[backend/federation] Basic mentions handling (ISH-38)

This implementation adds handling of incoming mentions, including rewriting non-canonical mentions of split domain users into their canonical form when inserting notes into the database.

Iceshrimp.Backend/Controllers/Mastodon/Renderers/NoteRenderer.cs

@@ -1,18 +1,41 @@
 using Iceshrimp.Backend.Controllers.Mastodon.Schemas.Entities;
 using Iceshrimp.Backend.Core.Configuration;
+using Iceshrimp.Backend.Core.Database;
 using Iceshrimp.Backend.Core.Database.Tables;
 using Iceshrimp.Backend.Core.Extensions;
 using Iceshrimp.Backend.Core.Helpers.LibMfm.Conversion;
+using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Options;
 
 namespace Iceshrimp.Backend.Controllers.Mastodon.Renderers;
 
-public class NoteRenderer(IOptions<Config.InstanceSection> config, UserRenderer userRenderer) {
+public class NoteRenderer(
-	public async Task<Status> RenderAsync(Note note, int recurse = 2) {
+	IOptions<Config.InstanceSection> config,
+	UserRenderer userRenderer,
+	MfmConverter mfmConverter,
+	DatabaseContext db
+) {
+	public async Task<Status> RenderAsync(Note note, List<Mention>? mentions = null, int recurse = 2) {
 		var uri     = note.Uri ?? $"https://{config.Value.WebDomain}/notes/{note.Id}";
-		var renote  = note.Renote != null && recurse > 0 ? await RenderAsync(note.Renote, --recurse) : null;
+		var renote  = note.Renote != null && recurse > 0 ? await RenderAsync(note.Renote, mentions, --recurse) : null;
 		var text    = note.Text; //TODO: append quote uri
-		var content = text != null ? await MfmConverter.ToHtmlAsync(text) : null;
+		var content = text != null ? await mfmConverter.ToHtmlAsync(text, note.MentionedRemoteUsers) : null;
+
+		if (mentions == null) {
+			mentions = await db.Users.Where(p => note.Mentions.Contains(p.Id))
+			                   .Select(u => new Mention {
+				                   Id       = u.Id,
+				                   Username = u.Username,
+				                   Acct     = u.Acct,
+				                   Url = (u.UserProfile != null
+						                   ? u.UserProfile.Url ?? u.Uri
+						                   : u.Uri) ?? $"https://{config.Value.WebDomain}/@{u.Username}"
+			                   })
+			                   .ToListAsync();
+		}
+		else {
+			mentions = [..mentions.Where(p => note.Mentions.Contains(p.Id))];
+		}
 
 		var res = new Status {
 			Id             = note.Id,
@@ -38,13 +61,30 @@ public class NoteRenderer(IOptions<Config.InstanceSection> config, UserRenderer
 			Visibility     = Status.EncodeVisibility(note.Visibility),
 			Content        = content,
 			Text           = text,
+			Mentions       = mentions,
 			IsPinned       = false //FIXME
 		};
 
 		return res;
 	}
 
+	private async Task<List<Mention>> GetMentions(IReadOnlyCollection<Note> notes) {
+		var ids = notes.SelectMany(n => n.Mentions).Distinct();
+		return await db.Users.Where(p => ids.Contains(p.Id))
+		               .Select(u => new Mention {
+			               Id       = u.Id,
+			               Username = u.Username,
+			               Acct     = u.Acct,
+			               Url = u.UserProfile != null
+				               ? u.UserProfile.Url ?? u.Uri ?? $"https://{config.Value.WebDomain}/@{u.Username}"
+				               : u.Uri ?? $"https://{config.Value.WebDomain}/@{u.Username}"
+		               })
+		               .ToListAsync();
+	}
+
 	public async Task<IEnumerable<Status>> RenderManyAsync(IEnumerable<Note> notes) {
-		return await notes.Select(RenderAsync).AwaitAllAsync();
+		var noteList = notes.ToList();
+		var mentions = await GetMentions(noteList);
+		return await noteList.Select(async p => await RenderAsync(p, mentions)).AwaitAllAsync();
 	}
 }

Iceshrimp.Backend/Controllers/Mastodon/Renderers/UserRenderer.cs

@@ -7,7 +7,7 @@ using Microsoft.Extensions.Options;
 
 namespace Iceshrimp.Backend.Controllers.Mastodon.Renderers;
 
-public class UserRenderer(IOptions<Config.InstanceSection> config) {
+public class UserRenderer(IOptions<Config.InstanceSection> config, MfmConverter mfmConverter) {
 	private readonly string _transparent = $"https://{config.Value.WebDomain}/assets/transparent.png";
 
 	public async Task<Account> RenderAsync(User user, UserProfile? profile) {
@@ -29,7 +29,7 @@ public class UserRenderer(IOptions<Config.InstanceSection> config) {
 			FollowersCount = user.FollowersCount,
 			FollowingCount = user.FollowingCount,
 			StatusesCount = user.NotesCount,
-			Note = await MfmConverter.ToHtmlAsync(profile?.Description ?? ""),
+			Note = await mfmConverter.ToHtmlAsync(profile?.Description ?? "", []),
 			Url = profile?.Url ?? user.Uri ?? $"https://{user.Host ?? config.Value.WebDomain}/@{user.Username}",
 			AvatarStaticUrl = user.AvatarUrl ?? _transparent, //TODO
 			HeaderUrl = user.BannerUrl ?? _transparent,

Iceshrimp.Backend/Controllers/Mastodon/Schemas/Entities/Mention.cs

@@ -0,0 +1,10 @@
+using J = System.Text.Json.Serialization.JsonPropertyNameAttribute;
+
+namespace Iceshrimp.Backend.Controllers.Mastodon.Schemas.Entities;
+
+public class Mention {
+	[J("id")]       public required string Id       { get; set; }
+	[J("username")] public required string Username { get; set; }
+	[J("acct")]     public required string Acct     { get; set; }
+	[J("url")]      public required string Url      { get; set; }
+}

Iceshrimp.Backend/Controllers/Mastodon/Schemas/Entities/Status.cs

@@ -40,9 +40,10 @@ public class Status : IEntity {
 	[JI(Condition = JsonIgnoreCondition.WhenWritingNull)]
 	public required bool? IsPinned { get; set; }
 
+	[J("mentions")] public required List<Mention> Mentions { get; set; }
+
 	[J("emojis")]            public object[] Emojis      => [];   //FIXME
 	[J("media_attachments")] public object[] Media       => [];   //FIXME
-	[J("mentions")]          public object[] Mentions    => [];   //FIXME
 	[J("tags")]              public object[] Tags        => [];   //FIXME
 	[J("reactions")]         public object[] Reactions   => [];   //FIXME
 	[J("filtered")]          public object[] Filtered    => [];   //FIXME

Iceshrimp.Backend/Core/Database/DatabaseContext.cs

@@ -531,7 +531,7 @@ public class DatabaseContext(DbContextOptions<DatabaseContext> options)
 			entity.Property(e => e.FileIds).HasDefaultValueSql("'{}'::character varying[]");
 			entity.Property(e => e.HasPoll).HasDefaultValue(false);
 			entity.Property(e => e.LocalOnly).HasDefaultValue(false);
-			entity.Property(e => e.MentionedRemoteUsers).HasDefaultValueSql("'[]'::text");
+			entity.Property(e => e.MentionedRemoteUsers).HasDefaultValueSql("'[]'::jsonb");
 			entity.Property(e => e.Mentions).HasDefaultValueSql("'{}'::character varying[]");
 			entity.Property(e => e.Reactions).HasDefaultValueSql("'{}'::jsonb");
 			entity.Property(e => e.RenoteCount).HasDefaultValue((short)0);

