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Media are reserved for one post, atomically

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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
This commit is contained in:
thepraandClaude Opus 5.5 committed 2026-10-07 10:34:16 +02:00
1 parent bb680e8cb8
commit 65938bb2a0
4 files changed
+117 -14
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PrivaPub/Domain/Statuses/StatusService.cs
+27 -10
View File
@@ -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)
await DB.Default.Update<MediaAttachment>().Match(m => ids.Contains(m.ID))
.Modify(m => m.PostId, postId).Modify(m => m.AttachedAt, DateTime.UtcNow).ExecuteAsync(token);
if (ids.Count == 0)
return true;
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()