Function create_record 

pub(crate) async fn create_record(
    __arg0: State<AppState>,
    headers: HeaderMap,
    __arg2: XrpcBody,
) -> Response

POST /xrpc/com.atproto.repo.createRecord

404 RepoNotFound when the repository is unknown, 400 when the collection is not one this deployment defines, the record’s $type disagrees with it, or the requested rkey is one the collection’s key strategy will not admit ; 400 InvalidSwap when the swapCommit named is not the commit the repository is at. The key in the returned uri is the one the lexicon decided, which is not always the one that was asked for. The response carries the uri and the cid the lexicon requires; see CreateRecordResponse.

A key already in use is refused

This is createRecord, and in atproto createRecord creates: a key already in use is a 400 and putRecord is the upsert. This server could not tell the two apart while there was no CID to condition a refusal on. There is one now, and compare-and-swap has landed. The write carries Precondition::Absent, which is the same check every other precondition goes through and is evaluated in the same place.

The refusal is InvalidRequest rather than the InvalidSwap the lexicon declares, and for a reason about names: InvalidSwap tells a client to re-read and retry, and this retry can never succeed. A caller that meant to replace the record wants putRecord.