Iceshrimp.Backend/Core/Database/Migrations/20240205190843_RefactorJsonbColumns.Designer.cs

@@ -4418,7 +4418,7 @@ namespace Iceshrimp.Backend.Core.Database.Migrations
                         .HasColumnName("location")
                         .HasComment("The location of the User.");
 
-                    b.Property<List<UserProfile.MentionedRemoteUsers>>("Mentions")
+                    b.Property<List<Note.MentionedUser>>("Mentions")
                         .IsRequired()
                         .ValueGeneratedOnAdd()
                         .HasColumnType("jsonb")

Iceshrimp.Backend/Core/Database/Migrations/20240207164121_AddFederationControl.Designer.cs

@@ -4455,7 +4455,7 @@ namespace Iceshrimp.Backend.Core.Database.Migrations
                         .HasColumnName("location")
                         .HasComment("The location of the User.");
 
-                    b.Property<List<UserProfile.MentionedRemoteUsers>>("Mentions")
+                    b.Property<List<Note.MentionedUser>>("Mentions")
                         .IsRequired()
                         .ValueGeneratedOnAdd()
                         .HasColumnType("jsonb")

Iceshrimp.Backend/Core/Database/Migrations/20240209170817_MoveDriveHashAlgoToSha256.Designer.cs

@@ -4454,7 +4454,7 @@ namespace Iceshrimp.Backend.Core.Database.Migrations
                         .HasColumnName("location")
                         .HasComment("The location of the User.");
 
-                    b.Property<List<UserProfile.MentionedRemoteUsers>>("Mentions")
+                    b.Property<List<Note.MentionedUser>>("Mentions")
                         .IsRequired()
                         .ValueGeneratedOnAdd()
                         .HasColumnType("jsonb")

Iceshrimp.Backend/Core/Database/Migrations/20240210151527_UserAlsoKnownAsToArray.Designer.cs

@@ -4454,7 +4454,7 @@ namespace Iceshrimp.Backend.Core.Database.Migrations
                         .HasColumnName("location")
                         .HasComment("The location of the User.");
 
-                    b.Property<List<UserProfile.MentionedRemoteUsers>>("Mentions")
+                    b.Property<List<Note.MentionedUser>>("Mentions")
                         .IsRequired()
                         .ValueGeneratedOnAdd()
                         .HasColumnType("jsonb")

Iceshrimp.Backend/Core/Database/Migrations/20240210233409_FixNoteMentionsColumnType.cs

@@ -0,0 +1,32 @@
+using System.Collections.Generic;
+using Iceshrimp.Backend.Core.Database.Tables;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Iceshrimp.Backend.Core.Database.Migrations
+{
+    /// <inheritdoc />
+    public partial class FixNoteMentionsColumnType : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+	        migrationBuilder.Sql("""
+	                             ALTER TABLE "note" ALTER COLUMN "mentionedRemoteUsers" DROP DEFAULT;
+	                             ALTER TABLE "note" ALTER COLUMN "mentionedRemoteUsers" TYPE jsonb USING "mentionedRemoteUsers"::jsonb;
+	                             ALTER TABLE "note" ALTER COLUMN "mentionedRemoteUsers" SET DEFAULT '[]'::jsonb;
+	                             """);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+	        migrationBuilder.Sql("""
+	                             ALTER TABLE "note" ALTER COLUMN "mentionedRemoteUsers" DROP DEFAULT;
+	                             ALTER TABLE "note" ALTER COLUMN "mentionedRemoteUsers" TYPE text USING "mentionedRemoteUsers"::text;
+	                             ALTER TABLE "note" ALTER COLUMN "mentionedRemoteUsers" SET DEFAULT '[]'::text;
+	                             """);
+        }
+    }
+}

Iceshrimp.Backend/Core/Database/Migrations/DatabaseContextModelSnapshot.cs

@@ -2417,12 +2417,12 @@ namespace Iceshrimp.Backend.Core.Database.Migrations
                         .HasDefaultValue(false)
                         .HasColumnName("localOnly");
 
-                    b.Property<string>("MentionedRemoteUsers")
+                    b.Property<List<Note.MentionedUser>>("MentionedRemoteUsers")
                         .IsRequired()
                         .ValueGeneratedOnAdd()
-                        .HasColumnType("text")
+                        .HasColumnType("jsonb")
                         .HasColumnName("mentionedRemoteUsers")
-                        .HasDefaultValueSql("'[]'::text");
+                        .HasDefaultValueSql("'[]'::jsonb");
 
                     b.Property<List<string>>("Mentions")
                         .IsRequired()
@@ -4451,7 +4451,7 @@ namespace Iceshrimp.Backend.Core.Database.Migrations
                         .HasColumnName("location")
                         .HasComment("The location of the User.");
 
-                    b.Property<List<UserProfile.MentionedRemoteUsers>>("Mentions")
+                    b.Property<List<Note.MentionedUser>>("Mentions")
                         .IsRequired()
                         .ValueGeneratedOnAdd()
                         .HasColumnType("jsonb")

Iceshrimp.Backend/Core/Database/Tables/Note.cs

@@ -5,6 +5,7 @@ using EntityFrameworkCore.Projectables;
 using Iceshrimp.Backend.Core.Helpers;
 using Microsoft.EntityFrameworkCore;
 using NpgsqlTypes;
+using J = System.Text.Json.Serialization.JsonPropertyNameAttribute;
 
 namespace Iceshrimp.Backend.Core.Database.Tables;
 
@@ -106,7 +107,8 @@ public class Note : IEntity {
 	[Column("mentions", TypeName = "character varying(32)[]")]
 	public List<string> Mentions { get; set; } = [];
 
-	[Column("mentionedRemoteUsers")] public string MentionedRemoteUsers { get; set; } = null!;
+	[Column("mentionedRemoteUsers", TypeName = "jsonb")] 
+	public List<Note.MentionedUser> MentionedRemoteUsers { get; set; } = [];
 
 	[Column("emojis", TypeName = "character varying(128)[]")]
 	public List<string> Emojis { get; set; } = [];
@@ -271,4 +273,11 @@ public class Note : IEntity {
 
 		return this;
 	}
+	
+	public class MentionedUser {
+		[J("uri")]      public required string  Uri      { get; set; }
+		[J("url")]      public          string? Url      { get; set; }
+		[J("username")] public required string  Username { get; set; }
+		[J("host")]     public required string  Host     { get; set; }
+	}
 }

Iceshrimp.Backend/Core/Database/Tables/UserProfile.cs

@@ -157,7 +157,7 @@ public class UserProfile {
 	[Column("preventAiLearning")] public bool PreventAiLearning { get; set; }
 
 	[Column("mentions", TypeName = "jsonb")]
-	public List<MentionedRemoteUsers> Mentions { get; set; } = null!;
+	public List<Note.MentionedUser> Mentions { get; set; } = null!;
 
