using System.Collections.Concurrent;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using ZB.MOM.WW.OtOpcUa.Commons.Observability;
using ZB.MOM.WW.OtOpcUa.Configuration.Entities;
using ZB.MOM.WW.OtOpcUa.Core.Authorization;

namespace ZB.MOM.WW.OtOpcUa.Host.OpcUa;

/// <summary>
///     Bounds what an <see cref="IApiKeyStore"/> can cost the OPC UA impersonation callback (arch-review
///     03/S18). Wraps <see cref="ConfigDbApiKeyStore"/> on a node that has a ConfigDb; the in-memory
///     <see cref="ArtifactApiKeyStore"/> needs neither half and is deliberately left unwrapped.
/// </summary>
/// <remarks>
///     <para>
///         <b>Why a deadline is the fix and the cache is not.</b> The SDK raises impersonation
///         synchronously under a <c>SessionManager</c>-wide lock, so a store that does not answer does not
///         stall one activation — it stalls every activation in the server. A cache lowers how OFTEN the
///         store is asked; only a deadline bounds how LONG one ask may hold that lock.
///     </para>
///     <para>
///         ⚠️ <b>Every bounded outcome DENIES.</b> A deadline exceeded, a faulted inner store and a
///         cancelled call all surface as an exception into
///         <see cref="ApiKeyOpcUaUserAuthenticator"/>'s catch-all, which returns the same opaque denial as
///         a bad credential. Nothing here can admit a session the inner store did not authorize — the
///         decorator can only return what the inner store returned, or throw.
///     </para>
///     <para>
///         ⚠️ <b>Faults and timeouts are NEVER cached</b>, and neither is the late answer of a lookup the
///         deadline abandoned. Caching a deny would be fail-closed and is still wrong: it would extend a
///         ConfigDb outage past the outage, denying valid keys for a further TTL after the database came
///         back. Caching an abandoned answer would be worse than that — it was taken before the deadline
///         and served after it. Only a lookup that completed BEFORE its deadline — a hit or an
///         authoritative miss — is stored.
///     </para>
/// </remarks>
public sealed class BoundedApiKeyStore : IApiKeyStore
{
    /// <summary>
    ///     Entries either cache may hold before it is cleared wholesale.
    /// </summary>
    /// <remarks>
    ///     Wholesale rather than per-entry eviction, for the reason <c>SessionAclDecisionCache</c> already
    ///     documents: picking WHICH entry to drop needs knowledge the cache does not have, and a
    ///     repopulate is cheap and cannot be wrong. The ceiling exists so a key-id guessing flood is a
    ///     bounded memory event — negative results are cached deliberately, so that a flood of unknown ids
    ///     does not become a SQL flood.
    /// </remarks>
    public const int MaxCacheEntries = 1024;

    private readonly IApiKeyStore _inner;
    private readonly IOptions<ApiKeyOptions> _options;
    private readonly ILogger<BoundedApiKeyStore> _log;
    private readonly TimeProvider _time;
    private readonly ConcurrentDictionary<string, Entry<ApiKeyLookup?>> _keys = new(StringComparer.Ordinal);
    private readonly ConcurrentDictionary<string, Entry<IReadOnlyList<NodeAcl>>> _grants =
        new(StringComparer.Ordinal);

    /// <summary>Initializes a new instance of the <see cref="BoundedApiKeyStore"/> class.</summary>
    /// <param name="inner">The store being bounded.</param>
    /// <param name="options">The <c>Security:ApiKeys</c> options carrying the deadline and TTL.</param>
    /// <param name="log">The logger.</param>
    /// <param name="time">
    ///     The shared clock. The same instance the authorizer reads — see
    ///     <c>DataPlaneAuthRegistration</c> for why a second clock here would be a hazard rather than a
    ///     detail.
    /// </param>
    public BoundedApiKeyStore(
        IApiKeyStore inner,
        IOptions<ApiKeyOptions> options,
        ILogger<BoundedApiKeyStore> log,
        TimeProvider time)
    {
        ArgumentNullException.ThrowIfNull(inner);
        ArgumentNullException.ThrowIfNull(options);
        ArgumentNullException.ThrowIfNull(log);
        ArgumentNullException.ThrowIfNull(time);

