76afa75efe
Nine of them -- `$sum`, `$avg`, `$min`, `$max`, `$first`, `$last`, `$push`,
`$addToSet`, `$count` -- and the corpus goes 9 pass / 10 fail to 18 pass /
1 fail. The one left is the compound `_id`, which needs the expression
evaluator.
Landed before that evaluator, against the tier order the design review set
out, and the corpus is why: every one of these takes a single value per
document, a path or a constant, so nine of its ten failures turned out to be
reachable without one. `classify_expr` already produced exactly that value.
What the recording caught, which is the argument for measuring expectations
rather than writing them:
- `$avg` over a group with no numeric value is **null**, not `0`. A divisor
that counted documents rather than numbers would pass every test anybody
would think to write by hand, and be wrong on the one group that matters.
- `$min`/`$max` compare across types in canonical BSON order, so the maximum
of `30`, `7` and `"not a number"` is the string.
- `$push` skips an absent field but would push an explicit null, so "resolved
to nothing" and "resolved to null" cannot be the same value internally --
which is why the accumulators take `?bson.Value` and not `.null`.
- `$first`/`$last` follow input order, including when the value is absent:
`$last` of a missing field is null, not the last present one.
`AccState` is one struct rather than a union: the fields are small and every
site already switches on the kind, so a union would add a tag test where a
switch was going to be anyway. Its arrays are the gpa's, the values inside them
the reply arena's -- they outlive the group and travel with the documents.
`numeric_value` is the int32-or-double narrowing MongoDB reports, shared now
between the accumulators and `cmd_aggregate`'s count fast path. It was written
twice before; a divergence between them would make `countDocuments` disagree
with the pipeline it is a shortcut for.
`$avg` and `$push` came out of the Tier 0 refusal test, replaced by
`$stdDevPop` and `$mergeObjects`. The refusal is a property of what is missing
rather than of a list, and the test should read that way.
191/191 unit tests in ReleaseFast and ReleaseSafe, 83/83 fuzz, e2e 49, e2e3 16,
e2e4 17, e2e6 72, e2e7 86, crud corpus unchanged at 201/90/196.
73 lines
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73 lines
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# The aggregation corpus
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`mongodb/specifications` has no aggregation suite. The thirteen
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`aggregate-*.json` files this project runs come from `crud` and test the
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aggregate *command* — cursors, read concern, the write stages, collation,
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`let`. They touch stages barely and expressions not at all: `$lookup`,
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`$unwind`, `$facet`, `$addFields` and `$replaceRoot` appear nowhere in the
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pinned corpus. That is PLAN amendment A6, and this directory is its
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consequence: M2.5 has to bring its own gate.
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## The one rule
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**Inputs are authored here; expectations are measured against a real mongod.**
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A corpus we write is a corpus that can encode our own bugs as expectations, and
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it would then agree with us forever. So `sources/*.json` holds documents and
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pipelines and nothing else, and `record.js` asks mongod 8.3.7 what each pipeline
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answers. It is the same discipline that corrected three assumptions in M1's
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session work and every error code in M2 — the alternative, in both cases, would
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have shipped.
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## Running it
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```sh
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node tests/spec/run.js --suite-dir tests/spec/aggregate
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```
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The same runner as the crud corpus, pointed elsewhere. Sharing it is the point:
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the entity model, the matchers, the skip accounting and `expectEvents` come for
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free, and a second runner would drift from the first exactly where it mattered.
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`--scorecard` is refused with `--suite-dir`: `tests/spec/scorecard.txt` is the
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crud corpus's record and the milestones are compared against it.
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## Re-recording
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```sh
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mongod --port 27099 --dbpath /tmp/mongo-corpus &
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node tests/spec/aggregate/record.js --mongod-port 27099
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```
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Writes `<name>.json` for every `sources/<name>.json`. The generated files are
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committed: they *are* the corpus, and regenerating them is how a disagreement
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with mongod gets re-measured rather than argued about.
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Two things to know when adding cases:
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- **End a `$group` pipeline with a `$sort`.** Group output order is unspecified,
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and a case that depended on it would fail for the wrong reason on either
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server.
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- **Errors record the code, not the message.** Message text is mongod's to
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change between releases; a corpus that pinned it would break for the wrong
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reason.
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Leave out any case whose answer depends on a server newer than the 4.4 this
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server reports — recording it from mongod 8.x and judging it against a 4.4
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answer measures the version gap, not the engine.
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## Where it stands
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Recorded against mongod 8.3.7. At the M2 tip it read 9 pass / 10 fail; with the
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accumulators in:
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```
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group-accumulators.json 18 pass 1 fail 0 skip
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```
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The one that remains is the compound `_id`, which needs the expression
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evaluator and is the next tier. The corpus found its first real disagreement on
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the way there: `$avg` over a group with no numeric value is `null`, not `0`,
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and a divisor that counted documents rather than numbers would have passed
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every test anybody would think to write by hand.
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