PLAN.md

@@ -467,7 +467,7 @@ answers, and the trade is only acceptable because the lie is removed first.
 | M1 | **Cursors + wire polish** | getMore / killCursors / batchSize; server-side cursor state with idle timeout; sessions plumbing (lsid accepted) as drivers send it; hello advertisement updates; **`moreToCome` on requests** (see the bug below); command-monitoring assertions in the spec runner | crud spec suite green; e2e green |
 | M2 | **The `aggregate` command surface** | `$out` and `$merge` (7 of the 13 failures), and refusing every pipeline construct the engine does not implement instead of answering `0` (amendment A6). The other 6 failures are blocked on M2.5, M4 and M8 — see `docs/M2_DESIGN_REVIEW.md` §7 | `aggregate-*.json`: 0 fail among the 7 reachable cases |
 | M2.5 | **The aggregation engine** | expression evaluator, per-stage document iterator, the accumulators, `$unwind`; `$lookup`/`$facet` explicitly out of the first cut (amendment A6) | a purpose-built stage corpus, every expectation measured against mongod |
-| M3 | **Update operators + index types** | $setOnInsert, $addToSet, $mul, $min/$max, $pop, $pullAll, $currentDate, pipeline updates; partial + hashed indexes | remaining crud coverage; e2e3/e2e4 green |
+| M3 | **Update operators + index types** | `distinct` (**done**); then $setOnInsert, $addToSet, $mul, $min/$max, $pop, $pullAll, $currentDate, `arrayFilters`, pipeline updates; partial + hashed indexes | remaining crud coverage; e2e3/e2e4 green |
 | M4 | **Sessions + transactions** | logical sessions, snapshot isolation on the mmap engine, write concern at commit | sessions + transactions spec suites green |
 | M5 | **Change streams** | change feed + resume tokens (likely log-seq based), getMore integration | change-streams spec suite green |
 | M6 | **Admin/ops commands** | dbStats, collStats, serverStatus, ping, buildInfo, listDatabases filters, dropDatabase durability (log it) | mongosh UX smoke; e2e green |
@@ -585,6 +585,11 @@ gaps (`bad update`, `update must be a document` — M3), unimplemented commands
 (`distinct`, `$merge`, `$out` — M2/M3), and result-shape mismatches in
 `bulkWrite`/`insertMany`. That list, not the total, is the milestone backlog.
 
+All three named commands have since landed: `$out`/`$merge` in M2, `distinct`
+as M3's first commit. The backlog the same grouping gives today is
+`arrayFilters` (14 cases), the `findOneAndUpdate`/`findOneAndReplace` shapes
+(~10), pipeline-form updates (~10), and `create-null-ids` (6).
+
 ---
 
 ## 4. Ground rules (inherited and new)
@@ -1040,6 +1045,33 @@ has to be its own commit with its own re-recorded scorecard.
   `$out`/`$merge` durability semantics, whether the expression evaluator is
   shared with M3's pipeline updates, and whether `allowDiskUse` has to stop
   being a lie.
+- **M3 update operators** — open. `distinct` landed first because it was a
+  whole missing command with no dependencies, and measuring it turned up three
+  things worth keeping, none of which are `distinct`'s to fix:
+  - **An unknown query operator matches nothing instead of erroring.**
+    `{x: {$bogus: 1}}` answers `ok: 1` with an empty result on `find`, `count`
+    and `aggregate` alike, where mongod answers `BadValue` (2) "unknown
+    operator: $bogus" on all three. Measured on both servers side by side.
+    This is the same class as M2's six silent wrong answers — a typo'd
+    operator reads as "no matches" — and it lives in the shared query path, so
+    it is one fix for every command rather than one per command. It predates
+    `distinct` and is not in its commit for that reason.
+  - **A non-string collection name is `BadValue` where mongod says
+    `InvalidNamespace` (73).** Dispatch refuses it while resolving the lock
+    target, before any handler runs, so this is one answer for the whole
+    command table. Worth correcting as its own commit, with the codes measured
+    per command — three `db.aggregate()` cases in the corpus fail on the same
+    message for an unrelated reason (they name no collection at all, M4).
+  - **`distinct` is unbounded in memory**, holding every value at the key
+    before deduping. Joins `$group` and `$sort` in that list; mongod caps the
+    reply at 16 MB, which is a different and cheaper answer than spilling.
+
+  Left deliberately: `distinct-comment.json`'s "pre 4.4, server error" case
+  wants a *document*-valued `comment` refused, because this server reports
+  4.4.0 and mongod only accepted one from 4.4.14. `estimatedDocumentCount`
+  already carries the identical standing failure, and emulating a bug fixed in
+  4.4.14 for one command would make the two disagree. `distinct-collation`
+  needs M8 and now fails with the honest "expected 1 elements, got 2".
 - **M4 transactions**: snapshot isolation over mmap (COW vs undo), read
   concern snapshot, conflict → TransientTransactionError semantics,
   retryable-writes interplay.