## Why
An Argon2 verify is **~8.9 ms of frozen world per login attempt** — more than half a 16 ms frame. Failed attempts cost exactly the same as successful ones, by design, so a credential-stuffing flood is a full-cost stall per packet without needing valid credentials. `SetPassword` derives a hash too, so `[password`, the admin gump and account creation each pay the same.
## What the measurement says
Off-loading does not delete the cost, it relocates it. Three things stay on the loop:
| Component | Measured |
|---|---:|
| Inline verify (today) | **8.92 ms** |
| Dispatch to the worker | 210 ns |
| Drain the continuation off `LoopContext` | 13 ns |
| Loop's own work slowed by shared-L3 eviction | **0.05 – 5.44 ms** |
Net gain **3.5 – 8.9 ms** of on-loop time per login. Harness in `ModernUO-Benchmarks` (`Benchmarks/Argon2OffLoop/`): it models the loop as a dependent-load pointer chase swept across working-set sizes, which is an upper bound on cache-latency sensitivity, and copies `EventLoopContext` so the hand-off cost is the real one.
Two results shaped the design:
- **The contention tax peaks in the middle of the working-set range**, not at the top — 5.44 ms at 8 MiB (a quarter of this chip's L3), but 0.76 ms at 30 MiB and 0.10 ms at 256 KiB. A tiny hot set has nothing in L3 to lose; a huge one is already DRAM-bound.
- **Per-login tax falls as concurrency rises** (5.44 → 2.56 → 1.60 ms at 1/2/4 hashers) while *total* loop damage rises. Contention is shared, not additive, so a login rush is not the disaster case — a single login is.
## Why exactly one worker
It is load-bearing three times over, which is also why it must not quietly become a pool:
- **Cost bound.** Off-loop loses to inline only if a hash steals ~82% of the loop's throughput. One hasher contending for one core leaves the loop ~50%. **A single background hasher cannot cost the loop more than the inline verify under any scheduling regime**, which is what lets the measurement hold on hardware we cannot inspect — AMD, VPS, oversubscribed VM. Four hashers drop the loop to ~20% and break it.
- **Memory.** Exactly one hashing arena is live at a time whatever the login volume.
- **Ordering.** Writes apply in dispatch order *only* because a single thread drains FIFO. A second worker would need ordering reintroduced; `WritesApplyInDispatchOrder` fails if that happens.
Throughput is ~110 verifies/sec. Only loop time matters, not login latency, so head-of-line blocking during a rush costs nothing.
## Making every protection safe off-thread
The worker was initially Argon2-only. That was the right call for the wrong reason — it was blamed on Argon2's salt RNG, which is a stateless syscall wrapper and was never a problem. The real blockers were elsewhere, and both are fixed at the source:
| Protection | Was | Now |
|---|---|---|
| MD5/SHA1/SHA2 | shared `HashAlgorithm.ComputeHash`, which carries the running digest across `HashCore`/`HashFinal` through process-wide singletons | static `HashData` into a `stackalloc` span — no state, no allocation, identical bytes |
| PBKDF2 | `Utility.RandomMinMax` → shared `System.Random`, thread-unsafe *and* game state | `RandomNumberGenerator.GetInt32`, matching the salt beside it |
| Argon2 | already safe (`Verify` is static + stackalloc) | unchanged, singleton reused |
Literal digests are pinned in a test **before** the change and still pass after it. These are compared as strings against every account database, so any casing or encoding drift would lock out every SHA and MD5 account at once.
With all three safe, the worker no longer knows which algorithm it runs and the dispatch conditions collapse to "is off-loop available".
## Correctness
- **Phrase derivation** moves to `AccountSecurity.DerivePhrase`, so verification (stored algorithm's rule) and rehash (target algorithm's rule) cannot disagree. Deriving with the wrong one is the shape of the lockout fixed in #2562.
- **Liveness** is checked at dequeue *and* at apply — a connection can drop while queued or while the result sits in the loop queue. A job with no connection attached, such as an admin password change, runs regardless.
