slp-modular-cms/src/SlpModularCms.Core.Tests/Hosting/DatabaseMigrationExtensionsTests.cs

using FluentAssertions;
using MySql.Data.MySqlClient;
using SlpModularCms.Core.Hosting;
using Xunit;

namespace SlpModularCms.Core.Tests.Hosting;

/// <summary>
/// Guards the failure classification of the startup migration.
/// </summary>
/// <remarks>
/// Startup migration distinguishes two failures that mean very different things:
/// a database that is not up yet (normal when the application and the database server start
/// together after a host reboot) versus a migration that is broken. Retrying the first is
/// correct; retrying the second only delays the inevitable and fills the log.
///
/// <c>MigrateCoreDatabase</c> itself needs a composed <c>WebApplication</c>, so the classifier is
/// exercised directly here; the composed startup path is verified at the phase-level Build and
/// Test stage by starting both hosts.
/// </remarks>
public class DatabaseMigrationExtensionsTests
{
    [Fact]
    public void MaxConnectionAttempts_ShouldAllowForAHostRebootWindow()
    {
        // Enough attempts that a database server starting alongside the application is tolerated,
        // few enough that a genuinely unreachable database still fails promptly.
        DatabaseMigrationExtensions.MaxConnectionAttempts.Should().BeGreaterThan(1);
        DatabaseMigrationExtensions.MaxConnectionAttempts.Should().BeLessThanOrEqualTo(10);
    }

    [Theory]
    [MemberData(nameof(TransientFailures))]
    public void IsTransientConnectionFailure_ShouldRecogniseConnectionProblems(Exception exception)
    {
        InvokeClassifier(exception).Should().BeTrue();
    }

    [Theory]
    [MemberData(nameof(NonTransientFailures))]
    public void IsTransientConnectionFailure_ShouldNotRecogniseMigrationProblems(Exception exception)
    {
        // A broken migration must fail immediately. Classifying it as transient would hide a
        // real fault behind a retry loop.
        InvokeClassifier(exception).Should().BeFalse();
    }

    public static TheoryData<Exception> TransientFailures() =>
    [
        MakeMySqlException(),
        new TimeoutException("Connect Timeout expired."),
        new InvalidOperationException("wrapper", MakeMySqlException()),
    ];

    public static TheoryData<Exception> NonTransientFailures() =>
    [
        new InvalidOperationException("There is already an object named 'Users' in the database."),
        new NotSupportedException("The migration cannot be applied."),
        new AggregateException(new InvalidOperationException("pending model changes")),
    ];

    private static bool InvokeClassifier(Exception exception)
    {
        var method = typeof(DatabaseMigrationExtensions).GetMethod(
            "IsTransientConnectionFailure",
            System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Static);

        method.Should().NotBeNull("the failure classifier is the behaviour under test");

        return (bool)method!.Invoke(null, [exception])!;
    }

    /// <summary>
    /// <see cref="MySqlException"/> has no public constructor, so one is produced through the
    /// framework's own factory path via reflection.
    /// </summary>
    private static Exception MakeMySqlException()
    {
        try
        {
            // Deliberately unreachable host and a very short timeout: this genuinely produces a
            // MySqlException rather than a hand-built stand-in, so the classifier is tested against
            // the real type it will encounter in production.
            using var connection = new MySqlConnection(
                "Server=localhost;Port=9;Database=none;Uid=none;Pwd=none;Connection Timeout=1");
            connection.Open();
        }
        catch (Exception ex)
        {
            return ex;
        }

        throw new InvalidOperationException("Expected the connection attempt to fail.");
    }
}