Rendezvous/src/FinalFactory.Rendezvous.Server/Observability/AuditTrail.cs

using System.Security.Cryptography;
using System.Text;
using Microsoft.Extensions.Options;

namespace FinalFactory.Rendezvous.Server.Observability;

internal sealed class AuditTrail
{
    private readonly object _gate = new();
    private readonly LinkedList<AuditEntry> _entries = [];
    private readonly AuditOptions _options;
    private readonly TimeProvider _timeProvider;
    private readonly ILogger<AuditTrail> _logger;
    private readonly RendezvousTelemetry _telemetry;

    public AuditTrail(
        IOptions<AuditOptions> options,
        ILogger<AuditTrail> logger,
        RendezvousTelemetry telemetry,
        TimeProvider? timeProvider = null)
    {
        _options = options.Value;
        _logger = logger;
        _telemetry = telemetry;
        _timeProvider = timeProvider ?? TimeProvider.System;
    }

    public void Record(
        string actorSubject,
        string action,
        string result,
        string targetKind,
        string targetIdentifier,
        string correlationId)
    {
        DateTimeOffset now = _timeProvider.GetUtcNow();
        AuditEntry entry = new(
            now,
            Fingerprint(actorSubject),
            action,
            result,
            targetKind,
            Fingerprint(targetIdentifier),
            correlationId);
        lock (_gate)
        {
            PurgeExpired(now);

            while (_entries.Count >= _options.MaxEntries)
            {
                _entries.RemoveFirst();
            }

            _entries.AddLast(entry);
        }

        _telemetry.RecordAudit(action, result);
        LogOperatorAction(
            _logger,
            entry.ActorFingerprint,
            action,
            result,
            targetKind,
            entry.TargetFingerprint,
            correlationId);
    }

    public IReadOnlyDictionary<string, long> GetAggregateCounts()
    {
        lock (_gate)
        {
            PurgeExpired(_timeProvider.GetUtcNow());
            return _entries
                .GroupBy(static entry => $"{entry.Action}:{entry.Result}", StringComparer.Ordinal)
                .ToDictionary(
                    static group => group.Key,
                    static group => (long)group.Count(),
                    StringComparer.Ordinal);
        }
    }

    internal IReadOnlyList<AuditEntry> GetEntriesForTests()
    {
        lock (_gate)
        {
            PurgeExpired(_timeProvider.GetUtcNow());
            return _entries.ToArray();
        }
    }

    private void PurgeExpired(DateTimeOffset now)
    {
        DateTimeOffset oldest = now.AddDays(-_options.RetentionDays);
        while (_entries.First is { Value.Timestamp: var timestamp }
               && timestamp < oldest)
        {
            _entries.RemoveFirst();
        }
    }

    private static string Fingerprint(string value)
    {
        byte[] digest = SHA256.HashData(Encoding.UTF8.GetBytes(value));
        return Convert.ToHexString(digest.AsSpan(0, 12));
    }

    private static readonly Action<ILogger, string, string, string, string, string, string, Exception?>
        OperatorAction = LoggerMessage.Define<string, string, string, string, string, string>(
            LogLevel.Information,
            new EventId(100, nameof(LogOperatorAction)),
            "Operator audit: actor {ActorFingerprint} action {Action} completed with {Result} for {TargetKind} target {TargetFingerprint}; correlation {CorrelationId}");

    private static void LogOperatorAction(
        ILogger logger,
        string actorFingerprint,
        string action,
        string result,
        string targetKind,
        string targetFingerprint,
        string correlationId) => OperatorAction(
            logger,
            actorFingerprint,
            action,
            result,
            targetKind,
            targetFingerprint,
            correlationId,
            null);
}

internal sealed record AuditEntry(
    DateTimeOffset Timestamp,
    string ActorFingerprint,
    string Action,
    string Result,
    string TargetKind,
    string TargetFingerprint,
    string CorrelationId)
{
    public override string ToString() =>
        $"[AuditEntry {Action}/{Result}; actor and target fingerprinted]";
}