Inboxintel/src/InboxIntel.Infrastructure/Gmail/GmailService.cs

using Google;
using Google.Apis.Gmail.v1;
using Google.Apis.Gmail.v1.Data;
using InboxIntel.Application.Abstractions;
using InboxIntel.Infrastructure.Configuration;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using Polly;
using Polly.Retry;
using System.Net;
using DomainLabel = InboxIntel.Domain.Entities.Label;

namespace InboxIntel.Infrastructure.Gmail;

/// <summary>
/// Gmail REST API wrapper. Every call is wrapped in a Polly retry pipeline that
/// applies exponential backoff with jitter on 429 / 5xx / transient errors,
/// honouring the configured max retry count. Responses are null-checked before use.
/// </summary>
public class GmailApiService : IGmailService
{
    private readonly GmailClientFactory _factory;
    private readonly GmailSyncOptions _options;
    private readonly ILogger<GmailApiService> _logger;
    private readonly ResiliencePipeline _pipeline;

    // Cache the authenticated client per user so parallel message fetches don't
    // each rebuild it (which would hit the shared DbContext concurrently).
    private readonly SemaphoreSlim _clientLock = new(1, 1);
    private Google.Apis.Gmail.v1.GmailService? _cachedClient;
    private Guid _cachedUserId;

    public GmailApiService(GmailClientFactory factory, IOptions<GmailSyncOptions> options, ILogger<GmailApiService> logger)
    {
        _factory = factory;
        _options = options.Value;
        _logger = logger;
        _pipeline = new ResiliencePipelineBuilder()
            .AddRetry(new RetryStrategyOptions
            {
                ShouldHandle = new PredicateBuilder()
                    .Handle<GoogleApiException>(IsTransient)
                    .Handle<HttpRequestException>(),
                MaxRetryAttempts = _options.MaxRetries,
                BackoffType = DelayBackoffType.Exponential,
                UseJitter = true,
                Delay = TimeSpan.FromMilliseconds(_options.BackoffBaseMs),
                OnRetry = args =>
                {
                    _logger.LogWarning("Gmail API transient failure, retry {Attempt} after {Delay}ms",
                        args.AttemptNumber, args.RetryDelay.TotalMilliseconds);
                    return default;
                }
            })
            .Build();
    }

    private static bool IsTransient(GoogleApiException ex) =>
        ex.HttpStatusCode is HttpStatusCode.TooManyRequests
            or HttpStatusCode.InternalServerError
            or HttpStatusCode.BadGateway
            or HttpStatusCode.ServiceUnavailable
            or HttpStatusCode.GatewayTimeout;

    /// <summary>Builds the Gmail client once per user and reuses it (thread-safe).</summary>
    private async Task<Google.Apis.Gmail.v1.GmailService> GetClientAsync(Guid userId, CancellationToken ct)
    {
        if (_cachedClient is not null && _cachedUserId == userId) return _cachedClient;
        await _clientLock.WaitAsync(ct);
        try
        {
            if (_cachedClient is null || _cachedUserId != userId)
            {
                _cachedClient = await _factory.CreateAsync(userId, ct);
                _cachedUserId = userId;
            }
            return _cachedClient;
        }
        finally { _clientLock.Release(); }
    }

    public async Task<string> GetProfileHistoryIdAsync(Guid userId, CancellationToken ct = default)
    {
        var client = await GetClientAsync(userId, ct);
        var profile = await _pipeline.ExecuteAsync(async token =>
            await client.Users.GetProfile("me").ExecuteAsync(token), ct);
        return profile?.HistoryId?.ToString() ?? throw new InvalidOperationException("Gmail profile returned no historyId.");
    }

    public async Task<GmailMessagePage> ListMessageIdsAsync(Guid userId, string? pageToken, CancellationToken ct = default)
    {
        var client = await GetClientAsync(userId, ct);
        var page = await _pipeline.ExecuteAsync(async token =>
        {
            var req = client.Users.Messages.List("me");
            req.MaxResults = _options.PageSize;
            req.PageToken = pageToken;
            // Include spam & trash so those folders aren't structurally empty; their state is
            // captured via the SPAM/TRASH labels (IsTrashed + EmailLabels) during upsert.
            req.IncludeSpamTrash = true;
            return await req.ExecuteAsync(token);
        }, ct);