- **Queue overflow rejects** a login rather than verifying inline; steering work back onto the loop is what a flood wants. A password change instead falls back to hashing inline, because unlike a login it must not be dropped.
- **Shutdown and crash** both just stop the thread, and pending jobs are dropped. No save is initiated once shutdown begins — saving is the operator's choice up front, via the admin gump's save/no-save variants, and `WaitForWriteCompletion` honours one already in flight — so a write applied during teardown would reach no disk. The crash path needs its own subscription because `HandleClosed` skips `InvokeShutdown` when crashed.
## Bounding
`MaxPending` is 4096 — a backstop, not a flood defense. `SentFirstPacket` holds a connection to one pending verify and the engine caps connections at 4096, so the queue is already bounded by construction and this can only trip if that invariant breaks. A cap low enough to blunt an attack would reject real players first; during a mass reconnect they *are* the queue. Flood defense belongs at the connection layer.
The real DoS improvement is elsewhere: today every attempt stalls the world, and after this a flood occupies one core while the loop keeps ticking.
## Gate
Release builds on 4+ cores. Below that there is no spare core to move work to, so off-loading buys nothing by construction; `DEBUG` is excluded because dev boxes and test shards have few logins. Both modes call the same code — the gate only chooses where it runs.
## Engine change
One property, `AccountLoginEventArgs.Deferred`, so a subscriber can say "no verdict yet". `EventSink.AccountLogin` is `Action<...>` with no continuation, and the packet handler replies in the same call. Approved separately since it touches `Projects/Server/`.
## Docs
`dev-docs/threading-model.md` and the threading skill gain a vetted-workers section. The forbidden-patterns table bans `new Thread`, `ConcurrentQueue<T>`, `Interlocked` and `volatile` in `UOContent`, and its exceptions covered only `Projects/Server/` — the existing Advanced Search fan-out already sat outside it. The new section leads with proving the need (measure on-loop time, not wall-clock; gate on core count; record the measurement), keeps game logic on the loop via chunking, and documents the hand-off protocol in both directions.
## Testing
698 UOContent tests, 810 Server tests, Release build clean.
Covered: verify and rehash outcomes, phrase rules for SHA1/SHA2 vs Argon2, stored-format stability for MD5/SHA1/SHA2, jobs with no connection attached, and dispatch ordering through the real queue. The liveness and ordering guards are mutation-verified.
## Summary
`AdvancedSearchThreadWorker.Execute` signals `_stopEvent` **before** clearing `_pause` and **before** reading the exit condition. `Sleep()` unblocks the instant that signal fires, so the owning thread can begin the next cycle while the worker is still finishing the previous one — and the worker's two trailing operations then land on the new cycle's state.
`SerializationThreadWorker` already orders the same handshake correctly and documents why (`Projects/Server/Serialization/SerializationThreadWorker.cs`):
```csharp
// The owning thread may start another pause cycle the moment _stopEvent is set
// (Exit does exactly that). Clear _pause and sample the exit condition before
// signaling, or the new cycle's pause request is clobbered / its Sleep orphaned.
var exiting = Core.Closing || worker._exit;
Volatile.Write(ref worker._pause, false);
worker._stopEvent.Set();
```
This applies the same ordering to the search worker. Three lines; no behavior change on the happy path.
## The two failures
**Reuse hang.** The next cycle's `Wake`/`Push`/`Sleep` writes `_pause = true`, then the worker's stale `_pause = false` lands on top of it. The inner loop never observes `pauseRequested`, its queue is already empty, and it spins on `Thread.Yield()` forever — so the owning thread's next `Sleep()` waits on a `_stopEvent` that is never set again. A single search wakes each worker exactly once, so this only surfaces once `_threadWorkers` is reused by a later search.
**Orphaned `Exit()`.** `Exit()` sets `_exit`, `Wake()`s, then `Sleep()`s — the moment the drain's `Sleep()` returns. Reading `_exit` *after* the signal, the worker can observe that fresh `_exit`, return without ever consuming the `Wake`, and leave `Exit()`'s `Sleep()` waiting on a signal nobody will send. The `_thread.IsAlive` guard doesn't close this: the thread passes the check and returns immediately after.
