Harden owner and suggestion invariants for concurrent writes

Endpoints/VoteWorkflowService.cs

@@ -2,6 +2,7 @@ using GameList.Contracts;
 using GameList.Data;
 using GameList.Domain;
 using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.ChangeTracking;
 
 namespace GameList.Endpoints;
 
@@ -71,26 +72,46 @@ internal sealed class VoteWorkflowService(AppDbContext db)
             .Where(v => v.PlayerId == playerId && linkedIds.Contains(v.SuggestionId))
             .ToListAsync();
 
-        foreach (var linkedSuggestionId in linkedIds)
+        for (var attempt = 0; attempt < 2; attempt++)
         {
-            var vote = existingVotes.FirstOrDefault(v => v.SuggestionId == linkedSuggestionId);
-            if (vote == null)
+            foreach (var linkedSuggestionId in linkedIds)
             {
-                db.Votes.Add(new Vote
+                var vote = existingVotes.FirstOrDefault(v => v.SuggestionId == linkedSuggestionId);
+                if (vote == null)
                 {
-                    PlayerId = playerId,
-                    SuggestionId = linkedSuggestionId,
-                    Score = score
-                });
+                    db.Votes.Add(new Vote
+                    {
+                        PlayerId = playerId,
+                        SuggestionId = linkedSuggestionId,
+                        Score = score
+                    });
+                }
+                else
+                {
+                    vote.Score = score;
+                }
             }
-            else
+
+            try
             {
-                vote.Score = score;
+                await db.SaveChangesAsync();
+                return Results.Ok(new VoteUpsertResponse(linkedIds, score));
+            }
+            catch (DbUpdateException ex) when (attempt == 0 && EndpointHelpers.IsSqliteConstraintViolation(ex))
+            {
+                DetachAddedVotes(db.ChangeTracker.Entries<Vote>());
+
+                await db.Votes
+                    .Where(v => v.PlayerId == playerId && linkedIds.Contains(v.SuggestionId))
+                    .ExecuteUpdateAsync(v => v.SetProperty(x => x.Score, score));
+
+                existingVotes = await db.Votes
+                    .Where(v => v.PlayerId == playerId && linkedIds.Contains(v.SuggestionId))
+                    .ToListAsync();
             }
         }
 
-        await db.SaveChangesAsync();
-        return Results.Ok(new VoteUpsertResponse(linkedIds, score));
+        return EndpointHelpers.ConflictError("Vote update conflict. Please retry.");
     }
 
     public async Task<IResult> SetFinalizeAsync(Guid playerId, bool final)
@@ -105,4 +126,13 @@ internal sealed class VoteWorkflowService(AppDbContext db)
         await db.SaveChangesAsync();
         return Results.Ok(new VoteFinalizeResponse(player.VotesFinal));
     }
+
+    private static void DetachAddedVotes(IEnumerable<EntityEntry<Vote>> voteEntries)
+    {
+        foreach (var entry in voteEntries)
+        {
+            if (entry.State == EntityState.Added)
+                entry.State = EntityState.Detached;
+        }
+    }
 }