        _inner = inner;
        _options = options;
        _log = log;
        _time = time;
    }

    /// <summary>The store being bounded.</summary>
    /// <remarks>
    ///     Exposed so the composition root's wiring can be asserted. A decorator registered in one branch
    ///     of a role conditional is the "built but never wired" shape, and a guard that cannot see what is
    ///     inside the wrapper cannot tell a correctly-wired node from one wrapping the wrong store.
    /// </remarks>
    public IApiKeyStore Inner => _inner;

    /// <summary>The clock this bound's deadline and TTL are measured on.</summary>
    /// <remarks>Same reason as <see cref="Inner"/>: the shared-clock invariant is only pinnable by reference.</remarks>
    public TimeProvider Clock => _time;

    /// <inheritdoc />
    /// <remarks>
    ///     Keyed by <paramref name="apiKeyId"/> alone. <paramref name="utcNow"/> is deliberately not part
    ///     of the key — it changes on every call, so including it would defeat the cache entirely — but a
    ///     cached hit IS re-checked against it before being served, so the TTL can never resurrect a key
    ///     that has expired by the clock in the meantime. The TTL therefore bounds how stale an
    ///     <c>Enabled</c> revocation may be, and nothing else.
    /// </remarks>
    public async Task<ApiKeyLookup?> FindAsync(string apiKeyId, DateTime utcNow, CancellationToken ct)
    {
        var ttl = Ttl;
        if (ttl > TimeSpan.Zero
            && TryServe(_keys, apiKeyId, ttl, out var cached)
            && !IsExpired(cached, utcNow))
        {
            // Metered HERE rather than inside TryServe: an entry that is inside the TTL but whose key has
            // since expired is not a hit — it falls through to a store read — so counting it in TryServe
            // would report cache hits that never happened.
            MeterCacheHit();
            return cached;
        }

        var fresh = await BoundAsync(token => _inner.FindAsync(apiKeyId, utcNow, token), ct)
            .ConfigureAwait(false);
        if (ttl > TimeSpan.Zero) Store(_keys, apiKeyId, fresh);
        return fresh;
    }

    /// <inheritdoc />
    /// <remarks>Keyed by principal — the anonymous activation path, which runs on every unauthenticated connect.</remarks>
    public async Task<IReadOnlyList<NodeAcl>> FindGrantsAsync(string principal, CancellationToken ct)
    {
        var ttl = Ttl;
        if (ttl > TimeSpan.Zero && TryServe(_grants, principal, ttl, out var cached))
        {
            MeterCacheHit();
            return cached;
        }

        var fresh = await BoundAsync(token => _inner.FindGrantsAsync(principal, token), ct)
            .ConfigureAwait(false);
        if (ttl > TimeSpan.Zero) Store(_grants, principal, fresh);
        return fresh;
    }

    /// <summary>The configured TTL; non-positive means the cache is off.</summary>
    private TimeSpan Ttl => TimeSpan.FromSeconds(_options.Value.StoreCacheSeconds);

    /// <summary>Whether a cached lookup has since expired by the clock.</summary>
    /// <param name="cached">The cached lookup, possibly null (an authoritative miss).</param>
    /// <param name="utcNow">The caller's current time.</param>
    /// <returns><see langword="true"/> when the cached key must not be served.</returns>
    private static bool IsExpired(ApiKeyLookup? cached, DateTime utcNow) =>
        cached?.Key.ExpiresUtc is { } expires && expires <= utcNow;

    /// <summary>Counts one lookup the cache actually answered.</summary>
    private static void MeterCacheHit() =>
        OtOpcUaTelemetry.ApiKeyStoreBounded.Add(1, new KeyValuePair<string, object?>("outcome", "cache_hit"));

