fix: handle race condition when posting link

Fix #30

Author: kyleratti

ApplicationData/ApplicationData.csproj

@@ -16,6 +16,7 @@
       <PackageReference Include="FruityFoundation.Base" Version="1.1.2" />
       <PackageReference Include="FruityFoundation.FsBase" Version="1.1.2" />
       <PackageReference Include="Microsoft.Extensions.Caching.Abstractions" Version="7.0.0" />
+      <PackageReference Include="ResultMonad" Version="1.0.1" />
       <PackageReference Include="SnooBrowser" Version="3.1.0" />
       <PackageReference Include="System.Linq.Async" Version="6.0.1" />
     </ItemGroup>

ApplicationData/Services/LinkProvider.cs

@@ -4,6 +4,7 @@
 using FruityFoundation.Base.Extensions;
 using FruityFoundation.Base.Structures;
 using Npgsql;
+using ResultMonad;
 
 namespace ApplicationData.Services;
 
@@ -92,29 +93,48 @@ public async Task DeleteLinkById(int linkId)
 		await tx.CommitAsync();
 	}
 
-	public async Task<int> CreateLink(NewLink link)
+	public abstract record CreateLinkError
 	{
-		await using var tx = await _db.CreateTransactionAsync();
-		var linkIdResult = (int?)await tx.ExecuteSqlScalarAsync(
-			@"INSERT INTO app.links (reddit_post_id, link_url, link_type, created_at, is_deleted, owner)
-				VALUES (@redditPostId, @linkUrl, @linkType, NOW(), false, @userId)
-				RETURNING link_id",
-			new NpgsqlParameter[]
-			{
-				new("@redditPostId", link.RedditPostId),
-				new("@linkUrl", link.LinkUrl),
-				new("@linkType", link.LinkType.RawValue),
-				new("@userId", link.OwnerUserId)
-			});
-
-		if (!linkIdResult.ToMaybe().Try(out var linkId))
-			throw new ApplicationException("Failed to create link");
-
-		await QueueRedditPostIdForUpdate(tx, link.RedditPostId);
+		public T Switch<T>(Func<T> linkAlreadyExists) => this switch
+		{
+			LinkAlreadyExists => linkAlreadyExists(),
+			_ => throw new NotImplementedException($"Type not implemented: {GetType().FullName}")
+		};
+	}
 
-		await tx.CommitAsync();
+	public record LinkAlreadyExists : CreateLinkError;
 
-		return linkId;
+	public async Task<Result<int, CreateLinkError>> CreateLink(NewLink link)
+	{
+		try
+		{
+			await using var tx = await _db.CreateTransactionAsync();
+			var linkIdResult = (int?)await tx.ExecuteSqlScalarAsync("""
+			INSERT INTO app.links (reddit_post_id, link_url, link_type, created_at, is_deleted, owner)
+			VALUES (@redditPostId, @linkUrl, @linkType, NOW(), false, @userId)
+			RETURNING link_id
+			""",
+				new NpgsqlParameter[]
+				{
+					new("@redditPostId", link.RedditPostId),
+					new("@linkUrl", link.LinkUrl),
+					new("@linkType", link.LinkType.RawValue),
+					new("@userId", link.OwnerUserId)
+				});
+
+			if (!linkIdResult.ToMaybe().Try(out var linkId))
+				throw new ApplicationException("Failed to create link");
+
+			await QueueRedditPostIdForUpdate(tx, link.RedditPostId);
+
+			await tx.CommitAsync();
+
+			return Result.Ok<int, CreateLinkError>(linkId);
+		}
+		catch (PostgresException ex) when (ex is { SqlState: PostgresErrorCodes.UniqueViolation, ConstraintName: "UQ_reddit_post_id_link_url_link_type_is_deleted_owner" })
+		{
+			return Result.Fail<int, CreateLinkError>(new LinkAlreadyExists());
+		}
 	}
 
 	public async Task<IReadOnlyCollection<Link>> GetAllLinksByUserId(int userId)

WebApi/Controllers/LinkController.cs

@@ -53,14 +53,6 @@ public async Task<IActionResult> SubmitLink([FromBody] SubmitLinkRequest linkReq
 		var validUrl = GetValidUriOrFail(linkRequest.LinkUrl);
 		var linkKind = GetLinkKindOrFail(linkRequest.LinkType!.Value);
 
-		if ((await _linkProvider.FindLink(UserId, linkRequest.RedditPostId, validUrl.OriginalString, linkKind)).HasValue)
-			return new ConflictObjectResult(new LinkAlreadyExistsError(
-				Message: TranslatedStrings.LinkController.LinkAlreadyExists,
-				RedditPostId: linkRequest.RedditPostId,
-				Url: validUrl.OriginalString,
-				LinkType: linkRequest.LinkType
-			));
-
 		var maybeSubmission = await _submissionBrowser.GetSubmission(LinkThing.CreateFromShortId(linkRequest.RedditPostId));
 
 		if (!maybeSubmission.Try(out var submission) || submission.IsArchived || submission.IsLocked)
@@ -69,13 +61,24 @@ public async Task<IActionResult> SubmitLink([FromBody] SubmitLinkRequest linkReq
 				RedditPostId: linkRequest.RedditPostId
 			));
 
-		await _linkProvider.CreateLink(new NewLink(
+		var createResult = await _linkProvider.CreateLink(new NewLink(
 			redditPostId: linkRequest.RedditPostId,
 			linkUrl: validUrl.OriginalString,
 			linkKind,
 			ownerUserId: UserId
 		));
 
+		if (!createResult.TrySuccess(out _, out var error))
+		{
+			return error!.Switch(
+				linkAlreadyExists: () => new ConflictObjectResult(new LinkAlreadyExistsError(
+					Message: TranslatedStrings.LinkController.LinkAlreadyExists,
+					RedditPostId: linkRequest.RedditPostId,
+					Url: validUrl.OriginalString,
+					LinkType: linkRequest.LinkType
+				)));
+		}
+
 		// We are technically violating the HTTP spec here but not sending back the location of the resource we just created, but oh well.
 		return new StatusCodeResult((int)HttpStatusCode.Created);
 	}

WebApi/Util/ResultExtensions.cs

@@ -0,0 +1,30 @@
+using ResultMonad;
+
+namespace WebApi.Util;
+
+/// <summary>
+/// ResultMonad extensions
+/// </summary>
+public static class ResultExtensions
+{
+	/// <summary>
+	/// Return true if the Result is a successful one.
+	/// Assign success and error out parameters.
+	/// </summary>
+	public static bool TrySuccess<TSuccess, TError>(this Result<TSuccess, TError> item, out TSuccess? success,
+		out TError? error)
+	{
+		if (item.IsSuccess)
+		{
+			success = item.Value;
+			error = default;
+		}
+		else
+		{
+			success = default;
+			error = item.Error;
+		}
+
+		return item.IsSuccess;
+	}
+}