 	[ForeignKey("PinnedPageId")]
 	[InverseProperty(nameof(Page.UserProfile))]
@@ -179,11 +179,4 @@ public class UserProfile {
 		[J("value")]    public required string Value      { get; set; }
 		[J("verified")] public          bool?  IsVerified { get; set; }
 	}
-
-	public class MentionedRemoteUsers {
-		[J("uri")]      public required string  Uri      { get; set; }
-		[J("url")]      public          string? Url      { get; set; }
-		[J("username")] public required string  Username { get; set; }
-		[J("host")]     public required string  Host     { get; set; }
-	}
 }

Iceshrimp.Backend/Core/Extensions/EnumerableExtensions.cs

@@ -4,4 +4,13 @@ public static class EnumerableExtensions {
 	public static async Task<IEnumerable<T>> AwaitAllAsync<T>(this IEnumerable<Task<T>> tasks) {
 		return await Task.WhenAll(tasks);
 	}
+	
+	public static async Task<List<T>> AwaitAllNoConcurrencyAsync<T>(this IEnumerable<Task<T>> tasks) {
+		var results = new List<T>();
+		foreach (var task in tasks) {
+			results.Add(await task);
+		}
+
+		return results;
+	}
 }

Iceshrimp.Backend/Core/Extensions/ServiceExtensions.cs

@@ -4,6 +4,7 @@ using Iceshrimp.Backend.Controllers.Schemas;
 using Iceshrimp.Backend.Core.Configuration;
 using Iceshrimp.Backend.Core.Database;
 using Iceshrimp.Backend.Core.Federation.WebFinger;
+using Iceshrimp.Backend.Core.Helpers.LibMfm.Conversion;
 using Iceshrimp.Backend.Core.Middleware;
 using Iceshrimp.Backend.Core.Services;
 using Microsoft.AspNetCore.DataProtection;
@@ -27,6 +28,7 @@ public static class ServiceExtensions {
 			.AddScoped<ActivityPub.NoteRenderer>()
 			.AddScoped<ActivityPub.UserResolver>()
 			.AddScoped<ActivityPub.ObjectResolver>()
+			.AddScoped<ActivityPub.MentionsResolver>()
 			.AddScoped<ActivityPub.ActivityDeliverService>()
 			.AddScoped<ActivityPub.FederationControlService>()
 			.AddScoped<ActivityPub.ActivityHandlerService>()
@@ -46,6 +48,7 @@ public static class ServiceExtensions {
 		// Singleton = instantiated once across application lifetime
 		services
 			.AddSingleton<HttpClient>()
+			.AddSingleton<MfmConverter>()
 			.AddSingleton<HttpRequestService>()
 			.AddSingleton<QueueService>()
 			.AddSingleton<ObjectStorageService>()

Iceshrimp.Backend/Core/Federation/ActivityPub/MentionsResolver.cs

@@ -0,0 +1,83 @@
+using Iceshrimp.Backend.Core.Configuration;
+using Iceshrimp.Backend.Core.Database;
+using Iceshrimp.Backend.Core.Database.Tables;
+using Iceshrimp.Backend.Core.Extensions;
+using Iceshrimp.Backend.Core.Helpers.LibMfm.Parsing;
+using Iceshrimp.Backend.Core.Helpers.LibMfm.Serialization;
+using Iceshrimp.Backend.Core.Helpers.LibMfm.Types;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Caching.Distributed;
+using Microsoft.Extensions.Options;
+
+namespace Iceshrimp.Backend.Core.Federation.ActivityPub;
+
+using SplitDomainMapping = IReadOnlyDictionary<(string usernameLower, string webDomain), string>;
+
+/// <summary>
+/// Resolves mentions into their canonical form. This is required for handling split domain mentions correctly, as it cannot be guaranteed that remote instances handle split domain users correctly.
+/// </summary>
+public class MentionsResolver(
+	DatabaseContext db,
+	IOptions<Config.InstanceSection> config,
+	IDistributedCache cache
+) {
+	public async Task<string> ResolveMentions(
+		string mfm, string? host,
+		List<Note.MentionedUser> mentionCache,
+		SplitDomainMapping splitDomainMapping
+	) {
+		var nodes = MfmParser.Parse(mfm);
+		nodes = await ResolveMentions(nodes, host, mentionCache, splitDomainMapping);
+		return MfmSerializer.Serialize(nodes);
+	}
+
+	public async Task<IEnumerable<MfmNode>> ResolveMentions(
+		IEnumerable<MfmNode> nodes, string? host,
+		List<Note.MentionedUser> mentionCache,
+		SplitDomainMapping splitDomainMapping
+	) {
+		var nodesList = nodes.ToList();
+		foreach (var mention in nodesList.SelectMany(p => p.Children.Append(p)).OfType<MfmMentionNode>())
+			await ResolveMention(mention, host, mentionCache, splitDomainMapping);
+
+		return nodesList;
+	}
+
+	private async Task ResolveMention(
+		MfmMentionNode node, string? host,
+		IEnumerable<Note.MentionedUser> mentionCache,
+		SplitDomainMapping splitDomainMapping
+	) {
+		var finalHost = node.Host ?? host;
+
+		if (finalHost == config.Value.AccountDomain || finalHost == config.Value.WebDomain)
+			finalHost = null;
+		if (finalHost != null &&
+		    splitDomainMapping.TryGetValue((node.Username.ToLowerInvariant(), finalHost), out var value))
+			finalHost = value;
+
+		var resolvedUser =
+			mentionCache.FirstOrDefault(p => string.Equals(p.Username, node.Username,
+			                                               StringComparison.InvariantCultureIgnoreCase) &&
+			                                 p.Host == finalHost);
+
+		if (resolvedUser != null) {
+			node.Username = resolvedUser.Username;
+			node.Host     = resolvedUser.Host;
+			node.Acct     = $"@{resolvedUser.Username}@{resolvedUser.Host}";
+		}
+		else {
+			async Task<string> FetchLocalUserCapitalization() {
+				var username = await db.Users.Where(p => p.UsernameLower == node.Username.ToLowerInvariant())
+				                       .Select(p => p.Username)
+				                       .FirstOrDefaultAsync();
+				return username ?? node.Username;
+			}
+
+			node.Username = await cache.FetchAsync($"localUserNameCapitalization:{node.Username.ToLowerInvariant()}",
+			                                       TimeSpan.FromHours(24), FetchLocalUserCapitalization);
+
+			node.Acct = $"@{node.Username}";
+		}
+	}
+}

Iceshrimp.Backend/Core/Federation/ActivityPub/NoteRenderer.cs

@@ -6,30 +6,30 @@ using Microsoft.Extensions.Options;
 
 namespace Iceshrimp.Backend.Core.Federation.ActivityPub;
 
-public class NoteRenderer(IOptions<Config.InstanceSection> config) {
+public class NoteRenderer(IOptions<Config.InstanceSection> config, MfmConverter mfmConverter) {
 	public async Task<ASNote> RenderAsync(Note note) {
 		var id     = $"https://{config.Value.WebDomain}/notes/{note.Id}";
 		var userId = $"https://{config.Value.WebDomain}/users/{note.User.Id}";
 		var replyId = note.ReplyId != null
-			? new ASIdObject($"https://{config.Value.WebDomain}/notes/{note.ReplyId}")
+			? new ASObjectBase($"https://{config.Value.WebDomain}/notes/{note.ReplyId}")
 			: null;
 
-		List<ASIdObject> to = note.Visibility switch {
+		List<ASObjectBase> to = note.Visibility switch {
 			Note.NoteVisibility.Public    => [new ASLink($"{Constants.ActivityStreamsNs}#Public")],
 			Note.NoteVisibility.Followers => [new ASLink($"{userId}/followers")],
 			Note.NoteVisibility.Specified => [], // FIXME
 			_                             => []
 		};
 
