DodoSSH/tests/DodoSSH.Client.Sync.Tests/FakeVaultServer.cs

using System.Globalization;
using DodoSSH.Client.Api;
using DodoSSH.Contracts;

namespace DodoSSH.Client.Sync.Tests;

/// <summary>
/// An in-memory vault server with the real sync semantics.
/// </summary>
/// <remarks>
/// <para>
/// A faithful reimplementation of <c>DodoSSH.Api.Features.Sync.SyncService</c>'s decision table: the
/// version check, the tombstone-beats-late-upsert rule, idempotent deletes, operation receipts, the
/// change log, and cursors that are opaque to the client. It is not a stub that returns canned answers —
/// if it were, none of the conflict tests would mean anything, because the interesting behaviour is
/// exactly the server's refusal to apply a stale write.
/// </para>
/// <para>
/// The duplication against the real service is deliberate and is the point of the exercise: two
/// independent expressions of the same rules, and <c>SyncEndpointTests</c> checks the other one against
/// real Postgres. A shared implementation would let a misreading of the protocol pass on both sides.
/// </para>
/// </remarks>
internal sealed class FakeVaultServer : ISyncApi
{
    private readonly Dictionary<Guid, Row> rows = [];
    private readonly List<LogEntry> log = [];
    private readonly Dictionary<Guid, Receipt> receipts = [];

    internal FakeVaultServer(Guid vaultId, uint keyGeneration = 1)
    {
        VaultId = vaultId;
        KeyGeneration = keyGeneration;
    }

    internal Guid VaultId { get; }

    internal uint KeyGeneration { get; set; }

    /// <summary>The server's clock, so a test can create skew deliberately.</summary>
    internal DateTimeOffset Now { get; set; } = DateTimeOffset.FromUnixTimeSeconds(1_750_000_000);

    /// <summary>Pull pages are capped here, as the real server clamps a client's requested limit.</summary>
    internal int MaxPullLimit { get; set; } = 500;

    /// <summary>Forces the next push to answer <see cref="SyncOperationStatus.Forbidden"/>.</summary>
    internal bool DenyWrites { get; set; }

    /// <summary>Pushes received, so a test can prove a retry did or did not happen.</summary>
    internal int PushCount { get; private set; }

    /// <summary>
    /// Runs just before a push is applied, so a test can land another client's write in the window
    /// between one client's pull and its push. That window is the whole subject of the cursor-gap test.
    /// </summary>
    internal Action? OnPush { get; set; }

    internal int RowCount => rows.Count(entry => !entry.Value.IsDeleted);

    /// <inheritdoc />
    public Task<SyncPullResponse> SyncPullAsync(
        Guid vaultId,
        SyncPullRequest request,
        CancellationToken cancellationToken)
    {
        var after = DecodeCursor(request.Cursor);
        var limit = Math.Clamp(request.Limit ?? MaxPullLimit, 1, MaxPullLimit);

        var page = log.Where(entry => entry.Sequence > after).Take(limit + 1).ToList();

        var hasMore = page.Count > limit;
        if (hasMore)
        {
            page.RemoveAt(page.Count - 1);
        }

        // When nothing came back the cursor must not move, or a write landing between this read and the
        // next would be skipped for ever.
        var next = page.Count > 0 ? page[^1].Sequence : after;

        return Task.FromResult(new SyncPullResponse(
            [.. page.Select(entry => Hydrate(entry))],
            EncodeCursor(next),
            hasMore,
            Now,
            KeyGeneration));
    }

    /// <inheritdoc />
    public Task<SyncPushResponse> SyncPushAsync(
        Guid vaultId,
        SyncPushRequest request,
        CancellationToken cancellationToken)
    {
        PushCount++;

        var interleaved = OnPush;
        OnPush = null;
        interleaved?.Invoke();

        var results = new List<SyncPushResult>(request.Operations.Count);

        foreach (var operation in request.Operations)
        {
            results.Add(Apply(operation));
        }

