Aleksey Shakhmatov
21489723a9
`nModified` counted every write, so an update that altered nothing was reported
as a modification. MongoDB counts a document as modified only if applying the
update changed it, and writes no oplog entry when it did not: `$set: {x: 11}`
on a document already holding `x: 11` is matched and not modified. The spec
suite says it plainly -- `bulkWrite` with four updateOne operations expects
matchedCount 2 and modifiedCount 1.
Decided in the engine rather than the command, because that is where the
document is already serialized: the comparison is against the bytes that would
actually be stored, and it lands before the log append, so a no-op costs no log
record, no fsync, no slab bytes and no garbage. `Engine.replace` returns
`Written.modified` or `.unchanged` and `cmd_update` counts the first.
That exposed a second difference. A replacement keeps `_id` at the front, so
replacing a document with itself was a byte-level change whenever `_id` was not
stored first -- and it usually was not: the Node driver fills a missing `_id` by
assigning the property, which in JavaScript appends it, so `insertOne({name,
age})` reaches the server as `{name, age, _id}` and we stored it that way.
MongoDB moves `_id` to the front whatever order it arrives in. Now so does
`serialize_with_id`, for every document rather than only the ones whose `_id` it
generates. Visible to clients as `_id` coming back first, as it does from
MongoDB.
spec scorecard 161 pass / 131 fail -> 163 pass / 129 fail
bulkWrite.json 8 pass / 2 fail -> 10 pass / 0 fail
e2e.js 45 checks -> 49
No spec file regressed. Mutation: delete the byte comparison in `upsert`'s
`.replace` arm -- red on the log growing, on the garbage counters moving, and on
`replace` claiming `.modified`.
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