-		List<ASIdObject> cc = note.Visibility switch {
+		List<ASObjectBase> cc = note.Visibility switch {
 			Note.NoteVisibility.Home => [new ASLink($"{Constants.ActivityStreamsNs}#Public")],
 			_                        => []
 		};
 
 		return new ASNote {
 			Id           = id,
-			Content      = note.Text != null ? await MfmConverter.ToHtmlAsync(note.Text) : null,
+			Content      = note.Text != null ? await mfmConverter.ToHtmlAsync(note.Text, []) : null,
-			AttributedTo = [new ASIdObject(userId)],
+			AttributedTo = [new ASObjectBase(userId)],
 			Type         = $"{Constants.ActivityStreamsNs}#Note",
 			MkContent    = note.Text,
 			PublishedAt  = note.CreatedAt,

Iceshrimp.Backend/Core/Federation/ActivityPub/ObjectResolver.cs

@@ -10,29 +10,29 @@ public class ObjectResolver(
 	DatabaseContext db,
 	FederationControlService federationCtrl
 ) {
-	public async Task<ASObject?> ResolveObject(ASIdObject idObj) {
+	public async Task<ASObject?> ResolveObject(ASObjectBase baseObj) {
-		if (idObj is ASObject obj) return obj;
+		if (baseObj is ASObject obj) return obj;
-		if (idObj.Id == null) {
+		if (baseObj.Id == null) {
 			logger.LogDebug("Refusing to resolve object with null id property");
 			return null;
 		}
 
-		if (await federationCtrl.ShouldBlockAsync(idObj.Id)) {
+		if (await federationCtrl.ShouldBlockAsync(baseObj.Id)) {
 			logger.LogDebug("Instance is blocked");
 			return null;
 		}
 
-		if (await db.Notes.AnyAsync(p => p.Uri == idObj.Id))
+		if (await db.Notes.AnyAsync(p => p.Uri == baseObj.Id))
-			return new ASNote { Id = idObj.Id };
+			return new ASNote { Id = baseObj.Id };
-		if (await db.Users.AnyAsync(p => p.Uri == idObj.Id))
+		if (await db.Users.AnyAsync(p => p.Uri == baseObj.Id))
-			return new ASActor { Id = idObj.Id };
+			return new ASActor { Id = baseObj.Id };
 
 		try {
-			var result = await fetchSvc.FetchActivityAsync(idObj.Id);
+			var result = await fetchSvc.FetchActivityAsync(baseObj.Id);
 			return result.FirstOrDefault();
 		}
 		catch (Exception e) {
-			logger.LogDebug("Failed to resolve object {id}: {error}", idObj.Id, e);
+			logger.LogDebug("Failed to resolve object {id}: {error}", baseObj.Id, e);
 			return null;
 		}
 	}

Iceshrimp.Backend/Core/Federation/ActivityPub/UserRenderer.cs

@@ -10,7 +10,7 @@ using Microsoft.Extensions.Options;
 
 namespace Iceshrimp.Backend.Core.Federation.ActivityPub;
 
-public class UserRenderer(IOptions<Config.InstanceSection> config, DatabaseContext db) {
+public class UserRenderer(IOptions<Config.InstanceSection> config, DatabaseContext db, MfmConverter mfmConverter) {
 	/// <summary>
 	/// This function is meant for compacting an actor into the @id form as specified in ActivityStreams
 	/// </summary>
@@ -58,17 +58,17 @@ public class UserRenderer(IOptions<Config.InstanceSection> config, DatabaseConte
 			Url = new ASLink($"https://{config.Value.WebDomain}/@{user.Username}"),
 			Username = user.Username,
 			DisplayName = user.DisplayName ?? user.Username,
-			Summary = profile?.Description != null ? await MfmConverter.FromHtmlAsync(profile.Description) : null,
+			Summary = profile?.Description != null ? await mfmConverter.FromHtmlAsync(profile.Description) : null,
 			MkSummary = profile?.Description,
 			IsCat = user.IsCat,
 			IsDiscoverable = user.IsExplorable,
 			IsLocked = user.IsLocked,
 			Endpoints = new ASEndpoints {
-				SharedInbox = new ASIdObject($"https://{config.Value.WebDomain}/inbox")
+				SharedInbox = new ASObjectBase($"https://{config.Value.WebDomain}/inbox")
 			},
 			PublicKey = new ASPublicKey {
 				Id        = $"{id}#main-key",
-				Owner     = new ASIdObject(id),
+				Owner     = new ASObjectBase(id),
 				PublicKey = keypair.PublicKey,
 				Type      = "Key"
 			}

Iceshrimp.Backend/Core/Federation/ActivityPub/UserResolver.cs

@@ -71,6 +71,10 @@ public class UserResolver(ILogger<UserResolver> logger, UserService userSvc, Web
 		return query;
 	}
 
+	public async Task<User> ResolveAsync(string username, string? host) {
+		return host != null ? await ResolveAsync($"acct:{username}@{host}") : await ResolveAsync($"acct:{username}");
+	}
+
 	public async Task<User> ResolveAsync(string query) {
 		query = NormalizeQuery(query);
 

Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASCollection.cs

@@ -6,7 +6,7 @@ using VC = Iceshrimp.Backend.Core.Federation.ActivityStreams.Types.ValueObjectCo
 
 namespace Iceshrimp.Backend.Core.Federation.ActivityStreams.Types;
 
-public class ASCollection<T>() : ASIdObject where T : ASObject {
+public class ASCollection<T>() : ASObjectBase where T : ASObject {
 	public ASCollection(string id) : this() {
 		Id = id;
 	}

Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASEndpoints.cs

@@ -5,8 +5,8 @@ namespace Iceshrimp.Backend.Core.Federation.ActivityStreams.Types;
 
 public class ASEndpoints {
 	[J("https://www.w3.org/ns/activitystreams#sharedInbox")]
-	[JC(typeof(LDIdObjectConverter))]
+	[JC(typeof(ASObjectBaseConverter))]
-	public ASIdObject? SharedInbox { get; set; }
+	public ASObjectBase? SharedInbox { get; set; }
 }
 
 public class ASEndpointsConverter : ASSerializer.ListSingleObjectConverter<ASEndpoints>;

Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASLink.cs

@@ -1,12 +1,18 @@
 using J = Newtonsoft.Json.JsonPropertyAttribute;
+using JC = Newtonsoft.Json.JsonConverterAttribute;
 
 namespace Iceshrimp.Backend.Core.Federation.ActivityStreams.Types;
 
-public class ASLink(string url) : ASIdObject(url) {
+public class ASLink(string url) : ASObjectBase(url) {
 	[J("https://www.w3.org/ns/activitystreams#href")]
-	public string? Href { get; set; }
+	[JC(typeof(ASObjectBaseConverter))]
+	public ASObjectBase? Href { get; set; }
 
-	public          string? Link       => Id ?? Href;
+	[J("https://www.w3.org/ns/activitystreams#name")]
+	[JC(typeof(ValueObjectConverter))]
+	public string? Name { get; set; }
+
+	public          string? Link       => Id ?? Href?.Id;
 	public override string? ToString() => Link;
 }
 

Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASNote.cs

@@ -32,17 +32,21 @@ public class ASNote : ASObject {
 	public ASNoteSource? Source { get; set; }
 
