fix: tolerate concurrent user creation

Morpheus.Tests/UsersServiceTests.cs

@@ -32,6 +32,29 @@ public async Task TryGetCreateUser_CreatesUserFromNonSocketUser()
         Assert.Single(await db.Users.ToListAsync());
     }
 
+    [Fact]
+    public async Task TryGetCreateUserAsync_WhenAnotherHandlerCreatesUser_ReturnsPersistedUser()
+    {
+        await using SqliteConnection connection = new("Data Source=:memory:");
+        await connection.OpenAsync();
+
+        DbContextOptions<DB> options = new DbContextOptionsBuilder<DB>()
+            .UseSqlite(connection)
+            .Options;
+        await using (DB setup = new(options))
+            await setup.Database.EnsureCreatedAsync();
+
+        await using RacingDb db = new(options);
+        db.InsertCompetingUserOnNextSave = true;
+        UsersService service = new(db, new LogsService(new LogQueue()));
+
+        User result = await service.TryGetCreateUserAsync(123, "first");
+
+        Assert.Equal((ulong)123, result.DiscordId);
+        Assert.Equal("concurrent", result.Username);
+        Assert.Equal(1, await db.Users.CountAsync());
+    }
+
     private static IUser CreateUser(ulong id, string username)
     {
         IUser user = DispatchProxy.Create<IUser, UserProxy>();
@@ -53,4 +76,31 @@ public class UserProxy : DispatchProxy
             _ => throw new NotSupportedException(targetMethod?.Name)
         };
     }
+
+    private sealed class RacingDb : DB
+    {
+        public RacingDb(DbContextOptions<DB> options) : base(options)
+        {
+        }
+
+        public bool InsertCompetingUserOnNextSave { get; set; }
+
+        public override async Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
+        {
+            if (InsertCompetingUserOnNextSave)
+            {
+                InsertCompetingUserOnNextSave = false;
+                ChangeTracker.Clear();
+                Users.Add(new User
+                {
+                    DiscordId = 123,
+                    Username = "concurrent"
+                });
+                await base.SaveChangesAsync(cancellationToken);
+                throw new DbUpdateException("Simulated concurrent unique-key conflict.");
+            }
+
+            return await base.SaveChangesAsync(cancellationToken);
+        }
+    }
 }

Services/UsersService.cs

@@ -7,25 +7,42 @@
 namespace Morpheus.Services;
 public class UsersService(DB dbContext, LogsService logsService)
 {
-    public async Task<User> TryGetCreateUser(IUser user)
+    public Task<User> TryGetCreateUser(IUser user) =>
+        TryGetCreateUserAsync(user.Id, user.Username);
+
+    internal async Task<User> TryGetCreateUserAsync(ulong discordId, string username)
     {
-        User? userDb = await dbContext.Users.FirstOrDefaultAsync(u => u.DiscordId == user.Id);
+        User? userDb = await dbContext.Users.FirstOrDefaultAsync(u => u.DiscordId == discordId);
 
         if (userDb != null)
             return userDb;
 
         userDb = new User()
         {
-            DiscordId = user.Id,
-            Username = user.Username,
+            DiscordId = discordId,
+            Username = username,
             InsertDate = DateTime.UtcNow,
             LastUsernameCheck = DateTime.UtcNow
         };
 
         await dbContext.Users.AddAsync(userDb);
-        await dbContext.SaveChangesAsync();
+        try
+        {
+            await dbContext.SaveChangesAsync();
+        }
+        catch (DbUpdateException)
+        {
+            // Another message handler may have created the same Discord user after our initial
+            // lookup. Re-query instead of failing activity processing on the unique index.
+            dbContext.ChangeTracker.Clear();
+            User? concurrentUser = await dbContext.Users.FirstOrDefaultAsync(u => u.DiscordId == discordId);
+            if (concurrentUser != null)
+                return concurrentUser;
+
+            throw;
+        }
 
-        logsService.Log($"New user created {user.Username}", Discord.LogSeverity.Verbose);
+        logsService.Log($"New user created {username}", Discord.LogSeverity.Verbose);
 
         return userDb;
     }