(ctx context.Context,
 func (s *RPCServer) UpdateBalance(ctx context.Context,
 	req *litrpc.UpdateAccountBalanceRequest) (*litrpc.Account, error) {
 
-	return nil, fmt.Errorf("not implemented")
+	var (
+		isAddition bool
+		amount     lnwire.MilliSatoshi
+	)
+
+	switch reqType := req.GetUpdate().(type) {
+	case *litrpc.UpdateAccountBalanceRequest_Add:
+		log.Infof("[addbalance] id=%s, label=%v, amount=%d",
+			req.Id, req.Label, reqType.Add)
+
+		isAddition = true
+		amount = lnwire.MilliSatoshi(reqType.Add * 1000)
+
+	case *litrpc.UpdateAccountBalanceRequest_Deduct:
+		log.Infof("[deductbalance] id=%s, label=%v, amount=%d",
+			req.Id, req.Label, reqType.Deduct)
+
+		isAddition = false
+		amount = lnwire.MilliSatoshi(reqType.Deduct * 1000)
+
+	}
+
+	if amount <= 0 {
+		return nil, fmt.Errorf("amount %v must be greater than 0",
+			int64(amount/1000))
+	}
+
+	accountID, err := s.findAccount(ctx, req.Id, req.Label)
+	if err != nil {
+		return nil, err
+	}
+
+	// Ask the service to update the account.
+	account, err := s.service.UpdateBalance(
+		ctx, accountID, amount, isAddition,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	return marshalAccount(account), nil
 }
 
 // ListAccounts returns all accounts that are currently stored in the account

accounts/service.go

@@ -345,6 +345,32 @@ func (s *InterceptorService) UpdateAccount(ctx context.Context,
 	return s.store.Account(ctx, accountID)
 }
 
+// UpdateBalance adds or deducts an amount from an existing account in the
+// account database.
+func (s *InterceptorService) UpdateBalance(ctx context.Context,
+	accountID AccountID, amount lnwire.MilliSatoshi,
+	isAddition bool) (*OffChainBalanceAccount, error) {
+
+	s.Lock()
+	defer s.Unlock()
+
+	// As this function updates account balances, we require that the
+	// service is running before we execute it.
+	if !s.isRunningUnsafe() {
+		// This case can only happen if the service is disabled while
+		// we're processing a request.
+		return nil, ErrAccountServiceDisabled
+	}
+
+	// Adjust the balance of the account in the db.
+	err := s.store.AdjustAccountBalance(ctx, accountID, amount, isAddition)
+	if err != nil {
+		return nil, fmt.Errorf("unable to update account: %w", err)
+	}
+
+	return s.store.Account(ctx, accountID)
+}
+
 // Account retrieves an account from the bolt DB and un-marshals it. If the
 // account cannot be found, then ErrAccNotFound is returned.
 func (s *InterceptorService) Account(ctx context.Context,