M3's row and §6 updated: the operator corpus is 125 cases across seven files,
all green, and the pipeline work also gave `aggregate` the `$replaceWith`
stage it never had -- the compiler is shared between the two.
Full matrix at this commit: 249/249 unit tests in ReleaseFast and ReleaseSafe,
83/83 fuzz, operators 125/0, positional 51/0, aggregation 70/0, pinned crud
228/63/196, e2e and crash-fuzz green.
`u` (and `findAndModify`'s `update`) may be an array of aggregation stages
instead of a document of operators. The stages themselves already existed --
M2 built `compile_rewrite` and `apply_rewrite` for `aggregate` -- so this is
mostly about which of them are allowed here and what happens around them.
Six are: `$addFields`/`$set`, `$project`, `$unset`, `$replaceRoot`/
`$replaceWith`. Each rewrites one document into one document, which is the
property that makes them usable: `$match` could drop it, `$group` and
`$unwind` could change how many there are, and `$sort` means nothing to one.
mongod refuses those four by name with InvalidOptions (72) and a name that is
no stage at all with 40324, and the difference is worth keeping -- "not here"
and "not at all" are different things to be told.
`$replaceWith` did not exist here at all and now does, in `compile_rewrite`,
so `aggregate` gets it too: it is `$replaceRoot` with the expression in place
of the `{newRoot: ...}` wrapper, one stage under two spellings.
**The `_id` survives every stage.** `$replaceRoot: {newRoot: "$t"}` drops it
and `$project: {_id: 0}` asks to, and it comes back either way, because a
pipeline update rewrites a document rather than replacing one document with
another. A stage that sets it to a *different* value is ImmutableField (66);
restating the same one is fine. Measured on all six stages, and the mutation
that reddens the test is deleting the restore -- the document becomes
unfindable by the id it is stored under.
`EvalCtx.coll` becomes optional, which is what lets the upsert path run a
pipeline over a document held in memory before the collection it will be
inserted into exists. Only an `.offsets` stream reads the slab; a `.docs` one
never touched the field.
`arrayFilters` beside a pipeline is FailedToParse (9), not ignored: an
identifier a pipeline cannot spell would make the update silently different
from the one the client wrote.
pipeline.json 0/23 -> 23/23, operator corpus 125/125.
Pinned crud scorecard **218 -> 228 pass, 73 -> 63 fail**: the five pipeline
files, the four `-rawdata` files that use pipelines, and
`findOneAndUpdate-comment`, which is two more pipeline cases.
249/249 unit tests in ReleaseFast and ReleaseSafe, 83/83 fuzz.
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.
