Follow requests nobody answers are sent again

A Follow was resent only when the persona followed again. Lemmy 1.0
sends nothing it queued for a server before it started sending there, so
the Accept of a community follow made on first contact was lost for good:
the village's persona stayed "requested" a day while Lemmy listed her as a
follower, and every post the community announced was refused as not
followed. A request still unanswered is now sent again, the same activity,
after 15 minutes, an hour, 6 hours, a day, two and four days
(FollowResender, every 15 minutes); a server that holds the follow answers
the copy.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Domain/Social/FollowResender.cs

@@ -0,0 +1,27 @@
+namespace PrivaPub.Domain.Social
+{
+	// sends again, now and then, the follow requests still unanswered (FollowService.ResendPending)
+	public sealed class FollowResender(IServiceScopeFactory scopes, ILogger<FollowResender> logger) : BackgroundService
+	{
+		static readonly TimeSpan Every = TimeSpan.FromMinutes(15);
+
+		protected override async Task ExecuteAsync(CancellationToken token)
+		{
+			using var timer = new PeriodicTimer(Every);
+			while (await timer.WaitForNextTickAsync(token))
+			{
+				try
+				{
+					using var scope = scopes.CreateScope();
+					var sent = await scope.ServiceProvider.GetRequiredService<IFollowService>().ResendPending(DateTime.UtcNow, token);
+					if (sent > 0)
+						logger.LogInformation("Sent {Count} unanswered follow requests again", sent);
+				}
+				catch (Exception ex) when (ex is not OperationCanceledException)
+				{
+					logger.LogWarning(ex, "Unanswered follow requests could not be sent again");
+				}
+			}
+		}
+	}
+}

PrivaPub/Domain/Social/FollowService.cs

@@ -29,6 +29,7 @@ namespace PrivaPub.Domain.Social
 		Task<Following> FollowAs(LocalActor follower, string target, bool showReblogs, CancellationToken token);
 		Task UnfollowAs(LocalActor follower, string target, CancellationToken token);
 		Task<bool> Decide(LocalActor me, string followerAccountId, bool accept, CancellationToken token);
+		Task<int> ResendPending(DateTime now, CancellationToken token);
 	}
 
 	public class FollowService : IFollowService
@@ -276,6 +277,37 @@ namespace PrivaPub.Domain.Social
 			return (default, actorUri == default ? default : await _remoteActors.GetActor(actorUri, refresh: false, token));
 		}
 
+		// how long after it was made, or last sent again, a request still unanswered is sent again: a server can take a Follow and
+		// lose its answer (Lemmy 1.0 sends nothing it queued for a server before it started sending there), and one that
+		// already holds the follow answers the copy again
+		public static readonly TimeSpan[] ResendAfter =
+		{
+			TimeSpan.FromMinutes(15), TimeSpan.FromHours(1), TimeSpan.FromHours(6), TimeSpan.FromDays(1), TimeSpan.FromDays(2), TimeSpan.FromDays(4)
+		};
+
+		// the requests to remote accounts due to be sent again; how many were
+		public async Task<int> ResendPending(DateTime now, CancellationToken token)
+		{
+			var sent = 0;
+			// (a request older than the count has no Resent yet: "not at least" matches it, "less than" would not)
+			var pending = await _dbEntities.Followings.Match(f => f.State == FollowState.Requested && !f.TargetIsLocal && !(f.Resent >= ResendAfter.Length))
+				.ExecuteAsync(token);
+			foreach (var following in pending.Where(f => (f.ResentAt ?? f.CreatedAt) + ResendAfter[f.Resent] <= now))
+			{
+				var follower = await _localActors.FindById(LocalActorKind.Person, following.AvatarId, token);
+				var inbox = following.TargetInboxURL ?? (await _remoteActors.GetActor(following.TargetActorURI, refresh: false, token))?.InboxURL;
+				if (follower != default && !string.IsNullOrEmpty(inbox))
+				{
+					await _delivery.Enqueue(follower, new[] { inbox }, FollowActivity(follower, following), token,
+						again: "resend-" + now.ToString("yyyyMMddHHmm", System.Globalization.CultureInfo.InvariantCulture));
+					sent++;
+				}
+				await DB.Default.Update<Following>().MatchID(following.ID).Modify(b => b.Inc(f => f.Resent, 1)).Modify(f => f.ResentAt, now)
+					.ExecuteAsync(token);
+			}
+			return sent;
+		}
+
 		static JsonObject FollowActivity(LocalActor follower, Following following) => new()
 		{
 			["@context"] = ActivityPubRenderer.ActivityStreams,