    /// <summary>Serves an entry if one is present and still inside the TTL.</summary>
    /// <typeparam name="T">The cached value type.</typeparam>
    /// <param name="cache">The cache to read.</param>
    /// <param name="key">The cache key.</param>
    /// <param name="ttl">How long an entry stays servable.</param>
    /// <param name="value">The cached value, when this returns true.</param>
    /// <returns><see langword="true"/> on a hit.</returns>
    private bool TryServe<T>(
        ConcurrentDictionary<string, Entry<T>> cache, string key, TimeSpan ttl, out T value)
    {
        if (cache.TryGetValue(key, out var entry) && _time.GetUtcNow() - entry.StampedUtc < ttl)
        {
            value = entry.Value;
            return true;
        }

        value = default!;
        return false;
    }

    /// <summary>Stores a COMPLETED lookup, clearing wholesale first if the ceiling has been reached.</summary>
    /// <typeparam name="T">The cached value type.</typeparam>
    /// <param name="cache">The cache to write.</param>
    /// <param name="key">The cache key.</param>
    /// <param name="value">The value to cache.</param>
    private void Store<T>(ConcurrentDictionary<string, Entry<T>> cache, string key, T value)
    {
        if (cache.Count >= MaxCacheEntries) cache.Clear();
        cache[key] = new Entry<T>(_time.GetUtcNow(), value);
    }

    /// <summary>One cached lookup and the instant it was taken.</summary>
    /// <typeparam name="T">The cached value type.</typeparam>
    /// <param name="StampedUtc">When the inner store answered.</param>
    /// <param name="Value">What it answered.</param>
    private readonly record struct Entry<T>(DateTimeOffset StampedUtc, T Value);

    /// <summary>Runs one inner call under the configured deadline.</summary>
    /// <typeparam name="T">The inner call's result type.</typeparam>
    /// <param name="call">The inner call, taking the deadline-linked token.</param>
    /// <param name="ct">The caller's token — in production always <c>None</c>, see remarks.</param>
    /// <returns>Whatever the inner store returned.</returns>
    /// <remarks>
    ///     <para>
    ///         The deadline is created HERE rather than received from the caller because there is no caller
    ///         to receive one from: <c>OpcUaApplicationHost</c> block-bridges the SDK's synchronous
    ///         impersonation callback with <see cref="CancellationToken.None"/>, so the <c>ct</c> the store
    ///         sees in production is always uncancellable.
    ///     </para>
    ///     <para>
    ///         ⚠️ <b>The deadline is a HARD race, not a cooperative token — and that distinction was a live
    ///         gate failure, not a theoretical one.</b> This used to be a
    ///         <c>CancellationTokenSource(TimeSpan, TimeProvider)</c> linked into the inner call, which
    ///         bounds the call only if the callee chooses to observe the token.
    ///         <c>Microsoft.Data.SqlClient</c> does not, while a connect or login is in flight on a
    ///         black-holed socket: the 2026-08-20 R3-05 gate paused the SQL container and watched a keyed
    ///         activation hang <b>42.9 s</b> past its 2000 ms bound (the Warning below fired only at
    ///         unpause), stalling an unrelated already-open subscription for <b>57 s</b> behind the
    ///         <c>SessionManager</c> lock — the exact hazard the bound exists to prevent. So the inner call
    ///         is now raced against a <see cref="Task.Delay(TimeSpan, TimeProvider, CancellationToken)"/>
    ///         on the injected clock and <b>abandoned</b> when the delay wins.
    ///     </para>
    ///     <para>
    ///         ⚠️ <b>An abandoned lookup's answer is DISCARDED, never cached.</b> It was taken before the
    ///         deadline and would be served after it, so a key revoked or expired in the meantime could be
    ///         admitted from a lookup nobody was still waiting for. The token is still cancelled — a
    ///         cooperative inner store should stop — and the abandoned task's outcome is observed by
    ///         <see cref="ObserveAbandoned{T}"/> so it can never surface as an
    ///         <c>UnobservedTaskException</c>.
    ///     </para>
    ///     <para>
    ///         ⚠️ <b>It throws rather than returning <see langword="null"/>.</b> <c>null</c> means
    ///         "no such usable key", which the authenticator logs as <i>denied — no enabled, unexpired key
    ///         with id X</i>. A timeout is not that, and an operator reading that line during a ConfigDb
    ///         outage would go hunting for a key problem that does not exist. The distinct Warning below is
    ///         the honest signal; the denial itself is unchanged either way — the authenticator's catch-all
    ///         turns any throw into the same opaque denial.
    ///     </para>
    /// </remarks>
    private async Task<T> BoundAsync<T>(Func<CancellationToken, Task<T>> call, CancellationToken ct)
    {
        var timeoutMs = _options.Value.StoreTimeoutMs;
        if (timeoutMs <= 0) return await call(ct).ConfigureAwait(false);

