AutopilotMonitor/src/Backend/AutopilotMonitor.Functions/Middleware/GlobalExceptionMiddleware.cs

using System.Net;
using Azure;
using AutopilotMonitor.Functions.Helpers;
using Microsoft.AspNetCore.Http;
using Microsoft.Azure.Functions.Worker;
using Microsoft.Azure.Functions.Worker.Middleware;
using Microsoft.Extensions.Logging;

namespace AutopilotMonitor.Functions.Middleware;

/// <summary>
/// Safety-net middleware that catches any exception escaping past function-level catch blocks
/// and returns a structured error response instead of a raw 500. Placed after
/// <see cref="CorrelationIdMiddleware"/> (needs correlation ID) and before
/// <see cref="AuthenticationMiddleware"/> (catches auth-layer exceptions too).
/// MCP clients receive richer error details via the X-Client-Source header detection.
/// </summary>
public class GlobalExceptionMiddleware : IFunctionsWorkerMiddleware
{
    private readonly ILogger<GlobalExceptionMiddleware> _logger;

    public GlobalExceptionMiddleware(ILogger<GlobalExceptionMiddleware> logger)
    {
        _logger = logger;
    }

    public async Task Invoke(FunctionContext context, FunctionExecutionDelegate next)
    {
        try
        {
            await next(context);
        }
        catch (Exception ex)
        {
            var httpContext = context.GetHttpContext();
            if (httpContext == null)
                throw; // Non-HTTP triggers: let it propagate to the Functions runtime

            if (httpContext.Response.HasStarted)
                throw; // Response already streaming: can't write a JSON body

            var correlationId = context.GetCorrelationId();
            var functionName = context.FunctionDefinition.Name;
            _logger.LogError(ex, "Unhandled exception in {Function} [CorrelationId={CorrelationId}]",
                functionName, correlationId);

            var isMcp = string.Equals(
                httpContext.Request.Headers["X-Client-Source"].FirstOrDefault(),
                "mcp", StringComparison.OrdinalIgnoreCase);

            var errorBody = new Dictionary<string, object?>
            {
                ["error"] = ResponseHelper.SanitizeErrorMessage(ex, functionName),
                ["correlationId"] = correlationId,
                ["exceptionType"] = ex.GetType().Name,
            };

            if (isMcp)
            {
                errorBody["operation"] = functionName;
                var hint = ResponseHelper.GetRecoveryHint(ex);
                if (hint != null) errorBody["hint"] = hint;
            }

            httpContext.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
            httpContext.Response.ContentType = "application/json";
            await httpContext.Response.WriteAsJsonAsync(errorBody);
        }
    }
}