[backend/core] Add keyed async locks in UserResolver to prevent insertion conflicts

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

@@ -1,3 +1,4 @@
+using AsyncKeyedLock;
 using Iceshrimp.Backend.Core.Database.Tables;
 using Iceshrimp.Backend.Core.Federation.WebFinger;
 using Iceshrimp.Backend.Core.Middleware;
@@ -12,6 +13,12 @@ public class UserResolver(
 	FollowupTaskService followupTaskSvc
 )
 {
+	private static readonly AsyncKeyedLocker<string> KeyedLocker = new(o =>
+	{
+		o.PoolSize        = 100;
+		o.PoolInitialFill = 5;
+	});
+
 	/*
 	 * The full web finger algorithm:
 	 *
@@ -119,9 +126,12 @@ public class UserResolver(
 		if (user != null)
 			return await GetUpdatedUser(user);
 
+		using (await KeyedLocker.LockAsync(uri))
+		{
 			// Pass the job on to userSvc, which will create the user
 			return await userSvc.CreateUserAsync(uri, acct);
 		}
+	}
 
 	private async Task<User> GetUpdatedUser(User user)
 	{

Iceshrimp.Backend/Core/Services/UserService.cs

@@ -66,17 +66,10 @@ public class UserService(
 	public async Task<User> CreateUserAsync(string uri, string acct)
 	{
 		logger.LogDebug("Creating user {acct} with uri {uri}", acct, uri);
-		var actor = await fetchSvc.FetchActorAsync(uri);
-		logger.LogDebug("Got actor: {url}", actor.Url);
-
-		actor.Normalize(uri, acct);
-
-		if (actor.PublicKey?.Id == null || actor.PublicKey?.PublicKey == null)
-			throw new GracefulException(HttpStatusCode.UnprocessableEntity, "Actor has no valid public key");
 
 		var user = await db.Users
 		                   .IncludeCommonProperties()
-		                   .FirstOrDefaultAsync(p => p.Uri != null && p.Uri == actor.Id);
+		                   .FirstOrDefaultAsync(p => p.Uri != null && p.Uri == uri);
 
 		if (user != null)
 		{
@@ -85,6 +78,17 @@ public class UserService(
 			return user;
 		}
 
+		var actor = await fetchSvc.FetchActorAsync(uri);
+		logger.LogDebug("Got actor: {url}", actor.Url);
+
+		actor.Normalize(uri, acct);
+
+		if (actor.Id != uri)
+			throw GracefulException.UnprocessableEntity("Uri doesn't match id of fetched actor");
+
+		if (actor.PublicKey?.Id == null || actor.PublicKey?.PublicKey == null)
+			throw GracefulException.UnprocessableEntity("Actor has no valid public key");
+
 		user = new User
 		{
 			Id            = IdHelpers.GenerateSlowflakeId(),

Iceshrimp.Backend/Iceshrimp.Backend.csproj

@@ -18,6 +18,7 @@
         <PackageReference Include="Amazon.S3" Version="0.31.2" />
         <PackageReference Include="AngleSharp" Version="1.1.0" />
         <PackageReference Include="Asp.Versioning.Http" Version="8.0.0" />
+        <PackageReference Include="AsyncKeyedLock" Version="6.3.4" />
         <PackageReference Include="Blurhash.ImageSharp" Version="3.0.0" />
         <PackageReference Include="cuid.net" Version="5.0.2" />
         <PackageReference Include="dotNetRdf.Core" Version="3.2.1-dev" />