## Summary
Many tests proved a specific change when it landed but don't need to run on every CI build forever. This removes **202 test methods and 9 files: 313 test cases, 2,247 → 1,934**. UOContent.Tests drops from about 14s to about 5s locally.
Removed:
- **Tautologies.** Asserting a constant, a constructor readback or a trivial getter, or testing BCL behavior.
- **Exact duplicates** of a surviving test on the same code path.
- **Obsolete regressions** whose failure can no longer be expressed.
- **Game-content values** shard owners customize (era drop rates, loot and skill tables, single-creature tuning). Each customization would otherwise force a test fix.
## How it was decided
1. Every test was classified, with a reason, by a per-shard sweep over all 287 test files.
2. Each proposed deletion was then checked by a second, adversarial reviewer against the surviving tests and the production code.
3. Anything that was the only coverage of a separate code path was **kept**, even where it looked like a duplicate. Examples: the unsigned `SpanReader`/`SpanWriter` methods, the at-location versus bounds enumerators, null-map enumerables, the security filters, archive-journal state transitions. About 130 proposed deletions were rejected this way.
Whole files removed:
- `FishingTests`, apart from 3 tests: the map-edge scan, serpent cleanup, and SOS chest consumption. The SOS test now stops at the first chest instead of measuring a catch rate. The multi-million-cast rate simulations are gone.
- `LordOaksTests` and `MeerEnragedTests` (single-creature tests).
- `AutoArchiveHelperTests`, `JsonDiscoverableTypeAttributeTests`, `DecimalSerializationTests` and `EnumConversionTests` (tautologies, or tests of their own local helpers).
- `NetworkStatsTests`.
- `MigratedDataLoadTests` (a subset of `AllSpawnFilesLoadTests`).
- `SpawnerFixtureCapture`: a capture tool, not a test, and its `version == 12` assert can no longer pass. The `.bin` fixtures it produced stay.
## Tests
- Server.Tests: 811 pass.
- UOContent.Tests: 1,123 pass.
A follow-up PR, stacked on this one, fixes how the remaining tests behave: shared state, CI-vacuous tests, harness duplication.
## 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.
Kamron Batman