 	[J("https://www.w3.org/ns/activitystreams#to")]
-	public List<ASIdObject> To { get; set; } = [];
+	public List<ASObjectBase> To { get; set; } = [];
 
 	[J("https://www.w3.org/ns/activitystreams#cc")]
-	public List<ASIdObject> Cc { get; set; } = [];
+	public List<ASObjectBase> Cc { get; set; } = [];
 
 	[J("https://www.w3.org/ns/activitystreams#attributedTo")]
-	public List<ASIdObject> AttributedTo { get; set; } = [];
+	public List<ASObjectBase> AttributedTo { get; set; } = [];
 
 	[J("https://www.w3.org/ns/activitystreams#inReplyTo")]
-	[JC(typeof(LDIdObjectConverter))]
+	[JC(typeof(ASObjectBaseConverter))]
-	public ASIdObject? InReplyTo { get; set; }
+	public ASObjectBase? InReplyTo { get; set; }
+
+	[J("https://www.w3.org/ns/activitystreams#tag")]
+	[JC(typeof(ASTagConverter))]
+	public List<ASTag>? Tags { get; set; }
 
 	public Note.NoteVisibility GetVisibility(ASActor actor) {
 		if (To.Any(p => p.Id == "https://www.w3.org/ns/activitystreams#Public"))
@@ -55,6 +59,17 @@ public class ASNote : ASObject {
 		return Note.NoteVisibility.Specified;
 	}
 
+	public List<string> GetRecipients(ASActor actor) {
+		return To.Concat(Cc)
+		         .Select(p => p.Id)
+		         .Distinct()
+		         .Where(p => p != $"{Constants.ActivityStreamsNs}#Public" &&
+		                     p != (actor.Followers?.Id ?? actor.Id + "/followers"))
+		         .Where(p => p != null)
+		         .Select(p => p!)
+		         .ToList();
+	}
+
 	public new static class Types {
 		private const string Ns = Constants.ActivityStreamsNs;
 

Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASObject.cs

@@ -7,11 +7,11 @@ using JR = Newtonsoft.Json.JsonRequiredAttribute;
 
 namespace Iceshrimp.Backend.Core.Federation.ActivityStreams.Types;
 
-public class ASObject : ASIdObject {
+public class ASObject : ASObjectBase {
 	[J("@id")] [JR] public new required string Id { get; set; }
 
 	[J("@type")]
-	[JC(typeof(LDTypeConverter))]
+	[JC(typeof(StringListSingleConverter))]
 	public string? Type { get; set; }
 
 	public bool IsUnresolved => GetType() == typeof(ASObject) && Type == null;
@@ -55,7 +55,7 @@ public class ASObject : ASIdObject {
 
 public class ASTombstone : ASObject;
 
-public sealed class LDTypeConverter : ASSerializer.ListSingleObjectConverter<string>;
+public sealed class StringListSingleConverter : ASSerializer.ListSingleObjectConverter<string>;
 
 internal sealed class ASObjectConverter : JsonConverter {
 	public override bool CanWrite => false;

Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASObjectBase.cs

@@ -2,8 +2,8 @@ using J = Newtonsoft.Json.JsonPropertyAttribute;
 
 namespace Iceshrimp.Backend.Core.Federation.ActivityStreams.Types;
 
-public class ASIdObject() {
+public class ASObjectBase() {
-	public ASIdObject(string? id) : this() {
+	public ASObjectBase(string? id) : this() {
 		Id = id;
 	}
 
@@ -14,4 +14,4 @@ public class ASIdObject() {
 	}
 }
 
-public sealed class LDIdObjectConverter : ASSerializer.ListSingleObjectConverter<ASIdObject>;
+public sealed class ASObjectBaseConverter : ASSerializer.ListSingleObjectConverter<ASObjectBase>;

Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASPublicKey.cs

@@ -6,8 +6,8 @@ namespace Iceshrimp.Backend.Core.Federation.ActivityStreams.Types;
 
 public class ASPublicKey : ASObject {
 	[J("https://w3id.org/security#owner")]
-	[JC(typeof(LDIdObjectConverter))]
+	[JC(typeof(ASObjectBaseConverter))]
-	public ASIdObject? Owner { get; set; }
+	public ASObjectBase? Owner { get; set; }
 	
 	[J("https://w3id.org/security#publicKeyPem")]
 	[JC(typeof(VC))]

Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASTag.cs

@@ -0,0 +1,75 @@
+using Iceshrimp.Backend.Core.Configuration;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+using J = Newtonsoft.Json.JsonPropertyAttribute;
+using JC = Newtonsoft.Json.JsonConverterAttribute;
+
+namespace Iceshrimp.Backend.Core.Federation.ActivityStreams.Types;
+
+public class ASTag : ASObjectBase {
+	[J("@type")]
+	[JC(typeof(StringListSingleConverter))]
+	public string? Type { get; set; }
+}
+
+public class ASTagLink : ASTag {
+	[J("https://www.w3.org/ns/activitystreams#href")]
+	[JC(typeof(ASObjectBaseConverter))]
+	public ASObjectBase? Href { get; set; }
+
+	[J("https://www.w3.org/ns/activitystreams#name")]
+	[JC(typeof(ValueObjectConverter))]
+	public string? Name { get; set; }
+}
+
+public class ASMention : ASTagLink;
+
+public class ASHashtag : ASTagLink;
+
+public class ASEmoji : ASTag {
+	//TODO
+}
+
+public sealed class ASTagConverter : JsonConverter {
+	public override bool CanWrite => false;
+
+	public override bool CanConvert(Type objectType) {
+		return true;
+	}
+
+	public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue,
+	                                 JsonSerializer serializer) {
+		if (reader.TokenType == JsonToken.StartObject) {
+			var obj = JObject.Load(reader);
+			return HandleObject(obj);
+		}
+
+		if (reader.TokenType == JsonToken.StartArray) {
+			var array  = JArray.Load(reader);
+			var result = new List<ASTag>();
+			foreach (var token in array) {
+				if (token is not JObject obj) return null;
+				var item = HandleObject(obj);
+				if (item == null) return null;
+				result.Add(item);
+			}
+
+			return result;
+		}
+
+		return null;
+	}
+
+	private ASTag? HandleObject(JToken obj) {
+		var link = obj.ToObject<ASTagLink?>();
+		if (link is not { Href: not null }) return obj.ToObject<ASEmoji?>();
+
+		return link.Type == $"{Constants.ActivityStreamsNs}#Mention"
+			? obj.ToObject<ASMention?>()
+			: obj.ToObject<ASHashtag?>();
+	}
+
+	public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer) {
+		throw new NotImplementedException();
+	}
+}

Iceshrimp.Backend/Core/Helpers/LibMfm/Conversion/MfmConverter.cs

@@ -4,15 +4,18 @@ using System.Web;
 using AngleSharp;
 using AngleSharp.Dom;
 using AngleSharp.Html.Parser;
+using Iceshrimp.Backend.Core.Configuration;
+using Iceshrimp.Backend.Core.Database.Tables;
 using Iceshrimp.Backend.Core.Helpers.LibMfm.Parsing;
 using Iceshrimp.Backend.Core.Helpers.LibMfm.Types;
-using static Iceshrimp.Backend.Core.Helpers.LibMfm.Parsing.HtmlParser;
+using Microsoft.Extensions.Options;
+using MfmHtmlParser = Iceshrimp.Backend.Core.Helpers.LibMfm.Parsing.HtmlParser;
 using HtmlParser = AngleSharp.Html.Parser.HtmlParser;
 
 namespace Iceshrimp.Backend.Core.Helpers.LibMfm.Conversion;
 
