submit ops failed due to version mismatched (code 5002)

[zbryikt]

I've noticed that when using sharedb-postgres, sometimes server produces following errors (code 5002) if ops are submitted in a high frequency ( 3 ~ 4 ops a second from different sources ) :

Uncaught Error: Op submit failed. Versions mismatched during op transform:
    at Connection.handleMessage (bundle.js:16893)
    at WebSocket.socket.onmessage (bundle.js:16846)

This error is gone if switching back to memory DB.

tracing the source code I found that when doing transformOp, getOps of sharedb-postgres returns incorrect range - it always returns ranges one version earlier, i.e., getOps("col", "doc", 10, 11) will give us the op with version 9.

I also tried with the patch by @billwashere in the pull request and it still had this issue.

another clue is that the version field in the op table in database is always plus 1 to the field of v in op.

so I went here and simply shift range by 1 like this:

SELECT version, operation FROM ops WHERE collection = $1 AND doc_id = $2 AND version > $3 AND version <= $4'

then it seems to have this issue fixed. Not quite sure but I guess there might be some inconsistent in versioning between sharedb and sharedb-postgres and the corresponding database field?

I've put the test code here in this repo so you can quickly verify it. It's adopted from rich-text example of sharedb itself, with a simple instruction in README.

[zopsicle]

Thanks! This bug report helped us tremendously fixing the same issue. 😄