fix(engagement): claim the seed guard atomically, and let a rule name a digest body

Both defects came out of Phase 13's acceptance walk, and neither was visible to
any test.

**1. The one-shot rule-group guard was not a guard.** `seedRuleGroup` and
`coreRules.seedGroup` both read their settings stamp, inserted the whole group,
and wrote the stamp AFTER the loop. Two instances booting in the same moment both
read "absent" and both insert -- the walk ended up with 52 module rules where the
module ships 26. `docker compose up --scale app=2` and a rolling restart both
start two instances on purpose, so this is ordinary rather than exotic.

`settings.db` gains `claim(key, value)`: the same `INSERT IGNORE` as
`seedDefault`, reporting its own `affectedRows`, so exactly one caller can win a
key. The atomicity is the PRIMARY KEY's -- no transaction, no lock, the same
bargain `engagementWorker`'s row claim already makes. Both seeders claim before
inserting.

The trade the code already documented is unchanged, only its order: a process
that dies mid-loop leaves the group stamped and partly seeded, and the missing
rules are an operator's visit to the "new rule" form. A duplicate rule is two
mails per event, for every rule in the group, which both functions' own comments
already call the worse outcome.

**Why no test caught it:** the stubs supplied `get` and `set` over a Map, which
cannot race, so they agreed with the bug -- Phase 4a's `foundRows` finding in a
different costume. The fake is now `claim`-shaped and decides without yielding,
exactly as the table does, and both suites gained a test that runs two seeders
with `Promise.all` and asserts one insert each. Verified by reverting the fix:
the module test then reports every rule inserted twice.

**2. A rule that names a `digest` body could not be saved.** `registries.js`
`checkSeedRule` permits `digest` in as many words -- it is the body
`teamDigestWorker` renders for a rule whose email channel an individual set to
digest mode, so it never appears in `channels` and never could -- and
MODULE_API.md 2.4 tells modules they may point `template_keys` at
`notify.digest`. `engagementRules.model.js` then rejected any key that was not
one of the rule's channels.

So every rule shipping a digest body answered an operator who opened it and
pressed Save with a 400 naming a key they had never typed: core's own Team and
news rules, and sixteen of module-uo's. The only way to save was to delete the
digest body, silently dropping digest support from that rule. The two validators
now agree; anything that is neither a channel nor `digest` is still refused, with
a test for each direction.

No MODULE_API bump: no member is added, removed or changed, and the documented
contract is what the code now honours rather than something new.

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

server/src/engagement/coreRules.js

@@ -159,8 +159,15 @@ const NEWS_RULES = [
 async function seedGroup(key, rules, note) {
   const summary = { inserted: 0, skipped: 0 }
   try {
-    const seen = await settingsDb.get(key)
-    if (seen) return { ...summary, skipped: rules.length }
+    // **Claimed BEFORE the loop, atomically**, and the stamp is the claim. A
+    // `get()` here with a `set()` after the inserts is not a guard when two
+    // instances boot together — both read "absent", both seed — and a duplicate
+    // rule is two mails per event. `claim()` is an `INSERT IGNORE` reporting its
+    // own `affectedRows`, so exactly one caller proceeds. See the note below on
+    // what a partial run costs: that trade is unchanged, only its ordering.
+    if (!(await settingsDb.claim(key, new Date().toISOString()))) {
+      return { ...summary, skipped: rules.length }
+    }
 
     for (const rule of rules) {
       try {
@@ -181,10 +188,10 @@ async function seedGroup(key, rules, note) {
         log.error('team rule seed failed', { trigger: rule.trigger_id, message: err.message })
       }
     }
-    // Stamped even on a partial run. Re-running would duplicate the rules that
-    // did insert, and a duplicate rule is two mails per event — a worse outcome
-    // than the one missing rule an operator can add from the screen.
-    await settingsDb.set(key, new Date().toISOString())
+    // Stamped even on a partial run — the claim above is the stamp. Re-running
+    // would duplicate the rules that did insert, and a duplicate rule is two
+    // mails per event, a worse outcome than the one missing rule an operator can
+    // add from the screen.
     if (summary.inserted) {
       log.info('seeded engagement rules, all disabled', { rules: summary.inserted, note })
     }

