secantus-mdb 0.5.3-beta.187 refuses a duplicate document _id and keeps a document a $merge could not replace

Summary: secantus-mdb 0.5.3-beta.187 fixes two bugs that left the wrong documents in a collection: a duplicate _id written as a different number type, and a $merge replacement that deleted the document it could not replace.

Upgrade if you run secantusd-rs. This release fixes two bugs that left the wrong documents in a collection.

• Two documents could share one _id. After {_id: {a: 1}}, the server accepted {_id: {a: 1.0}} and {_id: {a: NumberLong(1)}}. mongod compares the values and refuses both as duplicates. This affected an _id that is a document; a number or a string was already compared by value.
• A $merge could lose a document. With whenMatched: "replace", the server deleted the matched document and then inserted the new one. If a unique index refused the new one, neither was left. mongod keeps the original, and so does this server now. A replaced document also keeps its place in the collection, where it used to move to the end.

The fix for the first does not change anything already on disk. A store written by an earlier version opens and reads as before. The first time this version opens a store it records the _id of every document already there, so a duplicate of an older document is refused too. That happens once and reads only the documents whose _id is a document. If an earlier version already stored two documents with one _id, both stay.

We found both by sending 905 insert, delete, count and distinct commands to the server and to mongod 8.2.11, and comparing each reply and the documents left behind. The previous release differed on 162. This one differs on 32.

The same comparison found other writes that touched the wrong documents:

• A delete statement with no limit removed every match. mongod refuses the command, and nothing is removed.
• delete ran the statements ahead of one it then refused. It now checks every statement first.
• A limit: 1 delete ignored its hint. With {$natural: -1}, mongod removes the newest match. This server removed whichever one it met first.
• find with $in over an index returned documents in the order of the list. mongod returns them in the index's order. A limit: 1 delete with the same query removed a different document.
• insert stored documents mongod refuses: a $-prefixed field inside an _id below its top level, and a document nested deeper than 180 levels.

Some replies changed too. A duplicate-key error printed a date, a sub-document, binary data or a timestamp as Rust debug text; it prints them as mongod does. insert, delete, count and distinct refuse fields they do not take. Forty commands that take no writeConcern refuse one.

What this release does not do. $where is not run. The message for a hint that names no index is worded differently from mongod's. When a document repeats the keys of two unique indexes, the index named in the error can differ.

We ran the pymongo, Go and Node driver suites against this code. Each gave the same result as the release before.

This is a server for tests. It is single-node and it is a beta.

cargo install secantus-mdb --version 0.5.3-beta.187 builds it from crates.io. Binaries for Linux x86_64, macOS arm64 and Windows x86_64 are on the release.

Rust MongoDB server · secantus-mdb on crates.io · MongoDB binaries