-public static class MfmConverter {
+public class MfmConverter(IOptions<Config.InstanceSection> config) {
-	public static async Task<string?> FromHtmlAsync(string? html) {
+	public async Task<string?> FromHtmlAsync(string? html, List<Note.MentionedUser>? mentions = null) {
 		if (html == null) return null;
 
 		// Ensure compatibility with AP servers that send both <br> as well as newlines
@@ -23,45 +26,49 @@ public static class MfmConverter {
 		if (dom.Body == null) return "";
 
 		var sb     = new StringBuilder();
-		dom.Body.ChildNodes.Select(ParseNode).ToList().ForEach(s => sb.Append(s));
+		var parser = new MfmHtmlParser(mentions ?? []);
+		dom.Body.ChildNodes.Select(parser.ParseNode).ToList().ForEach(s => sb.Append(s));
 		return sb.ToString().Trim();
 	}
 
-	public static async Task<string> ToHtmlAsync(string mfm) {
+	public async Task<string> ToHtmlAsync(IEnumerable<MfmNode> nodes, List<Note.MentionedUser> mentions) {
-		var nodes = MfmParser.Parse(mfm);
-
 		var context  = BrowsingContext.New();
 		var document = await context.OpenNewAsync();
 		var element  = document.CreateElement("p");
 
-		foreach (var node in nodes) element.AppendNodes(document.FromMfmNode(node));
+		foreach (var node in nodes) element.AppendNodes(FromMfmNode(document, node, mentions));
 
 		await using var sw = new StringWriter();
 		await element.ToHtmlAsync(sw);
 		return sw.ToString();
 	}
 
-	private static INode FromMfmNode(this IDocument document, MfmNode node) {
+	public async Task<string> ToHtmlAsync(string mfm, List<Note.MentionedUser> mentions) {
+		var nodes = MfmParser.Parse(mfm);
+		return await ToHtmlAsync(nodes, mentions);
+	}
+
+	private INode FromMfmNode(IDocument document, MfmNode node, List<Note.MentionedUser> mentions) {
 		switch (node) {
 			case MfmBoldNode: {
 				var el = document.CreateElement("b");
-				el.AppendChildren(document, node);
+				AppendChildren(el, document, node, mentions);
 				return el;
 			}
 			case MfmSmallNode: {
 				var el = document.CreateElement("small");
-				el.AppendChildren(document, node);
+				AppendChildren(el, document, node, mentions);
 				return el;
 			}
 			case MfmStrikeNode: {
 				var el = document.CreateElement("del");
-				el.AppendChildren(document, node);
+				AppendChildren(el, document, node, mentions);
 				return el;
 			}
 			case MfmItalicNode:
 			case MfmFnNode: {
 				var el = document.CreateElement("i");
-				el.AppendChildren(document, node);
+				AppendChildren(el, document, node, mentions);
 				return el;
 			}
 			case MfmCodeBlockNode codeBlockNode: {
@@ -73,7 +80,7 @@ public static class MfmConverter {
 			}
 			case MfmCenterNode: {
 				var el = document.CreateElement("div");
-				el.AppendChildren(document, node);
+				AppendChildren(el, document, node, mentions);
 				return el;
 			}
 			case MfmEmojiCodeNode emojiCodeNode: {
@@ -84,8 +91,7 @@ public static class MfmConverter {
 			}
 			case MfmHashtagNode hashtagNode: {
 				var el = document.CreateElement("a");
-				//TODO: get url from config
+				el.SetAttribute("href", $"https://{config.Value.WebDomain}/tags/{hashtagNode.Hashtag}");
-				el.SetAttribute("href", $"https://example.org/tags/{hashtagNode.Hashtag}");
 				el.TextContent = $"#{hashtagNode.Hashtag}";
 				el.SetAttribute("rel", "tag");
 				return el;
@@ -108,18 +114,45 @@ public static class MfmConverter {
 			case MfmLinkNode linkNode: {
 				var el = document.CreateElement("a");
 				el.SetAttribute("href", linkNode.Url);
-				el.AppendChildren(document, node);
+				AppendChildren(el, document, node, mentions);
 				return el;
 			}
 			case MfmMentionNode mentionNode: {
 				var el = document.CreateElement("span");
+
+				if (mentionNode.Host == config.Value.AccountDomain || mentionNode.Host == config.Value.WebDomain)
+					mentionNode.Host = null;
+
+				var mention = mentionNode.Host == null
+					? new Note.MentionedUser {
+						Host     = config.Value.AccountDomain,
+						Uri      = $"https://{config.Value.WebDomain}/@{mentionNode.Username}",
+						Username = mentionNode.Username
+					}
+					: mentions.FirstOrDefault(p => string.Equals(p.Username, mentionNode.Username,
+					                                             StringComparison.InvariantCultureIgnoreCase) &&
+					                               string.Equals(p.Host, mentionNode.Host,
+					                                             StringComparison.InvariantCultureIgnoreCase));
+				if (mention == null) {
 					el.TextContent = mentionNode.Acct;
-				//TODO: Resolve mentions and only fall back to the above
+				}
+				else {
+					el.ClassList.Add("h-card");
+					el.SetAttribute("translate", "no");
+					var a = document.CreateElement("a");
+					a.ClassList.Add("u-url", "mention");
+					a.SetAttribute("href", mention.Url ?? mention.Uri);
+					var span = document.CreateElement("span");
+					span.TextContent = $"@{mention.Username}";
+					a.AppendChild(span);
+					el.AppendChild(a);
+				}
+
 				return el;
 			}
 			case MfmQuoteNode: {
 				var el = document.CreateElement("blockquote");
-				el.AppendChildren(document, node);
+				AppendChildren(el, document, node, mentions);
 				return el;
 			}
 			case MfmTextNode textNode: {
@@ -155,7 +188,7 @@ public static class MfmConverter {
 			}
 			case MfmPlainNode: {
 				var el = document.CreateElement("span");
-				el.AppendChildren(document, node);
+				AppendChildren(el, document, node, mentions);
 				return el;
 			}
 			default: {
@@ -164,7 +197,9 @@ public static class MfmConverter {
 		}
 	}
 
-	private static void AppendChildren(this INode element, IDocument document, MfmNode parent) {
+	private void AppendChildren(INode element, IDocument document, MfmNode parent,
-		foreach (var node in parent.Children) element.AppendNodes(document.FromMfmNode(node));
+	                            List<Note.MentionedUser> mentions
+	) {
+		foreach (var node in parent.Children) element.AppendNodes(FromMfmNode(document, node, mentions));
 	}
 }

Iceshrimp.Backend/Core/Helpers/LibMfm/Parsing/HtmlParser.cs

@@ -1,9 +1,11 @@
 using AngleSharp.Dom;
+using AngleSharp.Html.Dom;
+using Iceshrimp.Backend.Core.Database.Tables;
 
 namespace Iceshrimp.Backend.Core.Helpers.LibMfm.Parsing;
 