        return Task.FromResult(new SyncPushResponse(results, EncodeCursor(Head)));
    }

    /// <summary>Applies a change as if another client had made it.</summary>
    internal int ExternalUpsert(Guid entityId, EncryptedPayload payload, SyncPlaintextFields? fields)
    {
        var result = Apply(new SyncPushOperation(
            Guid.CreateVersion7(),
            SyncEntityType.Host,
            entityId,
            SyncOperation.Upsert,
            rows.TryGetValue(entityId, out var existing) && !existing.IsDeleted
                ? existing.Version
                : null,
            payload,
            fields ?? new SyncPlaintextFields()));

        if (result.Status != SyncOperationStatus.Applied)
        {
            throw new InvalidOperationException(
                $"The external write was not applied: {result.Status} — {result.Detail}.");
        }

        return result.Version!.Value;
    }

    /// <summary>Deletes as if another client had done it.</summary>
    internal void ExternalDelete(Guid entityId)
    {
        var existing = rows[entityId];

        var result = Apply(new SyncPushOperation(
            Guid.CreateVersion7(),
            SyncEntityType.Host,
            entityId,
            SyncOperation.Delete,
            existing.Version,
            null,
            null));

        if (result.Status != SyncOperationStatus.Applied)
        {
            throw new InvalidOperationException($"The external delete was not applied: {result.Status}.");
        }
    }

    internal Row? Find(Guid entityId) => rows.TryGetValue(entityId, out var row) ? row : null;

    private long Head => log.Count == 0 ? 0 : log[^1].Sequence;

    // ---- The decision table ----

    private SyncPushResult Apply(SyncPushOperation operation)
    {
        if (operation.EntityType != SyncEntityType.Host)
        {
            return Invalid(operation, $"Entity type {operation.EntityType} is not yet supported.");
        }

        if (receipts.TryGetValue(operation.OperationId, out var receipt))
        {
            return new SyncPushResult(
                operation.OperationId,
                SyncOperationStatus.Duplicate,
                receipt.Version,
                receipt.Sequence,
                null,
                null);
        }

        if (DenyWrites)
        {
            return new SyncPushResult(
                operation.OperationId, SyncOperationStatus.Forbidden, null, null, null, null);
        }

        rows.TryGetValue(operation.EntityId, out var existing);

        return operation.Operation == SyncOperation.Delete
            ? ApplyDelete(operation, existing)
            : ApplyUpsert(operation, existing);
    }

    private SyncPushResult ApplyUpsert(SyncPushOperation operation, Row? existing)
    {
        if (operation.Payload is null)
        {
            return Invalid(operation, "An upsert requires a payload.");
        }

        if (operation.Payload.WrappedDataKey.Length == 0 || operation.Payload.DataKeyId == Guid.Empty)
        {
            return Invalid(operation, "A payload requires its data key.");
        }

        var fields = operation.PlaintextFields ?? new SyncPlaintextFields();

        if (!fields.RelayEnabled && (fields.Hostname is not null || fields.Port is not null))
        {
            return Invalid(operation, "An address may only be supplied when relay is enabled.");
        }

        if (fields.RelayEnabled && (string.IsNullOrWhiteSpace(fields.Hostname) || fields.Port is null))
        {
            return Invalid(operation, "Relay-enabled hosts require both a hostname and a port.");
        }

        if (existing is null || existing.IsDeleted)
        {
            return Create(operation, existing, fields);
        }

        if (operation.ExpectedVersion != existing.Version)
        {
            return Conflict(operation, existing);
        }

        var updated = existing with
        {
            Version = existing.Version + 1,
            Payload = operation.Payload,
            Fields = fields,
            IsDeleted = false,
        };

        return Commit(operation, updated, SyncOperation.Upsert);
    }

    private SyncPushResult Create(SyncPushOperation operation, Row? existing, SyncPlaintextFields fields)
    {
        // A tombstone beats a late upsert. The client is told so it can resurrect the item deliberately
        // under a new id rather than silently undoing someone else's delete.
        if (existing?.IsDeleted == true)
        {
            return Conflict(operation, existing);
        }

        if (operation.ExpectedVersion is not null)
        {
            // The client believes it is updating something that does not exist here.
            return Conflict(operation, existing: null);
        }

