Media are reserved for one post, atomically

An upload's owner checked that it was free, then attached it in a second step, so two posts asking for the same
upload at once could both get it; a post now, or an edit, could take media a scheduled post held, which then failed
when its time came (only logged) and was deleted; and a persona deleted or banned after scheduling a post still
published it.

Now a post claims its media in one conditional update before anything is written (its id is minted first), and a
post that loses the race is refused with 422, the media it took put back. Scheduling reserves media the same way.
Media held by a scheduled post are that post's alone, and its job publishes only for a persona still there whose root
is neither deleted nor banned; otherwise what it held is trashed. MastodonScheduledStatusesTests: a post now and an
edit can't take scheduled media, two racing posts never both get an upload, a gone persona's scheduled post never
publishes.

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

PrivaPub.Tests/Http/MastodonScheduledStatusesTests.cs

@@ -99,5 +99,60 @@ namespace PrivaPub.Tests.Http
 			Assert.Equal(published.Text("id"), media.PostId);
 			Assert.Null(media.ScheduledStatusId);
 		}
+
+		async Task<string> Picture(Mastodon account)
+		{
+			var upload = MastodonHelpers.Multipart(("file", MastodonHelpers.JpegWithMetadata(16, 16), "image/jpeg", "a.jpg"));
+			return (await account.Client.Exchange(new HttpRequestMessage(HttpMethod.Post, "/api/v2/media") { Content = upload })).Ok().Body.Text("id");
+		}
+
+		// a scheduled post's media are its alone: a post now, or an edit, can't take them
+		[Fact]
+		public async Task A_scheduled_posts_media_cannot_be_taken_by_a_post_now()
+		{
+			var alice = await _host.Mastodon("alice");
+			var mediaId = await Picture(alice);
+			(await alice.Client.Post("/api/v1/statuses", ("status", "later"), ("media_ids[]", mediaId), ("scheduled_at", In(TimeSpan.FromMinutes(10))))).Ok();
+
+			Assert.Equal(HttpStatusCode.UnprocessableEntity, (await alice.Client.Post("/api/v1/statuses", ("status", "now"), ("media_ids[]", mediaId))).Status);
+			var plain = (await alice.Client.Post("/api/v1/statuses", ("status", "plain"))).Ok().Body.Text("id");
+			Assert.Equal(HttpStatusCode.UnprocessableEntity, (await alice.Client.Put($"/api/v1/statuses/{plain}", ("status", "edited"), ("media_ids[]", mediaId))).Status);
+			var media = await DB.Default.Find<MediaAttachment>().OneAsync(mediaId, Token);
+			Assert.Null(media.PostId);
+			Assert.NotNull(media.ScheduledStatusId);
+		}
+
+		// two posts asking for the same upload at once: exactly one gets it
+		[Fact]
+		public async Task Two_posts_racing_for_one_upload_never_both_get_it()
+		{
+			var alice = await _host.Mastodon("alice");
+			for (var round = 0; round < 5; round++)
+			{
+				var mediaId = await Picture(alice);
+				var answers = await Task.WhenAll(Enumerable.Range(0, 2).Select(i =>
+					alice.Client.Post("/api/v1/statuses", ("status", $"race {round} {i}"), ("media_ids[]", mediaId))));
+				Assert.Single(answers, a => a.Status == HttpStatusCode.OK);
+				Assert.Single(answers, a => a.Status == HttpStatusCode.UnprocessableEntity);
+			}
+		}
+
+		// a persona deleted since it scheduled a post publishes nothing, and what the post held is trashed
+		[Fact]
+		public async Task A_gone_personas_scheduled_post_never_publishes()
+		{
+			var gone = await _host.Mastodon($"gone{Guid.NewGuid():N}"[..12]);
+			var mediaId = await Picture(gone);
+			var id = (await gone.Client.Post("/api/v1/statuses", ("status", "from beyond"), ("media_ids[]", mediaId), ("scheduled_at", In(TimeSpan.FromMinutes(10))))).Ok().Object.Text("id");
+			await DB.Default.Update<Models.User.Avatar>().MatchID(gone.Persona.Id).Modify(a => a.DeletionAt, DateTime.UtcNow).ExecuteAsync(Token);
+
+			await DB.Default.Update<ScheduledStatus>().MatchID(id).Modify(s => s.ScheduledAt, DateTime.UtcNow.AddSeconds(-1)).ExecuteAsync(Token);
+			await DB.Default.Update<Job>().Match(j => j.Kind == JobKind.PublishScheduled && j.Payload == id).Modify(j => j.RunAt, DateTime.UtcNow).ExecuteAsync(Token);
+			await _host.Run(j => j.Kind == JobKind.PublishScheduled && j.Payload == id, Token);
+
+			Assert.False(await DB.Default.Find<Models.Post.Post>().Match(p => p.GroupUserId == gone.Persona.Id && p.Text == "from beyond").ExecuteAnyAsync(Token));
+			Assert.False(await DB.Default.Find<ScheduledStatus>().Match(s => s.ID == id).ExecuteAnyAsync(Token));
+			Assert.NotNull((await DB.Default.Find<MediaAttachment>().OneAsync(mediaId, Token)).TrashedAt);
+		}
 	}
 }

