PLib/tests/PLib.Tests/Metadata/RemoteImageCacheTests.cs

using Microsoft.Extensions.Logging.Abstractions;
using NSubstitute;
using PLib.Infrastructure.Metadata;
using PLib.Infrastructure.Storage;
using Shouldly;

namespace PLib.Tests.Metadata;

/// <summary>
/// A picture host that will not answer must cost a few attempts, not one per candidate.
/// </summary>
public sealed class RemoteImageCacheTests : IDisposable
{
    private readonly TempPaths _paths = new();
    private readonly CountingHandler _handler = new();

    public void Dispose()
    {
        _paths.Dispose();
        _handler.Dispose();
    }

    [Fact]
    public async Task A_host_that_keeps_failing_is_left_alone_after_a_few_tries()
    {
        var cache = Create();
        var problems = new List<string>();

        for (var attempt = 0; attempt < 6; attempt++)
        {
            var image = await cache.GetOrCreateAsync($"https://cdn.example/{attempt}.jpg", Token);

            image.Path.ShouldBeNull();

            if (image.Problem is { } problem)
            {
                problems.Add(problem);
            }
        }

        // A library-wide run produces a cover per candidate, all from the same host. Without a
        // cut-off, a host that answers its headers and then stalls holds a connection open for
        // the full timeout on every single one of them.
        _handler.Requests.ShouldBe(3);

        // Said once, at the moment the host is given up on. Every miss would put the same line
        // beside every candidate; never saying it leaves rows of empty squares unexplained.
        problems.ShouldHaveSingleItem().ShouldContain("cdn.example");
    }

    [Fact]
    public async Task A_different_host_is_judged_on_its_own_behaviour()
    {
        var cache = Create();

        for (var attempt = 0; attempt < 4; attempt++)
        {
            await cache.GetOrCreateAsync($"https://broken.example/{attempt}.jpg", Token);
        }

        await cache.GetOrCreateAsync("https://other.example/a.jpg", Token);

        // Three to the broken host, then it is skipped; the fourth request is the other host.
        _handler.Requests.ShouldBe(4);
    }

    [Fact]
    public async Task An_address_that_is_not_a_web_address_is_never_fetched()
    {
        var cache = Create();

        // The URL comes from somebody else's server, so file:// would turn "fetch a picture"
        // into "read a path of the server's choosing".
        (await cache.GetOrCreateAsync(@"file:///C:/Windows/win.ini", Token)).Path.ShouldBeNull();
        (await cache.GetOrCreateAsync("не адрес", Token)).Path.ShouldBeNull();

        _handler.Requests.ShouldBe(0);
    }

    private static CancellationToken Token => TestContext.Current.CancellationToken;

    private HttpRemoteImageCache Create()
    {
        var factory = Substitute.For<IHttpClientFactory>();
        factory.CreateClient(Arg.Any<string>()).Returns(_ => new HttpClient(_handler, disposeHandler: false));

        return new HttpRemoteImageCache(factory, _paths, NullLogger<HttpRemoteImageCache>.Instance);
    }

    /// <summary>Fails every request at once, and counts how many it was asked to make.</summary>
    private sealed class CountingHandler : HttpMessageHandler
    {
        private int _requests;

        public int Requests => _requests;

        protected override Task<HttpResponseMessage> SendAsync(
            HttpRequestMessage request,
            CancellationToken cancellationToken)
        {
            Interlocked.Increment(ref _requests);
            throw new HttpRequestException("no route to host");
        }
    }

    private sealed class TempPaths : IAppPaths, IDisposable
    {
        public TempPaths()
        {
            DataDirectory = Path.Combine(Path.GetTempPath(), $"plib-images-{Guid.CreateVersion7()}");
            ThumbnailDirectory = Path.Combine(DataDirectory, "thumbnails");
            PreviewDirectory = Path.Combine(DataDirectory, "previews");
            RemoteImageDirectory = Path.Combine(DataDirectory, "images");
            DatabaseFile = Path.Combine(DataDirectory, "library.db");

            Directory.CreateDirectory(RemoteImageDirectory);
        }

        public string DataDirectory { get; }

        public string ThumbnailDirectory { get; }

        public string PreviewDirectory { get; }

        public string RemoteImageDirectory { get; }

        public string DatabaseFile { get; }

        public void Dispose()
        {
            if (Directory.Exists(DataDirectory))
            {
                Directory.Delete(DataDirectory, recursive: true);
            }
        }
    }
}