refactor: add Result<T> pattern and global exception handler for consistent error handling

octo-fiesta/Middleware/GlobalExceptionHandler.cs

@@ -0,0 +1,88 @@
+using Microsoft.AspNetCore.Diagnostics;
+
+namespace octo_fiesta.Middleware;
+
+/// <summary>
+/// Global exception handler that catches unhandled exceptions and returns appropriate Subsonic API error responses
+/// </summary>
+public class GlobalExceptionHandler : IExceptionHandler
+{
+    private readonly ILogger<GlobalExceptionHandler> _logger;
+
+    public GlobalExceptionHandler(ILogger<GlobalExceptionHandler> logger)
+    {
+        _logger = logger;
+    }
+
+    public async ValueTask<bool> TryHandleAsync(
+        HttpContext httpContext,
+        Exception exception,
+        CancellationToken cancellationToken)
+    {
+        _logger.LogError(exception, "Unhandled exception occurred: {Message}", exception.Message);
+
+        var (statusCode, subsonicErrorCode, errorMessage) = MapExceptionToResponse(exception);
+
+        httpContext.Response.StatusCode = statusCode;
+        httpContext.Response.ContentType = "application/json";
+
+        var response = CreateSubsonicErrorResponse(subsonicErrorCode, errorMessage);
+        await httpContext.Response.WriteAsJsonAsync(response, cancellationToken);
+
+        return true;
+    }
+
+    /// <summary>
+    /// Maps exception types to HTTP status codes and Subsonic error codes
+    /// </summary>
+    private (int statusCode, int subsonicErrorCode, string message) MapExceptionToResponse(Exception exception)
+    {
+        return exception switch
+        {
+            // Not Found errors (404)
+            FileNotFoundException => (404, 70, "Resource not found"),
+            DirectoryNotFoundException => (404, 70, "Directory not found"),
+            
+            // Authentication errors (401)
+            UnauthorizedAccessException => (401, 40, "Wrong username or password"),
+            
+            // Bad Request errors (400)
+            ArgumentNullException => (400, 10, "Required parameter is missing"),
+            ArgumentException => (400, 10, "Invalid request"),
+            FormatException => (400, 10, "Invalid format"),
+            InvalidOperationException => (400, 10, "Operation not valid"),
+            
+            // External service errors (502)
+            HttpRequestException => (502, 0, "External service unavailable"),
+            TimeoutException => (504, 0, "Request timeout"),
+            
+            // Generic server error (500)
+            _ => (500, 0, "An internal server error occurred")
+        };
+    }
+
+    /// <summary>
+    /// Creates a Subsonic-compatible error response
+    /// Subsonic error codes:
+    /// 0 = Generic error
+    /// 10 = Required parameter missing
+    /// 20 = Incompatible Subsonic REST protocol version
+    /// 30 = Incompatible Subsonic REST protocol version (server)
+    /// 40 = Wrong username or password
+    /// 50 = User not authorized
+    /// 60 = Trial period for the Subsonic server is over
+    /// 70 = Requested data was not found
+    /// </summary>
+    private object CreateSubsonicErrorResponse(int code, string message)
+    {
+        return new Dictionary<string, object>
+        {
+            ["subsonic-response"] = new
+            {
+                status = "failed",
+                version = "1.16.1",
+                error = new { code, message }
+            }
+        };
+    }
+}