PrivaPub/Api/Mastodon/Controllers/StatusesController.cs

@@ -116,9 +116,21 @@ namespace PrivaPub.Api.Mastodon.Controllers
 				return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: Media attachments are not yours, already posted or too many");
 
 			var scheduled = new ScheduledStatus { AvatarId = MyId, ScheduledAt = at, Params = asked };
-			await DB.Default.SaveAsync(scheduled, token);
+			scheduled.ID = (string)scheduled.GenerateNewID();
+			// reserved in one conditional update: a post or another scheduled post taking one meanwhile wins
 			if (mediaIds.Count > 0)
-				await DB.Default.Update<Models.Media.MediaAttachment>().Match(m => mediaIds.Contains(m.ID)).Modify(m => m.ScheduledStatusId, scheduled.ID).ExecuteAsync(token);
+			{
+				var reserved = await DB.Default.Update<Models.Media.MediaAttachment>()
+					.Match(m => mediaIds.Contains(m.ID) && m.PostId == null && m.ScheduledStatusId == null && m.TrashedAt == null)
+					.Modify(m => m.ScheduledStatusId, scheduled.ID).ExecuteAsync(token);
+				if (reserved.MatchedCount != mediaIds.Count)
+				{
+					await DB.Default.Update<Models.Media.MediaAttachment>().Match(m => m.ScheduledStatusId == scheduled.ID)
+						.Modify(m => m.ScheduledStatusId, null).ExecuteAsync(token);
+					return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: Media attachments are not yours, already posted or too many");
+				}
+			}
+			await DB.Default.SaveAsync(scheduled, token);
 			await _jobs.EnqueueMany(new[] { ScheduledStatuses.JobFor(scheduled, new Uri(Me.BaseAddress).Host) }, token);
 			if (cacheKey != default)
 				_cache.Set(cacheKey + ":scheduled", scheduled.ID, TimeSpan.FromHours(1));

PrivaPub/Domain/Statuses/ScheduledStatuses.cs

@@ -7,6 +7,8 @@ using PrivaPub.Models.Jobs;
 using PrivaPub.Models.Media;
 using PrivaPub.Models.Post;
 using PrivaPub.Models.Social;
+using PrivaPub.Models.User;
+using PrivaPub.Domain.Media;
 
 namespace PrivaPub.Domain.Statuses
 {
@@ -19,8 +21,9 @@ namespace PrivaPub.Domain.Statuses
 		public const int MaxWaiting = 300;
 		public const int MaxPerDay = 25;
 
-		public static StatusDraft Draft(ScheduledParams p) => new()
+		public static StatusDraft Draft(ScheduledParams p, string scheduledId = default) => new()
 		{
+			ScheduledStatusId = scheduledId,
 			Text = p.Text,
 			PlainText = true,
 			SpoilerText = p.SpoilerText,
@@ -66,6 +69,15 @@ namespace PrivaPub.Domain.Statuses
 			RunAt = scheduled.ScheduledAt
 		};
 