-internal static class HtmlParser {
+internal class HtmlParser(IEnumerable<Note.MentionedUser> mentions) {
-	internal static string? ParseNode(INode node) {
+	internal string? ParseNode(INode node) {
 		if (node.NodeType is NodeType.Text)
 			return node.TextContent;
 		if (node.NodeType is NodeType.Comment or NodeType.Document)
@@ -14,8 +16,20 @@ internal static class HtmlParser {
 				return "\n";
 			}
 			case "A": {
-				//TODO: implement parsing of links & mentions (automatically correct split domain mentions for the latter)
+				if (node is HtmlElement el) {
-				return null;
+					var href = el.GetAttribute("href");
+					if (href == null) return $"<plain>{el.TextContent}</plain>";
+
+					if (el.ClassList.Contains("u-url") && el.ClassList.Contains("mention")) {
+						var mention = mentions.FirstOrDefault(p => p.Uri == href || p.Url == href);
+						if (mention != null) {
+							return $"@{mention.Username}@{mention.Host}";
+						}
+					}
+
+					return $"[{el.TextContent}]({href})";
+				}
+				return node.TextContent;
 			}
 			case "H1": {
 				return $"【{ParseChildren(node)}】\n";
@@ -74,7 +88,7 @@ internal static class HtmlParser {
 		}
 	}
 
-	private static string ParseChildren(INode node) {
+	private string ParseChildren(INode node) {
 		return string.Join(null, node.ChildNodes.Select(ParseNode));
 	}
 }

Iceshrimp.Backend/Core/Helpers/LibMfm/Parsing/MfmParser.cs

@@ -1,10 +1,11 @@
+using System.Collections.Immutable;
 using System.Text.RegularExpressions;
 using Iceshrimp.Backend.Core.Helpers.LibMfm.Types;
 
 namespace Iceshrimp.Backend.Core.Helpers.LibMfm.Parsing;
 
 public static class MfmParser {
-	private static readonly List<INodeParser> Parsers = [
+	private static readonly ImmutableList<INodeParser> Parsers = [
 		new PlainNodeParser(),
 		new ItalicNodeParser(),
 		new BoldNodeParser(),

Iceshrimp.Backend/Core/Helpers/LibMfm/Serialization/MfmSerializer.cs

@@ -0,0 +1,125 @@
+using System.Text;
+using Iceshrimp.Backend.Core.Helpers.LibMfm.Types;
+
+namespace Iceshrimp.Backend.Core.Helpers.LibMfm.Serialization;
+
+public static class MfmSerializer {
+	public static string Serialize(IEnumerable<MfmNode> nodes) {
+		var result = new StringBuilder();
+
+		foreach (var node in nodes) {
+			switch (node) {
+				case MfmCodeBlockNode mfmCodeBlockNode: {
+					result.Append($"```{mfmCodeBlockNode.Language ?? ""}\n");
+					result.Append(mfmCodeBlockNode.Code);
+					result.Append("```");
+					break;
+				}
+				case MfmMathBlockNode mfmMathBlockNode: {
+					result.Append(@"\[");
+					result.Append(mfmMathBlockNode.Formula);
+					result.Append(@"\]");
+					break;
+				}
+				case MfmSearchNode mfmSearchNode: {
+					throw new NotImplementedException();
+					break;
+				}
+				case MfmBoldNode mfmBoldNode: {
+					result.Append("**");
+					result.Append(Serialize(node.Children));
+					result.Append("**");
+					break;
+				}
+				case MfmCenterNode mfmCenterNode: {
+					result.Append("<center>");
+					result.Append(Serialize(node.Children));
+					result.Append("</center>");
+					break;
+				}
+				case MfmEmojiCodeNode mfmEmojiCodeNode: {
+					result.Append($":{mfmEmojiCodeNode.Name}:");
+					break;
+				}
+				case MfmFnNode mfmFnNode: {
+					throw new NotImplementedException();
+					break;
+				}
+				case MfmHashtagNode mfmHashtagNode: {
+					result.Append($"#{mfmHashtagNode.Hashtag}");
+					break;
+				}
+				case MfmInlineCodeNode mfmInlineCodeNode: {
+					result.Append($"`{mfmInlineCodeNode.Code}`");
+					break;
+				}
+				case MfmItalicNode mfmItalicNode: {
+					result.Append("~~");
+					result.Append(Serialize(node.Children));
+					result.Append("~~");
+					break;
+				}
+				case MfmLinkNode mfmLinkNode: {
+					if (mfmLinkNode.Silent) result.Append('?');
+					result.Append('[');
+					result.Append(Serialize(node.Children));
+					result.Append(']');
+					result.Append($"({mfmLinkNode.Url})");
+					break;
+				}
+				case MfmMathInlineNode mfmMathInlineNode: {
+					result.Append(@"\(");
+					result.Append(mfmMathInlineNode.Formula);
+					result.Append(@"\)");
+					break;
+				}
+				case MfmMentionNode mfmMentionNode: {
+					result.Append($"@{mfmMentionNode.Username}");
+					if (mfmMentionNode.Host != null)
+						result.Append($"@{mfmMentionNode.Host}");
+					break;
+				}
+				case MfmPlainNode: {
+					result.Append(node.Children.OfType<MfmTextNode>().Select(p => p.Text));
+					break;
+				}
+				case MfmSmallNode: {
+					result.Append("<small>");
+					result.Append(Serialize(node.Children));
+					result.Append("</small>");
+					break;
+				}
+				case MfmStrikeNode: {
+					result.Append("~~");
+					result.Append(Serialize(node.Children));
+					result.Append("~~");
+					break;
+				}
+				case MfmTextNode mfmTextNode: {
+					result.Append(mfmTextNode.Text);
+					break;
+				}
+				case MfmUnicodeEmojiNode mfmUnicodeEmojiNode: {
+					result.Append(mfmUnicodeEmojiNode.Emoji);
+					break;
+				}
+				case MfmUrlNode mfmUrlNode: {
+					if (mfmUrlNode.Brackets)
+						result.Append($"<{mfmUrlNode.Url}>");
+					else
+						result.Append(mfmUrlNode.Url);
+					break;
+				}
+				case MfmQuoteNode mfmQuoteNode: {
+					throw new NotImplementedException();
+					break;
+				}
+				default: {
+					throw new Exception("Unknown node type");
+				}
+			}
+		}
+
+		return result.ToString();
+	}
+}

Iceshrimp.Backend/Core/Services/NoteService.cs

@@ -13,6 +13,12 @@ using Microsoft.Extensions.Options;
 
 namespace Iceshrimp.Backend.Core.Services;
 
+using MentionQuad =
+	(List<string> mentionedUserIds,
+	List<Note.MentionedUser> mentions,
+	List<Note.MentionedUser> remoteMentions,
+	Dictionary<(string usernameLower, string webDomain), string> splitDomainMapping);
+
 [SuppressMessage("ReSharper", "SuggestBaseTypeForParameterInConstructor",
                  Justification = "We need IOptionsSnapshot for config hot reload")]
 public class NoteService(
@@ -23,7 +29,9 @@ public class NoteService(
 	ActivityFetcherService fetchSvc,
 	ActivityDeliverService deliverSvc,
 	NoteRenderer noteRenderer,
-	UserRenderer userRenderer
+	UserRenderer userRenderer,
+	MentionsResolver mentionsResolver,
+	MfmConverter mfmConverter
 ) {
 	private readonly List<string> _resolverHistory = [];
 	private          int          _recursionLimit  = 100;
@@ -117,14 +125,15 @@ public class NoteService(
 		if (user.IsSuspended)
 			throw GracefulException.Forbidden("User is suspended");
 
-		//TODO: validate AP object type
 		//TODO: resolve anything related to the note as well (attachments, emoji, etc)
 