server/src/engagement/moduleSeeds.js

@@ -100,10 +100,11 @@ async function seedModuleTemplates(owner, templates, deps = {}) {
 /**
  * Seed one named rule group, once, under its own guard.
  *
- * Mirrors `coreRules.seedGroup` deliberately, including the stamp-on-partial
- * behaviour: re-running would duplicate the rules that DID insert, and a
- * duplicate rule is two mails per event — worse than the one missing rule an
- * operator can add from the Rules screen.
+ * Mirrors `coreRules.seedGroup` deliberately, including the claim-before-insert
+ * ordering and what it costs: re-running would duplicate the rules that DID
+ * insert, and a duplicate rule is two mails per event — worse than the one
+ * missing rule an operator can add from the Rules screen. The guard is taken
+ * atomically for the same reason; see the comment at the claim.
  */
 async function seedRuleGroup(owner, group, deps = {}) {
   const rules_ = deps.rulesDb || rulesDb
@@ -111,8 +112,23 @@ async function seedRuleGroup(owner, group, deps = {}) {
   const summary = { inserted: 0, skipped: 0 }
   const key = guardKey(owner, group.key)
   try {
-    const seen = await settings_.get(key)
-    if (seen) return { ...summary, skipped: group.rules.length }
+    // **Claim BEFORE inserting, not after.** The guard used to be a `get()` here
+    // and a `set()` after the loop, which is not a guard under concurrency: two
+    // processes starting in the same moment both read "absent" and both insert
+    // the whole group. That is not hypothetical — `docker compose up
+    // --scale app=2` and a rolling restart both boot two instances deliberately,
+    // and Phase 13's acceptance walk hit it with two, ending up with 52 module
+    // rules where the module ships 26. `claim()` is an `INSERT IGNORE` reporting
+    // its own `affectedRows`, so exactly one caller wins.
+    //
+    // The cost is the one this function already accepted below: a process that
+    // dies mid-loop leaves the group stamped and partly seeded, and the missing
+    // rules are an operator's visit to the "new rule" form. Duplicates are the
+    // worse failure — two mails per event, for every rule in the group — which
+    // is why the order is this way round rather than the other.
+    if (!(await settings_.claim(key, new Date().toISOString()))) {
+      return { ...summary, skipped: group.rules.length }
+    }
 
     for (const rule of group.rules) {
       try {
@@ -127,7 +143,6 @@ async function seedRuleGroup(owner, group, deps = {}) {
         })
       }
     }
-    await settings_.set(key, new Date().toISOString())
     if (summary.inserted) {
       log.info('seeded module engagement rules, all disabled', {
         owner,

server/src/model/engagement/engagementRules.model.js

@@ -40,6 +40,12 @@ const MAX_DELAY_SECONDS = 86_400
 // what makes rules-as-data safe (§7.1 Q3).
 const MAX_SENDS_PER_HOUR = 10_000
 
+// The one key in `template_keys` that is not a delivery channel. `teamDigestWorker`
+// renders it for a rule whose email channel an individual has set to digest mode,
+// so it belongs to a MODE rather than to the rule's channel list and can never
+// appear there.
+const DIGEST_SLOT = 'digest'
+
 const isPlainObject = (v) => v !== null && typeof v === 'object' && !Array.isArray(v)
 
 /**
@@ -91,7 +97,17 @@ async function validate(input, { existing = null } = {}) {
     errors.push('templateKeys must be an object of { channel: templateKey }')
   } else {
     for (const [channel, key] of Object.entries(raw.templateKeys || {})) {
-      if (!wanted.includes(channel)) {
+      // **`digest` is a template SLOT, not a channel**, and it is legal here for
+      // exactly the reason `registries.js` `checkSeedRule` says it is: it names
+      // the body `teamDigestWorker` renders for a rule whose email channel an
+      // individual has set to digest mode, so it never appears in `channels` and
+      // never could. Rejecting it made every rule that ships one unsaveable from
+      // the Rules screen — core's own team and news rules included, and sixteen
+      // of module-uo's — with a 400 naming a key the operator never typed, whose
+      // only remedy was deleting the digest body and silently dropping digest
+      // support. Found by Phase 13's acceptance walk; the two validators now
+      // agree about what `digest` is.
+      if (channel !== DIGEST_SLOT && !wanted.includes(channel)) {
         errors.push(`templateKeys names "${channel}", which is not one of this rule's channels`)
         continue
       }

server/src/model/settings/settings.db.js

@@ -36,6 +36,24 @@ async function seedDefault(key, value) {
   await query('INSERT IGNORE INTO settings (`key`, value) VALUES (?, ?)', [key, value])
 }
 
+/**
+ * Take a one-shot guard, atomically. `true` means THIS caller wrote the row.
+ *
+ * The same `INSERT IGNORE` as `seedDefault`, and the difference is the whole
+ * point: this one reports whether it won. A guard read with `get()` and written
+ * later with `set()` is not a guard at all under concurrency — two processes
+ * both read "absent" and both proceed — and this is used where proceeding twice
+ * means seeding a rule group twice, i.e. two mails per event.
+ *
+ * The atomicity is the PRIMARY KEY's: exactly one INSERT can create a given
+ * `key`, so exactly one caller sees `affectedRows === 1`. No transaction and no
+ * lock, the same bargain `engagementWorker`'s claim makes.
+ */
+async function claim(key, value) {
+  const res = await query('INSERT IGNORE INTO settings (`key`, value) VALUES (?, ?)', [key, value])
+  return Number(res && res.affectedRows) === 1
+}
+
 // Delete a settings row. "Reset to defaults" for the theming/nav keys is the
 // *absence* of a row, not a stored copy of the defaults — see
 // docs/website/THEMING_AND_NAV.md §2. Deleting a key that was never set is a
@@ -44,4 +62,4 @@ async function remove(key) {
   await query('DELETE FROM settings WHERE `key` = ?', [key])
 }
 
-module.exports = { getAll, get, getRow, set, seedDefault, remove }
+module.exports = { getAll, get, getRow, set, seedDefault, claim, remove }