        var ids = page?.Messages?.Select(m => m.Id).Where(id => id is not null).ToList() ?? new List<string>();
        return new GmailMessagePage(ids!, page?.NextPageToken, (int)(page?.ResultSizeEstimate ?? 0));
    }

    public async Task<GmailMessageDetail> GetMessageAsync(Guid userId, string gmailMessageId, CancellationToken ct = default)
    {
        var client = await GetClientAsync(userId, ct);
        var msg = await _pipeline.ExecuteAsync(async token =>
        {
            var req = client.Users.Messages.Get("me", gmailMessageId);
            req.Format = UsersResource.MessagesResource.GetRequest.FormatEnum.Full;
            return await req.ExecuteAsync(token);
        }, ct);

        if (msg is null) throw new InvalidOperationException($"Gmail returned null for message {gmailMessageId}.");
        return GmailMessageParser.Parse(msg);
    }

    public async Task<GmailHistoryPage> ListHistoryAsync(Guid userId, string startHistoryId, string? pageToken, CancellationToken ct = default)
    {
        var client = await GetClientAsync(userId, ct);
        var history = await _pipeline.ExecuteAsync(async token =>
        {
            var req = client.Users.History.List("me");
            req.StartHistoryId = ulong.Parse(startHistoryId);
            req.PageToken = pageToken;
            return await req.ExecuteAsync(token);
        }, ct);

        var changed = new List<string>();
        var deleted = new List<string>();
        foreach (var h in history?.History ?? Enumerable.Empty<History>())
        {
            if (h.MessagesAdded is not null) changed.AddRange(h.MessagesAdded.Select(m => m.Message.Id));
            if (h.MessagesDeleted is not null) deleted.AddRange(h.MessagesDeleted.Select(m => m.Message.Id));
        }
        return new GmailHistoryPage(changed, deleted, history?.NextPageToken, history?.HistoryId?.ToString());
    }

    public async Task<IReadOnlyList<DomainLabel>> ListLabelsAsync(Guid userId, CancellationToken ct = default)
    {
        var client = await GetClientAsync(userId, ct);
        var resp = await _pipeline.ExecuteAsync(async token =>
            await client.Users.Labels.List("me").ExecuteAsync(token), ct);

        return resp?.Labels?.Select(l => new DomainLabel
        {
            UserId = userId,
            GmailLabelId = l.Id,
            Name = l.Name,
            Type = l.Type ?? "user"
        }).ToList() ?? new List<DomainLabel>();
    }

    public async Task BatchModifyAsync(Guid userId, IEnumerable<string> messageIds, IEnumerable<string> addLabelIds, IEnumerable<string> removeLabelIds, CancellationToken ct = default)
    {
        var client = await GetClientAsync(userId, ct);
        var body = new BatchModifyMessagesRequest
        {
            Ids = messageIds.ToList(),
            AddLabelIds = addLabelIds.ToList(),
            RemoveLabelIds = removeLabelIds.ToList()
        };
        await _pipeline.ExecuteAsync(async token =>
        {
            await client.Users.Messages.BatchModify(body, "me").ExecuteAsync(token);
            return true;
        }, ct);
    }

    public async Task BatchTrashAsync(Guid userId, IEnumerable<string> messageIds, CancellationToken ct = default)
    {
        var client = await GetClientAsync(userId, ct);
        foreach (var id in messageIds)
        {
            await _pipeline.ExecuteAsync(async token =>
            {
                await client.Users.Messages.Trash("me", id).ExecuteAsync(token);
                return true;
            }, ct);
        }
    }

    public async Task BatchDeleteAsync(Guid userId, IEnumerable<string> messageIds, CancellationToken ct = default)
    {
        var client = await GetClientAsync(userId, ct);
        var body = new BatchDeleteMessagesRequest { Ids = messageIds.ToList() };
        await _pipeline.ExecuteAsync(async token =>
        {
            await client.Users.Messages.BatchDelete(body, "me").ExecuteAsync(token);
            return true;
        }, ct);
    }
}