+		// whether a persona may still publish: neither deleted nor its root deleted or banned
+		public static async Task<bool> MayPublish(string avatarId, CancellationToken token)
+		{
+			if (!await DB.Default.Find<Avatar>().Match(a => a.ID == avatarId && !a.DeletionAt.HasValue).ExecuteAnyAsync(token))
+				return false;
+			var rootIds = (await DB.Default.Find<RootToAvatar>().Match(r => r.AvatarId == avatarId).ExecuteAsync(token)).Select(r => r.RootId).ToList();
+			return await DB.Default.Find<RootUser>().Match(u => rootIds.Contains(u.ID) && u.DeletedAt == null && !u.IsBanned).ExecuteAnyAsync(token);
+		}
+
 		// the post's media go back to being plain uploads (the janitor's after a day), or are attached to the post by now
 		public static Task Release(string scheduledId, CancellationToken token) =>
 			DB.Default.Update<MediaAttachment>().Match(m => m.ScheduledStatusId == scheduledId).Modify(m => m.ScheduledStatusId, null).ExecuteAsync(token);
@@ -93,10 +105,17 @@ namespace PrivaPub.Domain.Statuses
 			if (scheduled == default || scheduled.ScheduledAt > DateTime.UtcNow.AddSeconds(30))
 				return JobOutcome.Done;//dropped, or moved to a later time that has its own job
 			using var scope = _scopes.CreateScope();
+			// a persona deleted or banned since it was scheduled publishes nothing, and what it held is trashed
+			if (!await ScheduledStatuses.MayPublish(scheduled.AvatarId, token))
+			{
+				await scope.ServiceProvider.GetRequiredService<IMediaService>().Trash(m => m.ScheduledStatusId == scheduled.ID, "its author is gone", token);
+				await DB.Default.DeleteAsync<ScheduledStatus>(scheduled.ID);
+				return JobOutcome.Done;
+			}
 			var author = await scope.ServiceProvider.GetRequiredService<ILocalActorService>().FindById(LocalActorKind.Person, scheduled.AvatarId, token);
 			if (author != default)
 			{
-				var outcome = await scope.ServiceProvider.GetRequiredService<IStatusService>().Publish(author, ScheduledStatuses.Draft(scheduled.Params), token);
+				var outcome = await scope.ServiceProvider.GetRequiredService<IStatusService>().Publish(author, ScheduledStatuses.Draft(scheduled.Params, scheduled.ID), token);
 				if (!outcome.Ok)
 					_logger.LogWarning("Scheduled status {Id} of {Avatar} could not be published: {Error}", scheduled.ID, scheduled.AvatarId, outcome.Error);
 			}

PrivaPub/Domain/Statuses/StatusService.cs

@@ -38,6 +38,8 @@ namespace PrivaPub.Domain.Statuses
 		public string ConversationId { get; init; }
 		public IReadOnlyList<string> Recipients { get; init; } = Array.Empty<string>();
 		public IReadOnlyList<string> MediaIds { get; init; }
+		// the scheduled post being published, whose media are reserved for it alone
+		public string ScheduledStatusId { get; init; }
 		public double? Latitude { get; init; }
 		public double? Longitude { get; init; }
 		public double? RangeKm { get; init; }
@@ -109,7 +111,7 @@ namespace PrivaPub.Domain.Statuses
 		{
 			if (TooLong(draft) is { } tooLong)
 				return tooLong;
-			var media = await Media(author, draft.MediaIds, default, token);
+			var media = await Media(author, draft.MediaIds, default, draft.ScheduledStatusId, token);
 			if (media == default)
 				return StatusOutcome.Fail(StatusCodes.Status422UnprocessableEntity, "Validation failed: Media attachments are invalid");
 			if (string.IsNullOrWhiteSpace(draft.Text) && media.Count == 0 && draft.Poll == default)
@@ -225,6 +227,9 @@ namespace PrivaPub.Domain.Statuses
 			post.ID = (string)post.GenerateNewID();
 			post.ObjectURI = author.PostUri(post.ID);
 			post.Url = author.PostHtmlUrl(post.ID);
+			// the media are this post's before anything is written: another post claiming them meanwhile wins, and this one is refused
+			if (!await Claim(media, post.ID, draft.ScheduledStatusId, token))
+				return StatusOutcome.Fail(StatusCodes.Status422UnprocessableEntity, "Validation failed: Media attachments are invalid");
 			post.LocalQuotePolicy = QuotePolicies.IsKnown(draft.QuotePolicy) ? draft.QuotePolicy : author.Settings.QuotePolicy ?? QuotePolicies.Public;
 			if (quoted is { IsFederatedCopy: false } && post.QuoteState == QuoteState.Accepted
 				&& await _localActors.FindById(LocalActorKind.Person, quoted.GroupUserId, token) is { } quotedAuthor)
@@ -270,7 +275,6 @@ namespace PrivaPub.Domain.Statuses
 			post.ActivityURI = create?["id"]?.GetValue<string>();
 