        var created = new Row(operation.EntityId, 1, 0, operation.Payload!, fields, false);
        return Commit(operation, created, SyncOperation.Upsert);
    }

    private SyncPushResult ApplyDelete(SyncPushOperation operation, Row? existing)
    {
        if (existing is null)
        {
            return Invalid(operation, "Cannot delete an item that does not exist.");
        }

        if (existing.IsDeleted)
        {
            // Idempotent: a client retrying a delete it is unsure about should not have to tell these
            // two situations apart.
            return new SyncPushResult(
                operation.OperationId,
                SyncOperationStatus.Applied,
                existing.Version,
                existing.ChangeSequence,
                null,
                null);
        }

        if (operation.ExpectedVersion is not null && operation.ExpectedVersion != existing.Version)
        {
            return Conflict(operation, existing);
        }

        var tombstone = existing with
        {
            Version = existing.Version + 1,
            IsDeleted = true,
            // The address goes with the item, or the server stays able to resolve a host the user
            // believes they deleted.
            Fields = new SyncPlaintextFields(),
        };

        return Commit(operation, tombstone, SyncOperation.Delete);
    }

    private SyncPushResult Commit(SyncPushOperation operation, Row row, SyncOperation change)
    {
        var sequence = Head + 1;

        log.Add(new LogEntry(sequence, row.EntityId, change, row.Version, Now));
        rows[row.EntityId] = row with { ChangeSequence = sequence };
        receipts[operation.OperationId] = new Receipt(row.Version, sequence);

        return new SyncPushResult(
            operation.OperationId, SyncOperationStatus.Applied, row.Version, sequence, null, null);
    }

    private SyncPushResult Conflict(SyncPushOperation operation, Row? existing) =>
        new(
            operation.OperationId,
            SyncOperationStatus.Conflict,
            existing?.Version,
            existing?.ChangeSequence,
            existing is null ? null : ToChange(existing),
            null);

    private static SyncPushResult Invalid(SyncPushOperation operation, string detail) =>
        new(operation.OperationId, SyncOperationStatus.Invalid, null, null, null, detail);

    private SyncChange Hydrate(LogEntry entry)
    {
        var row = rows[entry.EntityId];
        return ToChange(row, entry.Sequence, entry.Revision, entry.OccurredAt);
    }

    private SyncChange ToChange(Row row, long? sequence = null, int? version = null, DateTimeOffset? at = null) =>
        new(
            SyncEntityType.Host,
            row.EntityId,
            row.IsDeleted ? SyncOperation.Delete : SyncOperation.Upsert,
            version ?? row.Version,
            sequence ?? row.ChangeSequence,
            // A delete carries no payload: there is nothing left to decrypt, and shipping the pre-delete
            // ciphertext would undermine the point of the tombstone.
            row.IsDeleted ? null : row.Payload,
            row.IsDeleted ? null : row.Fields,
            at ?? Now);

    // ---- Cursors ----

    /// <remarks>
    /// Prefixed and non-numeric so a client that tried to compute one would produce something this
    /// rejects. The real server HMAC-tags them; the property that matters to the client is only that it
    /// must round-trip what it is given.
    /// </remarks>
    private static string EncodeCursor(long sequence) =>
        "fake-v1:" + sequence.ToString(CultureInfo.InvariantCulture);

    private static long DecodeCursor(string? cursor)
    {
        if (string.IsNullOrEmpty(cursor))
        {
            return 0;
        }

        if (!cursor.StartsWith("fake-v1:", StringComparison.Ordinal)
            || !long.TryParse(cursor.AsSpan(8), CultureInfo.InvariantCulture, out var sequence))
        {
            throw new InvalidOperationException($"A client sent a cursor it should not have: '{cursor}'.");
        }

        return sequence;
    }

    internal sealed record Row(
        Guid EntityId,
        int Version,
        long ChangeSequence,
        EncryptedPayload Payload,
        SyncPlaintextFields Fields,
        bool IsDeleted);

    private sealed record LogEntry(
        long Sequence,
        Guid EntityId,
        SyncOperation Operation,
        int Revision,
        DateTimeOffset OccurredAt);

    private sealed record Receipt(int Version, long Sequence);
}