Add transfer allowance functionality with error handling

backend/api_test.go

@@ -1039,7 +1039,7 @@ func TestTransferInsufficientFunds(t *testing.T) {
 	transfer := map[string]interface{}{"from": 1, "to": 2, "amount": 10}
 	resp := e.POST("/transfer").WithJSON(transfer).Expect().Status(400).JSON().Object()
 	// Error text should mention insufficient funds
-	resp.Value("error").String().Contains("insufficient")
+	resp.Value("error").String().ContainsFold("insufficient")
 }
 
 func TestTransferNotFound(t *testing.T) {

backend/db.go

@@ -746,7 +746,7 @@ func (db *Db) TransferAllowance(fromId int, toId int, amount float64) error {
 
 	// Ensure same owner
 	if fromUserId != toUserId {
-		return fmt.Errorf("allowances do not belong to the same user")
+		return fmt.Errorf(ErrDifferentUsers)
 	}
 
 	// Calculate how much the 'to' goal still needs
@@ -764,7 +764,7 @@ func (db *Db) TransferAllowance(fromId int, toId int, amount float64) error {
 
 	// Ensure 'from' has enough balance
 	if fromBalance < transfer {
-		return fmt.Errorf("insufficient funds in source allowance")
+		return fmt.Errorf(ErrInsufficientFunds)
 	}
 
 	// Perform updates

backend/main.go

@@ -25,6 +25,8 @@ const (
 	ErrInvalidUserID       = "Invalid user ID"
 	ErrUserNotFound        = "User not found"
 	ErrCheckingUserExist   = "Error checking user existence: %v"
+	ErrInsufficientFunds   = "Insufficient funds in source allowance"
+	ErrDifferentUsers      = "Allowances do not belong to the same user"
 )
 
 // ServerConfig holds configuration for the server.
@@ -644,6 +646,32 @@ func getHistory(c *gin.Context) {
 	c.IndentedJSON(http.StatusOK, history)
 }
 
+func transfer(c *gin.Context) {
+	var transferRequest TransferRequest
+	if err := c.ShouldBindJSON(&transferRequest); err != nil {
+		log.Printf("Error parsing request body: %v", err)
+		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid request body"})
+		return
+	}
+
+	err := db.TransferAllowance(transferRequest.From, transferRequest.To, transferRequest.Amount)
+	if err != nil {
+		if errors.Is(err, mysqlite.ErrNoRows) {
+			c.JSON(http.StatusNotFound, gin.H{"error": "Allowance not found"})
+			return
+		}
+		if err.Error() == ErrInsufficientFunds || err.Error() == ErrDifferentUsers {
+			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+			return
+		}
+		log.Printf("Error transferring allowance: %v", err)
+		c.JSON(http.StatusInternalServerError, gin.H{"error": ErrInternalServerError})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{"message": "Transfer successful"})
+}
+
 /*
 Initialises the database, and then starts the server.
 If the context gets cancelled, the server is shutdown and the database is closed.
@@ -679,6 +707,7 @@ func start(ctx context.Context, config *ServerConfig) {
 	router.PUT("/api/task/:taskId", putTask)
 	router.DELETE("/api/task/:taskId", deleteTask)
 	router.POST("/api/task/:taskId/complete", completeTask)
+	router.POST("/api/transfer", transfer)
 
 	srv := &http.Server{
 		Addr:    config.Addr,