using System.Collections.Concurrent;

namespace w4c_workflows.Services;

/// <summary>
/// Serializes concurrent workflow syncs for the same <c>(tenant, repo)</c> within
/// one process. Two overlapping <c>POST /sync</c> calls would otherwise both
/// insert the same deterministic workflow/task ids and collide on the primary key
/// (<c>DbUpdateException</c>). The gate makes the second call wait, so it observes
/// the first call's committed rows and upserts instead of duplicating.
///
/// For multi-replica deployments this is combined with the distributed
/// <see cref="ILeaseService"/> lease in <see cref="WorkflowSyncService"/>.
/// </summary>
public sealed class SyncGate
{
    private readonly ConcurrentDictionary<string, SemaphoreSlim> _gates = new(StringComparer.Ordinal);

    public async Task<IDisposable> AcquireAsync(string tenantId, string repoName, CancellationToken ct)
    {
        var gate = _gates.GetOrAdd(tenantId + "|" + repoName, _ => new SemaphoreSlim(1, 1));
        await gate.WaitAsync(ct);
        return new Releaser(gate);
    }

    private sealed class Releaser(SemaphoreSlim gate) : IDisposable
    {
        private int _released;

        public void Dispose()
        {
            if (Interlocked.Exchange(ref _released, 1) == 0)
                gate.Release();
        }
    }
}