An update document may be an *array* of aggregation stages instead of a
document of operators. The pinned crud corpus has five cases, one per command,
and not one of them is a refusal -- so which stages are allowed, what happens
to `_id`, and what an upsert does are all unmeasured there.
23 cases into the existing operator corpus rather than an eighth directory:
this is another shape of update document, and it shares the recorder, the
masking and the README.
Recorded red, 0/23. What it settled:
- **`_id` always survives**, through `$replaceRoot: {newRoot: "$t"}` and
through `$project: {_id: 0}` alike. A stage that sets it to a *different*
value is ImmutableField (66); restating the same one is fine.
- `$match`, `$group`, `$sort` and `$unwind` are real stages refused
specifically here: InvalidOptions (72), "$X is not allowed to be used
within an update". A name that is no stage at all is 40324, and two
stages packed into one array element is 40323.
- `arrayFilters` beside a pipeline is FailedToParse (9).
- an upsert runs the pipeline over the document the filter implies.
Three shapes were authored and removed: `{b: 1, $set: {...}}`, an empty
pipeline, and a pipeline holding a non-document. The driver rejects all three
before they reach a server, so there is nothing to record.
PLAN §3 lists eight operators for M3 -- `$setOnInsert`, `$addToSet`, `$mul`,
`$min`, `$max`, `$pop`, `$pullAll`, `$currentDate` -- and the pinned crud
corpus says almost nothing about any of them. A probe running the identical
update against mongod 8.3.7 and this server found all eight answering
`bad update`, code 2, one message for every question.
It also found the thing this directory exists for: `$push`'s `$slice`,
`$position` and `$sort` are **silently ignored**. `{$each: [3, 4], $slice: -3}`
appends both values, slices nothing, and answers ok: 1 with modifiedCount: 1.
A missing operator is an error the client can see; a modifier that is parsed,
accepted and then dropped is the same class of wrong answer the positional
operators were.
103 cases in six files, 18 pass / 84 fail -- red by construction, like
`tests/spec/aggregate/expressions.json` at 1/26 and the positional corpus at
15/36. Inputs authored in `sources/`, every expectation measured.
Two things here cannot be recorded as values, and both become a `$$type`
assertion rather than being left out: a `$currentDate` field is whatever the
clock said (named per case in `volatile`, so a real stored date can still be
pinned one day), and an upsert that inserts gets a generated ObjectId (that
one automatic -- no source authors an ObjectId). Everything else is compared
exactly. Files are canonical extended JSON, which the runner already parses
that way: `$mul` overflowing an int32 produces an int64, and writing
`4000000000` as a bare number would not have said so.
What recording it settled, none of it guessable:
- `$mul` of a missing field writes **0**, not the operand; of a non-numeric
field, or by one, TypeMismatch (14).
- `$min`/`$max` are not numeric operators. They compare in BSON canonical
order, so `$min: {s: 5}` on `s: "b"` writes 5, and a missing field is
always written.
- two operators writing one field is **ConflictingUpdateOperators (40)** --
`$min`+`$max`, `$set`+`$inc`, `$setOnInsert`+`$set`. A whole error class
this server does not have.
- `$addToSet` compares documents whole, **field order included**:
`{a:1,b:2}` and `{b:2,a:1}` are two values. But `2` and `2.0` are one.
- `$push` modifiers are only modifiers when `$each` is there: `{$slice: 1}`
alone is a value to push. With it, the order is position, then sort the
whole array, then slice.
- `$currentDate` with `false` still writes a date.
- `$setOnInsert` may write `_id` on an insert, where `$set` may not.
- an unknown modifier is FailedToParse (9), not BadValue.
One case was authored and then removed: `{b: 1, $set: {c: 1}}` never reaches a
server -- the driver rejects it -- so there was no answer to record and the
case would have asserted nothing.
