fix: more race condition handling

Author: kyleratti

ApplicationData/Services/LinkProvider.cs

@@ -81,7 +81,7 @@ FROM app.links l
 
 	public async Task DeleteLinkById(int linkId)
 	{
-		await using var tx = await _db.CreateTransactionAsync();
+		await using var tx = await _db.CreateTransactionAsync(System.Data.IsolationLevel.Serializable);
 		var redditPostId = (string)await tx.ExecuteSqlScalarAsync(
 			@"
 				UPDATE app.links SET is_deleted = true WHERE link_id = @linkId AND is_deleted = false
@@ -108,7 +108,7 @@ public async Task<Result<int, CreateLinkError>> CreateLink(NewLink link)
 	{
 		try
 		{
-			await using var tx = await _db.CreateTransactionAsync();
+			await using var tx = await _db.CreateTransactionAsync(System.Data.IsolationLevel.Serializable);
 			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)

BackgroundProcessor/Processors/LinkProcessor.cs

@@ -50,7 +50,7 @@ public async Task Process(CancellationToken cancellationToken)
 		{
 			var links = await _linkProvider.GetLinksByRedditPostId(item.RedditPostId, cancellationToken).ToArrayAsync(cancellationToken);
 			var maybeExistingComment = await _commentProvider.FindCommentIdByPostId(item.RedditPostId);
-			var tx = await _dbTxFactory.CreateTransaction();
+			var tx = await _dbTxFactory.CreateTransaction(System.Data.IsolationLevel.Serializable);
 
 			if (!links.Any())
 			{

Core/DbConnection.fs

@@ -99,10 +99,10 @@ module private DbConnection_impl =
             return! cmd.ExecuteReaderAsync ()
         }
 
-    let createTransactionAsync (db : NpgsqlConnection) =
+    let createTransactionAsync (db : NpgsqlConnection) (isolationLevel : IsolationLevel) =
         task {
             let! _ = db |> connectToDbIfNotConnectedAsync
-            let! tx = db.BeginTransactionAsync ()
+            let! tx = db.BeginTransactionAsync (isolationLevel)
             return new DbTransactionWrapper (tx)
         }
 
@@ -130,8 +130,9 @@ type DbConnection (settings : DbSettings) =
         (sql, queryParams)
         ||> DbConnection_impl.executeReaderAsync db
 
-    member _.CreateTransactionAsync () : DbTransactionWrapper Task =
-        DbConnection_impl.createTransactionAsync db
+    member _.CreateTransactionAsync (isolationLevel : IsolationLevel) : DbTransactionWrapper Task =
+        isolationLevel
+        |> DbConnection_impl.createTransactionAsync db
 
     interface IDbConnection with
         member this.ExecuteSqlNonQuery sql queryParams = task { return! this.ExecuteSqlNonQueryAsync(sql, queryParams) }
@@ -143,4 +144,4 @@ type DbConnection (settings : DbSettings) =
             db.Dispose ()
 
 type DbTransactionFactory (db : DbConnection) =
-    member _.CreateTransaction () : DbTransactionWrapper Task = DbConnection_impl.createTransactionAsync db.__RawDb
+    member _.CreateTransaction (isolationLevel : IsolationLevel) : DbTransactionWrapper Task = isolationLevel |> DbConnection_impl.createTransactionAsync db.__RawDb