 			await DB.Default.SaveAsync(post, token);
-			await Attach(media, post.ID, token);
 			if (parent != default && Counted.Reply(post))
 				await DB.Default.Update<PostEntity>().MatchID(parent.ID).Modify(b => b.Inc(p => p.RepliesCount, 1)).ExecuteAsync(token);
 			await _fanout.Distribute(post, token);
@@ -297,7 +301,7 @@ namespace PrivaPub.Domain.Statuses
 				return StatusOutcome.Fail(StatusCodes.Status404NotFound, "Record not found");
 			if (TooLong(draft) is { } tooLong)
 				return tooLong;
-			var media = draft.MediaIds == default ? default : await Media(author, draft.MediaIds, post.ID, token);
+			var media = draft.MediaIds == default ? default : await Media(author, draft.MediaIds, post.ID, default, token);
 			if (draft.MediaIds != default && media == default)
 				return StatusOutcome.Fail(StatusCodes.Status422UnprocessableEntity, "Validation failed: Media attachments are invalid");
 			if (string.IsNullOrWhiteSpace(draft.Text) && (media ?? new List<MediaAttachment>()).Count == 0 && post.Media.Count == 0)
@@ -318,8 +322,9 @@ namespace PrivaPub.Domain.Statuses
 			var rendered = plain ? await _content.PlainText(draft.Text ?? string.Empty, token) : await _content.Markdown(draft.Text ?? string.Empty, token);
 			if (media != default)
 			{
+				if (!await Claim(media, post.ID, default, token))
+					return StatusOutcome.Fail(StatusCodes.Status422UnprocessableEntity, "Validation failed: Media attachments are invalid");
 				post.Media = media.Select(ToPostMedia).ToList();
-				await Attach(media, post.ID, token);
 				// what the edit left out is no longer held by anything
 				var kept = media.Select(m => m.ID).ToList();
 				await _media.Trash(m => m.PostId == post.ID && !kept.Contains(m.ID), "edited out", token);
@@ -688,7 +693,8 @@ namespace PrivaPub.Domain.Statuses
 			};
 		}
 
-		async Task<List<MediaAttachment>> Media(LocalActor author, IReadOnlyList<string> ids, string postId, CancellationToken token)
+		// the author's own uploads, not yet another post's (nor held by another scheduled post), nor trashed, nor pictures
+		async Task<List<MediaAttachment>> Media(LocalActor author, IReadOnlyList<string> ids, string postId, string scheduledId, CancellationToken token)
 		{
 			if (ids == default || ids.Count == 0)
 				return new List<MediaAttachment>();
@@ -697,17 +703,28 @@ namespace PrivaPub.Domain.Statuses
 			var wanted = ids.Distinct().ToList();
 			var found = await DB.Default.Find<MediaAttachment>()
 				.Match(m => wanted.Contains(m.ID) && m.OwnerAvatarId == author.Id && (m.PostId == null || m.PostId == postId)
-					&& m.TrashedAt == null && m.ProfileOfAvatarId == null)
+					&& (m.ScheduledStatusId == null || m.ScheduledStatusId == scheduledId) && m.TrashedAt == null && m.ProfileOfAvatarId == null)
 				.ExecuteAsync(token);
 			return found.Count == wanted.Count ? wanted.Select(id => found.First(m => m.ID == id)).ToList() : default;
 		}
 
-		static async Task Attach(IEnumerable<MediaAttachment> media, string postId, CancellationToken token)
+		// attaches the media in one conditional update, true when all of them are the post's; when another post took one
+		// meanwhile, the ones this call took go back as they were
+		static async Task<bool> Claim(IReadOnlyList<MediaAttachment> media, string postId, string scheduledId, CancellationToken token)
 		{
 			var ids = media.Select(m => m.ID).ToList();
-			if (ids.Count > 0)
+			if (ids.Count == 0)
-				await DB.Default.Update<MediaAttachment>().Match(m => ids.Contains(m.ID))
+				return true;
-					.Modify(m => m.PostId, postId).Modify(m => m.AttachedAt, DateTime.UtcNow).ExecuteAsync(token);
+			var claimed = await DB.Default.Update<MediaAttachment>()
+				.Match(m => ids.Contains(m.ID) && (m.PostId == null || m.PostId == postId) && m.TrashedAt == null
+					&& (m.ScheduledStatusId == null || m.ScheduledStatusId == scheduledId))
+				.Modify(m => m.PostId, postId).Modify(m => m.AttachedAt, DateTime.UtcNow).ExecuteAsync(token);
+			if (claimed.MatchedCount == ids.Count)
+				return true;
+			var taken = media.Where(m => m.PostId == null).Select(m => m.ID).ToList();
+			await DB.Default.Update<MediaAttachment>().Match(m => taken.Contains(m.ID) && m.PostId == postId)
+				.Modify(m => m.PostId, null).Modify(m => m.AttachedAt, null).ExecuteAsync(token);
+			return false;
 		}
 
 		PostMedia ToPostMedia(MediaAttachment attachment) => new()