docs: measure WHY login takes 18s — it is not 468 call sites

The SOTU scopes the child-process DB bridge as 468 call sites and multi-week.
Measurement says otherwise.

  bare node -e '0'  on Kit 0 (4 vCPU, load 20): 1.70-1.99s
  bare node -e '0'  on Veron 1 (24 cores, idle): 0.01s

170x, and node startup is essentially the whole per-query cost — adding pg
connect and a real query to a bare node boot adds only ~0.2-1.2s on top of
~1.8s of interpreter start.

Login makes 9 such forks. 9 x 1.8s = 16s. Observed: 17-25s.

Two independent multipliers compound: the adapter forks per query (9x) and the
box is saturated so each fork costs 1.8s instead of 0.01s (170x).

The consequence: you do not need to touch 468 call sites. querySyncViaChild is
ONE function and its interface does not change — replace the per-query
execFileSync with a persistent worker holding a pg.Pool. All 468 call sites get
faster without being edited, including the mail-lookup that caused today's
outage.

Also measured: 54 containers, 301% CPU of 400%, load 20. dev/demo is 55% of
that — real relief, honestly not a fix. 42 non-dev containers on 4 cores is the
actual condition.

Not implemented here on purpose: sync-over-async with Atomics in the most
critical file in the ecosystem, at the end of a long session, on a box that
already had an outage today, deserves a fresh session and a load test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/incidents/2026-08-12-login-latency-analysis.md

@@ -0,0 +1,103 @@
+# Why login takes 18 seconds — measured, 2026-08-12
+
+The SOTU calls the child-process DB bridge *"the single largest stability
+liability under #8"* and scopes the fix as **468 call sites, multi-week**.
+
+That scoping is wrong, and the measurements below say so. **You do not need to
+touch 468 call sites.** You need to stop spawning a node process per query,
+which is one function.
+
+## The measurements
+
+**One login makes 9 synchronous queries**, each spawning `node -e` via
+`execFileSync` (`postgres-adapter.ts:114`):
+
+| # | query | source |
+|---|---|---|
+| 1 | `findUserByEmail` | `statements.ts:11` |
+| 2 | `mfa_secrets` lookup | `auth.ts` inline |
+| 3 | `findDevice` | `statements.ts:24` |
+| 4 | `touchDevice` *or* `countDevices` + `addDevice` | `statements.ts:25-35` |
+| 5 | `updateUserSeen` | `statements.ts:20` |
+| 6 | `generateTokens` → scope rows | `auth.ts:220` |
+| 7 | `generateTokens` → product rows | `auth.ts:229` |
+| 8 | `getDeviceList` | `auth.ts:384` |
+| 9 | `logAuditEvent` INSERT | `identity-service.ts:43` |
+
+**What each fork actually costs, measured inside the running container:**
+
+| | Kit 0 (4 vCPU, load ~20) | Veron 1 (24 cores, idle) |
+|---|---|---|
+| bare `node -e "0"` | **1.70 – 1.99 s** | **0.01 s** |
+| node + pg connect + `SELECT 1` | 1.89 – 3.19 s | — |
+
+**Node startup is 170× slower on Kit 0, and it is essentially the entire cost.**
+Adding a Postgres connect and a real query to a bare node start adds only
+~0.2–1.2 s on top of ~1.8 s of interpreter boot.
+
+    9 forks x ~1.8 s = ~16 s.   Observed login: 17–25 s.
+
+## The conclusion that matters
+
+There are **two independent multipliers**, and they compound:
+
+1. **The adapter forks a process per query** — 9x on the login path.
+2. **The box is saturated**, so each fork costs 1.8 s instead of 0.01 s — 170x.
+
+Either one alone is survivable. Together they turn a sub-second operation into
+eighteen seconds, and on 2026-08-12 they turned a retry loop into an
+ecosystem-wide auth outage.
+
+**Connection pooling barely helps.** The connection is not the cost; the
+interpreter boot is. PgBouncer, `pg.Pool` on the sync path, or a warmer
+Postgres would all leave ~1.8 s per query untouched.
+
+## Box census (measured same day)
+
+    54 containers, 301% CPU of 400% available, load average 20 on 4 vCPU
+
+| | containers | CPU |
+|---|---|---|
+| dev / demo / test | 12 | **55%** |
+| everything else | 42 | 246% |
+
+Top consumers: `windymail-migrate-stalwart` 31%, `scenemachine-db-prod` 30%,
+`account-server-account-postgres` 27%, `windy-synapse` 25%, `windy-directory`
+20%.
+
+**Be honest about this number:** stopping every dev/demo container reclaims 55%
+of 301% — it takes the box from 75% to 61% steady CPU. That is real relief and
+it is *not* a fix. Forty-two non-dev containers on four cores is the actual
+condition.
+
+## What to do, in value order
+
+1. **Stop forking a node process per query.** `querySyncViaChild`
+   (`postgres-adapter.ts:114`) is one function, and its interface —
+   `querySync(sql, params)` — does not change. Replace the per-query
+   `execFileSync` with a persistent worker holding a `pg.Pool`, using
+   `worker_threads` + `SharedArrayBuffer` + `Atomics.wait` for the synchronous
+   block. **Every one of the 468 call sites gets faster without being edited**,
+   including the mail-lookup route that caused today's outage.
+   Expected: login 18 s → well under 1 s, on this box, unchanged.
+2. **Then** migrate route families to the async path at leisure. The sync path
+   is still labelled legacy and should still die — but as cleanup, not as an
+   emergency.
+3. **Separately, unload Kit 0.** Move dev/demo off the box that runs identity,
+   the CA, mail, Matrix and the broker. This is worth doing on its own merits
+   regardless of the adapter.
+
+**Do not do #2 first.** Migrating handlers one family at a time is weeks of
+edits to the most critical code in the ecosystem, and it leaves every
+un-migrated call site paying 1.8 s a query the whole time.
+
+## Why this was not implemented in this session
+
+Replacing the sync bridge is a subtle change (`Atomics.wait`, structured-clone
+limits, worker lifecycle, failure fallback) in the single most critical file in
+the ecosystem, made at the end of a long session, on a box that had already had
+one outage that day. It deserves a fresh session, a real load test, and a
+rehearsed rollback — not a tired commit.
+
+The measurement is the deliverable. It converts a "multi-week, 468 call sites"
+job into a one-function change, and that is worth more than a rushed attempt.