ScadaBridge/tests/ZB.MOM.WW.ScadaBridge.NotificationService.Tests/SmtpErrorClassifierTests.cs

using System.Net.Sockets;
using MailKit;
using MailKit.Net.Smtp;

namespace ZB.MOM.WW.ScadaBridge.NotificationService.Tests;

/// <summary>
/// NS-002/NS-003: Tests for the shared SMTP error classification policy. This
/// policy is correctness-relevant — it decides whether a delivery failure is
/// retried (transient) or returned to the caller (permanent) — and is shared
/// by the central outbox's <c>EmailNotificationDeliveryAdapter</c>, so it
/// deserves direct coverage.
/// </summary>
public class SmtpErrorClassifierTests
{
    [Theory]
    [InlineData(421)] // service not available
    [InlineData(450)] // mailbox unavailable (busy)
    [InlineData(451)] // local error in processing
    [InlineData(452)] // insufficient system storage
    public void Classify_Smtp4xxCommand_IsTransient(int statusCode)
    {
        var ex = new SmtpCommandException(
            SmtpErrorCode.MessageNotAccepted, (SmtpStatusCode)statusCode, "rejected");

        Assert.Equal(SmtpErrorClass.Transient, SmtpErrorClassifier.Classify(ex, CancellationToken.None));
    }

    [Theory]
    [InlineData(500)] // syntax error
    [InlineData(550)] // mailbox unavailable (rejected)
    [InlineData(553)] // mailbox name not allowed
    [InlineData(554)] // transaction failed
    public void Classify_Smtp5xxCommand_IsPermanent(int statusCode)
    {
        var ex = new SmtpCommandException(
            SmtpErrorCode.MessageNotAccepted, (SmtpStatusCode)statusCode, "rejected");

        Assert.Equal(SmtpErrorClass.Permanent, SmtpErrorClassifier.Classify(ex, CancellationToken.None));
    }

    [Fact]
    public void Classify_SmtpCommandWithUnusualCode_IsUnknown()
    {
        // A status code outside the 4xx/5xx bands is not classifiable.
        var ex = new SmtpCommandException(
            SmtpErrorCode.UnexpectedStatusCode, (SmtpStatusCode)250, "ok-ish");

        Assert.Equal(SmtpErrorClass.Unknown, SmtpErrorClassifier.Classify(ex, CancellationToken.None));
    }

    [Fact]
    public void Classify_SmtpProtocolException_IsTransient()
    {
        Assert.Equal(
            SmtpErrorClass.Transient,
            SmtpErrorClassifier.Classify(new SmtpProtocolException("protocol error"), CancellationToken.None));
    }

    [Fact]
    public void Classify_ServiceNotConnectedException_IsTransient()
    {
        Assert.Equal(
            SmtpErrorClass.Transient,
            SmtpErrorClassifier.Classify(new ServiceNotConnectedException(), CancellationToken.None));
    }

    [Fact]
    public void Classify_SocketException_IsTransient()
    {
        Assert.Equal(
            SmtpErrorClass.Transient,
            SmtpErrorClassifier.Classify(new SocketException(), CancellationToken.None));
    }

    [Fact]
    public void Classify_TimeoutException_IsTransient()
    {
        Assert.Equal(
            SmtpErrorClass.Transient,
            SmtpErrorClassifier.Classify(new TimeoutException(), CancellationToken.None));
    }

    [Fact]
    public void Classify_RequestedCancellation_IsUnknown()
    {
        using var cts = new CancellationTokenSource();
        cts.Cancel();

        Assert.Equal(
            SmtpErrorClass.Unknown,
            SmtpErrorClassifier.Classify(new OperationCanceledException(), cts.Token));
    }

    [Fact]
    public void Classify_OperationCanceledWithoutRequestedCancellation_IsUnknown()
    {
        // Not a recognised SMTP error, and cancellation was not requested.
        Assert.Equal(
            SmtpErrorClass.Unknown,
            SmtpErrorClassifier.Classify(new OperationCanceledException(), CancellationToken.None));
    }

    [Fact]
    public void Classify_UnrecognisedException_IsUnknown()
    {
        Assert.Equal(
            SmtpErrorClass.Unknown,
            SmtpErrorClassifier.Classify(new InvalidOperationException("bad credential triple"), CancellationToken.None));
    }

    [Theory]
    [InlineData(450, true)]
    [InlineData(550, false)]
    [InlineData(250, false)]
    public void IsTransient_MatchesClassification(int statusCode, bool expectedTransient)
    {
        var ex = new SmtpCommandException(
            SmtpErrorCode.MessageNotAccepted, (SmtpStatusCode)statusCode, "x");

        Assert.Equal(expectedTransient, SmtpErrorClassifier.IsTransient(ex, CancellationToken.None));
    }

    [Fact]
    public void IsTransient_RequestedCancellation_IsFalse()
    {
        using var cts = new CancellationTokenSource();
        cts.Cancel();

        Assert.False(SmtpErrorClassifier.IsTransient(new OperationCanceledException(), cts.Token));
    }
}