## Verification
Verified with two throwaway timing tests — 25k reuse cycles and 2k drain-then-`Exit` cycles, each under a bounded wait:
| ordering | result |
|---|---|
| previous | `Failed: 2, Passed: 3` — both reproduce, cleanly at the 20s bound |
| this PR | 3 consecutive runs, 5/5, ~0.6s |
**Those tests are deliberately not included.** Their reproduction threshold is a property of one machine's scheduler — at 2k and 200 cycles the buggy build passed — so as permanent tests they'd cost ~560ms and 2000 thread creations on every suite run for a guarantee that may not hold on a CI runner. The ordering is protected the same way `SerializationThreadWorker`'s is: by the comment at the call site.
`UOContent.Tests`: **597/597**.
## Summary
Hardens the **Advanced Search** engine (`Projects/UOContent/Engines/Advanced Search/`) — the GM entity finder that fans searches across background worker threads. A code review surfaced 14 defects (A–N), including a shard-crasher reachable from a single admin typo and a path that silently disables autosave for the rest of the shard's uptime. Each behavioral fix ships with a test.
Full `UOContent.Tests` suite: **530/530 green** (21 new AdvancedSearch tests).
## Fixes
### Crash / data-loss
- **A — Shard crash on a malformed Property Test.** `AdvancedSearchThreadWorker.Execute` had no `try/catch` and the worker `Thread` is foreground, so a parse throw (`Hits>abc`, `Layer=onehanded` — `Enum.Parse` was case-sensitive, `Hits>1@` — empty sub-expression indexing) terminated the process. Now: `ParseValue`/`CompareValues` use `TryParse`/`Enum.TryParse(ignoreCase)` and return no-match instead of throwing; the per-entity filter is wrapped in `try/catch` (logs + skips); empty expressions are guarded.
- **C — Overlapping searches corrupt state + brick autosave.** `_threadWorkers`/`_threadId` were `static` but `DoSearch` is an instance method; a second search (double-click / two admins) stomped shared worker state and could leave a drain waiting forever on the shared `AutoResetEvent`, so `AutoSave.SavesEnabled` was never restored. Now: an `Interlocked` re-entrancy guard rejects concurrent searches.
- **G — Autosave restore not guaranteed.** The restore lived only in the success callback. Now it's in a `finally` (plus an outer `catch` covering the synchronous setup and a `catch` on the drain body), so autosave + the guard are always released.
### Wrong results
- **D — `@`/`|` operator precedence.** `a@b|c` evaluated as `a && (b || c)` instead of `(a && b) || c`. OR now binds looser than AND (`AdvancedSearchUtilities.EvaluateBoolean`, unit-tested).
- **E — Descending sort, partial last page rendered blank** (the index decreased in descending mode and the `break` early-out killed the loop). Now a bounded `VisibleCount`-driven loop renders the last page in both directions.
- **F — Deleted entities** were not skipped (ghost rows). Now `DoEntitySearch` skips `entity.Deleted`.
- **N — Reference-type comparisons** threw (`Comparer<T>.Default.Compare` on non-`IComparable`) and compared references to a string. Now equality is by value and ordering is guarded to `IComparable` (no throw).
### Worker perf / hardening
- **H** busy-spin → `Thread.Yield()` in the drain; **I** `GetProperties()` cached per `Type`; **J** `HandleValidInternal` moved behind the cheap map/range/region filters; **K** worker threads are `IsBackground` + `Exit()` tolerates an already-terminated worker; **L** `_filter == null` guard; **M** consistent `Volatile` access on `_pause`/`_exit`.
### Documented
- **B** — the residual worker/event-loop read race is documented on `AdvancedSearchThreadWorker`: workers read live entity state concurrently with the loop, so value-type reads may be stale-but-safe and getter exceptions are swallowed; fully eliminating it would require snapshotting entity fields on the main thread (deferred).