        // Linked with the caller's token so a genuine caller cancellation is still honoured — and so the
        // winner check below can tell the two apart. NOT linked to the deadline: the deadline is the race,
        // and this token is only the courtesy signal to an inner store that happens to be cooperative.
        var innerCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
        Task<T> inner;
        try
        {
            inner = call(innerCts.Token);
        }
        catch
        {
            innerCts.Dispose();
            throw;
        }

        // The delay runs on the INJECTED clock, so it is the same TimeProvider the authorizer and the
        // revalidation timer read. Its own CTS exists only to stop the timer when the inner call wins.
        using var delayCts = new CancellationTokenSource();
        var deadline = Task.Delay(TimeSpan.FromMilliseconds(timeoutMs), _time, delayCts.Token);

        if (await Task.WhenAny(inner, deadline).ConfigureAwait(false) == inner)
        {
            delayCts.Cancel();
            innerCts.Dispose();
            return await inner.ConfigureAwait(false);
        }

        innerCts.Cancel();
        ObserveAbandoned(inner, innerCts);

        OtOpcUaTelemetry.ApiKeyStoreBounded.Add(
            1, new KeyValuePair<string, object?>("outcome", "timeout"));
        _log.LogWarning(
            "ApiKeyStore: the key store did not answer within {TimeoutMs} ms — denying this activation. "
            + "The lookup is abandoned and its result, if it ever arrives, is discarded. Every OPC UA "
            + "session activation on this server is serialized behind this call; check ConfigDb health.",
            timeoutMs);
        throw new OperationCanceledException(
            $"The API key store did not answer within {timeoutMs} ms; the lookup was abandoned.");
    }

    /// <summary>
    ///     Watches an abandoned inner lookup to its end so its outcome is observed and its resources are
    ///     released, and discards whatever it produced.
    /// </summary>
    /// <typeparam name="T">The abandoned call's result type.</typeparam>
    /// <param name="abandoned">The lookup the deadline gave up on.</param>
    /// <param name="cts">The cancellation source that call was handed; disposed once it finishes.</param>
    /// <remarks>
    ///     Nobody awaits <paramref name="abandoned"/>, so without this its fault would be unobserved.
    ///     Debug rather than Warning: the timeout has already been reported at Warning, and a ConfigDb
    ///     outage would otherwise produce two lines per activation for one event.
    /// </remarks>
    private void ObserveAbandoned<T>(Task<T> abandoned, CancellationTokenSource cts) =>
        _ = abandoned.ContinueWith(
            finished =>
            {
                cts.Dispose();
                var error = finished.Exception; // The observation itself — do not remove.
                if (error is not null)
                {
                    _log.LogDebug(
                        error,
                        "ApiKeyStore: an abandoned key store lookup faulted after its deadline; discarded.");
                }
                else
                {
                    _log.LogDebug(
                        "ApiKeyStore: an abandoned key store lookup finished after its deadline; discarded.");
                }
            },
            CancellationToken.None,
            TaskContinuationOptions.ExecuteSynchronously,
            TaskScheduler.Default);
}