+		var (mentionedUserIds, mentions, remoteMentions, splitDomainMapping) = await ResolveNoteMentionsAsync(note);
+
 		var dbNote = new Note {
 			Id     = IdHelpers.GenerateSlowflakeId(),
 			Uri    = note.Id,
 			Url    = note.Url?.Id, //FIXME: this doesn't seem to work yet
-			Text   = note.MkContent ?? await MfmConverter.FromHtmlAsync(note.Content),
+			Text   = note.MkContent ?? await mfmConverter.FromHtmlAsync(note.Content, mentions),
 			UserId = user.Id,
 			CreatedAt = note.PublishedAt?.ToUniversalTime() ??
 			            throw GracefulException.UnprocessableEntity("Missing or invalid PublishedAt field"),
@@ -137,6 +146,22 @@ public class NoteService(
 		if (dbNote.Text is { Length: > 100000 })
 			throw GracefulException.UnprocessableEntity("Content cannot be longer than 100.000 characters");
 
+		if (dbNote.Text is not null) {
+			dbNote.Mentions             = mentionedUserIds;
+			dbNote.MentionedRemoteUsers = remoteMentions;
+			if (dbNote.Visibility == Note.NoteVisibility.Specified) {
+				var visibleUserIds = note.GetRecipients(actor).Concat(mentionedUserIds).ToList();
+				if (dbNote.ReplyUserId != null)
+					visibleUserIds.Add(dbNote.ReplyUserId);
+
+				dbNote.VisibleUserIds = visibleUserIds.Distinct().ToList();
+			}
+
+			dbNote.Text =
+				await mentionsResolver.ResolveMentions(dbNote.Text, dbNote.UserHost, remoteMentions,
+				                                       splitDomainMapping);
+		}
+
 		user.NotesCount++;
 		await db.Notes.AddAsync(dbNote);
 		await db.SaveChangesAsync();
@@ -144,6 +169,43 @@ public class NoteService(
 		return dbNote;
 	}
 
+	private async Task<MentionQuad> ResolveNoteMentionsAsync(ASNote note) {
+		var mentionTags = note.Tags?.OfType<ASMention>().Where(p => p.Href != null) ?? [];
+		var users = await mentionTags
+		                  .Select(async p => await userResolver.ResolveAsync(p.Href!.Id!))
+		                  .AwaitAllNoConcurrencyAsync();
+
+		var userIds = users.Select(p => p.Id).ToList();
+
+		var remoteUsers = users.Where(p => p is { Host: not null, Uri: not null })
+		                       .ToList();
+
+		var localUsers = users.Where(p => p.Host is null)
+		                      .ToList();
+
+		var splitDomainMapping = remoteUsers.Where(p => new Uri(p.Uri!).Host != p.Host)
+		                                    .DistinctBy(p => p.Host)
+		                                    .ToDictionary(p => (p.UsernameLower, new Uri(p.Uri!).Host), p => p.Host!);
+
+		var localMentions = localUsers.Select(p => new Note.MentionedUser {
+			Host     = config.Value.AccountDomain,
+			Username = p.Username,
+			Uri      = $"https://{config.Value.WebDomain}/users/{p.Id}",
+			Url      = $"https://{config.Value.WebDomain}/@{p.Username}"
+		});
+
+		var remoteMentions = remoteUsers.Select(p => new Note.MentionedUser {
+			Host     = p.Host!,
+			Uri      = p.Uri!,
+			Username = p.Username,
+			Url      = p.UserProfile?.Url
+		}).ToList();
+
+		var mentions = remoteMentions.Concat(localMentions).ToList();
+
+		return (userIds, mentions, remoteMentions, splitDomainMapping);
+	}
+
 	public async Task<Note?> ResolveNoteAsync(string uri) {
 		//TODO: is this enough to prevent DoS attacks?
 		if (_recursionLimit-- <= 0)

Iceshrimp.Backend/Core/Services/UserService.cs

@@ -24,9 +24,10 @@ public class UserService(
 	ILogger<UserService> logger,
 	DatabaseContext db,
 	ActivityFetcherService fetchSvc,
-	DriveService driveSvc
+	DriveService driveSvc,
+	MfmConverter mfmConverter
 ) {
-	private (string Username, string? Host) AcctToTuple(string acct) {
+	private static (string Username, string? Host) AcctToTuple(string acct) {
 		if (!acct.StartsWith("acct:")) throw new GracefulException(HttpStatusCode.BadRequest, "Invalid query");
 
 		var split = acct[5..].Split('@');
@@ -43,7 +44,9 @@ public class UserService(
 				       throw GracefulException.NotFound("User not found");
 			}
 			else {
-				return await db.Users.IncludeCommonProperties().FirstOrDefaultAsync(p => p.Uri == query);
+				return await db.Users
+				               .IncludeCommonProperties()
+				               .FirstOrDefaultAsync(p => p.Uri != null && p.Uri.ToLower() == query.ToLowerInvariant());
 			}
 
 		var tuple = AcctToTuple(query);
@@ -51,7 +54,8 @@ public class UserService(
 			tuple.Host = null;
 		return await db.Users
 		               .IncludeCommonProperties()
-		               .FirstOrDefaultAsync(p => p.Username == tuple.Username && p.Host == tuple.Host);
+		               .FirstOrDefaultAsync(p => p.UsernameLower == tuple.Username.ToLowerInvariant() &&
+		                                         p.Host == tuple.Host);
 	}
 
 	public async Task<User> CreateUserAsync(string uri, string acct) {
@@ -93,7 +97,7 @@ public class UserService(
 
 		var profile = new UserProfile {
 			User        = user,
-			Description = actor.MkSummary ?? await MfmConverter.FromHtmlAsync(actor.Summary),
+			Description = actor.MkSummary ?? await mfmConverter.FromHtmlAsync(actor.Summary),
 			//Birthday = TODO,
 			//Location = TODO,
 			//Fields = TODO,
@@ -151,7 +155,7 @@ public class UserService(
 
 		var processPendingDeletes = await ResolveAvatarAndBanner(user, actor);
 
-		user.UserProfile.Description = actor.MkSummary ?? await MfmConverter.FromHtmlAsync(actor.Summary);
+		user.UserProfile.Description = actor.MkSummary ?? await mfmConverter.FromHtmlAsync(actor.Summary);
 		//user.UserProfile.Birthday = TODO;
 		//user.UserProfile.Location = TODO;
 		//user.UserProfile.Fields = TODO;

Iceshrimp.Tests/Parsing/MfmTests.cs

@@ -1,5 +1,6 @@
 using Iceshrimp.Backend.Core.Helpers.LibMfm.Conversion;
 using Iceshrimp.Backend.Core.Helpers.LibMfm.Parsing;
+using Microsoft.Extensions.DependencyInjection;
 
 namespace Iceshrimp.Tests.Parsing;
 
@@ -24,8 +25,11 @@ public class MfmTests {
 		return;
 
 		async Task<double> Benchmark() {
+			var provider  = MockObjects.ServiceProvider;
+			var converter = provider.GetRequiredService<MfmConverter>();
+
 			var pre = DateTime.Now;
-			await MfmConverter.ToHtmlAsync(Mfm);
+			await converter.ToHtmlAsync(Mfm, []);
 			var post = DateTime.Now;
 			var ms   = (post - pre).TotalMilliseconds;
 			Console.WriteLine($"Took {ms} ms");