## Notes
- New test-only seams (`TryBeginSearch`/`EndSearch`/`IsSearchInProgress`/`VisibleCount`/`TryParseValue`/`EvaluateBoolean`) are `internal` via the existing `InternalsVisibleTo("UOContent.Tests")`.
- Dead `public ParseValue<T>` removed.
- `ConcurrentDictionary` for the reflection cache is intentional — these workers are genuinely parallel.
> [!Important]
> **Developer Note**
> This code change will **completely move gumps out of the core**
### Summary
- Adds `GetGumps()` convenience which exposes methods to Find/Close/Send multiple gumps. This helper is a performance improvement by eliminating the Dictionary<Player, List> lookup for gumps.
# New Gump API
We are pleased to release a new API that is faster, allocates nearly zero memory, and still feels very similar to the original API. The API is broken into 3 types of gumps, dynamic, static with placeholders, and static without placeholders.
### Dynamic Gumps
These gumps will inherit `DynamicGump` and are meant for gumps that have a dynamic layout. This includes specifying dynamic arguments to HtmlLocalized entries.
## Static Gumps
Static gumps are those where the function to the build the layout is called only once and cached forever. They can optionally have placeholders. These placeholders allow the developer to specify the string values later, dynamically in a `BuildStrings` method on the gump. If a gump does not have any placeholders, the string entries will also be cached forever.
## Benchmarks
To make sure we were going in the right direction and not wasting time, we took copious benchmarks. Here are the final benchmarks for a really simple gump.
Note:
* The majority of creating a gump is compressing the layout and the strings. Compressing each section takes ~6,000ns (12us total).
```cs
| Method | Mean | Error | StdDev | Median | Ratio | RatioSD | Gen0 | Allocated | Alloc Ratio |
|------------------------------- |-------------:|-------------:|-------------:|-------------:|------:|--------:|-------:|----------:|------------:|
| OldGump | 13,308.29 ns | 1,059.695 ns | 1,883.608 ns | 14,330.72 ns | 1.000 | 0.00 | 0.1526 | 2400 B | 1.00 |
| DynamicLayoutGump | 13,357.86 ns | 129.144 ns | 226.185 ns | 13,323.60 ns | 1.029 | 0.17 | - | 48 B | 0.02 |
| StaticLayoutDynamicStringsGump | 6,653.10 ns | 81.815 ns | 143.292 ns | 6,617.45 ns | 0.514 | 0.09 | - | 40 B | 0.02 |
| StaticLayoutGump | 86.33 ns | 0.760 ns | 1.350 ns | 86.07 ns | 0.007 | 0.00 | 0.0020 | 32 B | 0.01 |
```
# Non-Breaking Changes
* All gump components in the core have been moved to `Gumps/Legacy`.
* All legacy gumps will still inherit `Gump`, which now inherits `BaseGump`
# Special Thanks
Thank you to @stefanomerotta for considerable contributions/benchmarking/testing to make this effort a reality! We collectively went through over 10 iterations, but it is finally ready.
### Summary
- Fixes `[AdvancedSearch` being accessible by players 😱
- Adds `[GenCommands` to generate the same commands html page on https://muo.gg/commands.
- Fixes `[helpinfo` so all commands properly show up!
> [!WARNING]
> ### Developer Warning:
> Commands must now be registered in the `Configure` bootup phase.
> If a command is not registered early enough, it may not be available to systems like [helpinfo
> that cache their information.
> [!NOTE]
> ### Developer Note:
> Various commands related to generating content have been changed to _Developer_ and above access level.
### Screenshots
<img width="673" alt="image" src="https://github.com/modernuo/ModernUO/assets/3953314/b105b5c9-5eb4-4ace-93ff-1bfb31e7132f">
<img width="547" alt="image" src="https://github.com/modernuo/ModernUO/assets/3953314/e97487e8-47a5-4aa7-89cc-9fe3deda584d">
