using System.Text.Json;
using Akka.Actor;
using Microsoft.EntityFrameworkCore;
using Shouldly;
using Xunit;
using ZB.MOM.WW.OtOpcUa.Commons.Interfaces;
using ZB.MOM.WW.OtOpcUa.Commons.Messages.Deploy;
using ZB.MOM.WW.OtOpcUa.Commons.Messages.Fleet;
using ZB.MOM.WW.OtOpcUa.Commons.Types;
using ZB.MOM.WW.OtOpcUa.Configuration;
using ZB.MOM.WW.OtOpcUa.Configuration.Entities;
using ZB.MOM.WW.OtOpcUa.Configuration.Enums;
using ZB.MOM.WW.OtOpcUa.Runtime.Drivers;
using ZB.MOM.WW.OtOpcUa.Runtime.Tests.Harness;

namespace ZB.MOM.WW.OtOpcUa.Runtime.Tests.Drivers;

/// <summary>
///     Perf roadmap Task 26 — the <c>Direct</c>-mode artifact read is I/O (an EF read of a potentially
///     multi-MB gzip blob out of central SQL) and must not run on the actor thread.
/// </summary>
/// <remarks>
///     <para>
///         The <c>FetchAndCache</c> path has always piped its artifact fetch back to <c>Self</c> for
///         exactly this reason. <c>Direct</c> read the blob INLINE inside the receive, so every value
///         batch, diagnostics request and driver command queued behind central SQL for the whole read —
///         on a WAN link or a loaded server, for as long as that read takes.
///     </para>
///     <para>
///         The load-bearing assertion is therefore that the host answers an <b>unrelated</b> message
///         while the read is still in flight, not that the ACK eventually arrives: a blocking read still
///         acks, just late and behind everything else.
///     </para>
/// </remarks>
public sealed class DriverHostActorDirectApplyOffMailboxTests : RuntimeActorTestBase
{
    private static readonly NodeId TestNode = NodeId.Parse("driver-test");
    private static readonly RevisionHash RevA = RevisionHash.Parse(new string('a', 64));

    [Fact]
    public void A_slow_direct_artifact_read_does_not_block_the_host_mailbox()
    {
        var inner = NewInMemoryDbFactory();
        var db = new GatedConfigDbFactory(inner);
        var deploymentId = SeedDeployment(inner, RevA);

        var coordinator = CreateTestProbe();
        var actor = Sys.ActorOf(DriverHostActor.Props(
            db, TestNode, coordinator.Ref,
            localRoles: new HashSet<string> { "driver" }));

        try
        {
            // Barrier: an answered GetDiagnostics proves PreStart (and its bootstrap ConfigDb read) is
            // done, so the allowance below counts only the apply's own contexts.
            actor.Tell(new GetDiagnostics(CorrelationId.NewId()), coordinator.Ref);
            coordinator.ExpectMsg<NodeDiagnosticsSnapshot>(TimeSpan.FromSeconds(5));

            // Let ONE more context through — the apply's Applying-row upsert, which is a deliberate
            // on-thread write (bounded, one row) — and block the next, which is the artifact read.
            // Both misconfigurations of this number fail LOUDLY rather than passing vacuously: too
            // small blocks the upsert and the assertion below times out even on a fixed build; too
            // large never engages the gate and the presence assertion on Blocked fails.
            db.CloseAfter(allowThrough: 1);

            actor.Tell(new DispatchDeployment(deploymentId, RevA, CorrelationId.NewId()));

            // The read was entered and has NOT completed — the gate is still closed.
            AwaitAssert(() => db.Blocked.ShouldBe(1), duration: PresenceBudget);
            db.Unblocked.ShouldBe(0);

            // THE assertion: with the artifact read still in flight the host is still processing its
            // mailbox. Read inline on the actor thread, this times out.
            actor.Tell(new GetDiagnostics(CorrelationId.NewId()), coordinator.Ref);
            coordinator.ExpectMsg<NodeDiagnosticsSnapshot>(TimeSpan.FromSeconds(3));

            // Releasing the read completes the apply exactly as before — same Applied ACK, same revision.
            db.Open();
            coordinator.ExpectMsg<ApplyAck>(TimeSpan.FromSeconds(10)).Outcome.ShouldBe(ApplyAckOutcome.Applied);

            actor.Tell(new GetDiagnostics(CorrelationId.NewId()), coordinator.Ref);
            coordinator.ExpectMsg<NodeDiagnosticsSnapshot>(TimeSpan.FromSeconds(5)).CurrentRevision.ShouldBe(RevA);
        }
        finally
        {
            // Never leave the gate closed: a parked read would hang ActorSystem shutdown.
            db.Open();
        }
    }

    /// <summary>Seeds a sealed, parseable deployment and returns its id.</summary>
    private static DeploymentId SeedDeployment(IDbContextFactory<OtOpcUaConfigDbContext> db, RevisionHash rev)
    {
        var artifact = JsonSerializer.SerializeToUtf8Bytes(new { DriverInstances = Array.Empty<object>() });
        var id = DeploymentId.NewId();
        using var ctx = db.CreateDbContext();
        ctx.Deployments.Add(new Deployment
        {
            DeploymentId = id.Value,
            RevisionHash = rev.Value,
            Status = DeploymentStatus.Sealed,
            CreatedBy = "test",
            SealedAtUtc = DateTime.UtcNow,
            ArtifactBlob = artifact,
        });
        ctx.SaveChanges();
        return id;
    }

    /// <summary>
    ///     A ConfigDb factory that parks the caller on a gate — the fake shape of a slow central-SQL
    ///     read. It blocks the calling THREAD (not a task), because that is what an EF read does and it
    ///     is the only shape that can prove which thread the read runs on.
    /// </summary>
    private sealed class GatedConfigDbFactory(IDbContextFactory<OtOpcUaConfigDbContext> inner)
        : IDbContextFactory<OtOpcUaConfigDbContext>
    {
        private readonly ManualResetEventSlim _gate = new(initialState: true);
        private int _allowance = int.MaxValue;
        private int _blocked;
        private int _unblocked;

        /// <summary>How many callers have reached the closed gate.</summary>
        public int Blocked => Volatile.Read(ref _blocked);

        /// <summary>How many of those have been let through again.</summary>
        public int Unblocked => Volatile.Read(ref _unblocked);

        /// <summary>Closes the gate, after letting <paramref name="allowThrough"/> more contexts past.</summary>
        public void CloseAfter(int allowThrough)
        {
            Volatile.Write(ref _allowance, allowThrough);
            _gate.Reset();
        }

        /// <summary>Opens the gate for good — also the test-teardown safety net.</summary>
        public void Open() => _gate.Set();

        public OtOpcUaConfigDbContext CreateDbContext()
        {
            if (Interlocked.Decrement(ref _allowance) < 0 && !_gate.IsSet)
            {
                Interlocked.Increment(ref _blocked);
                // Bounded so a mis-wired test fails rather than hanging the whole assembly.
                _gate.Wait(TimeSpan.FromSeconds(30));
                Interlocked.Increment(ref _unblocked);
            }

            return inner.CreateDbContext();
        }
    }
}