`upsert`: the response from FerretDB is not the same as from proxy

[rumyantseva]

FerretDB version (commit) to reproduce the problem: 0c96f230f5359602d99ac440b6b66a9c91ff539e OS: MacOS FerretDB is run via task run

How to reproduce

Perform findAndModify with upsert, for example:

db.hello.findAndModify({ query: { _id: 12345}, update: {"$set": {"v": 43}}, upsert: true})

Expected result

The responses from proxy and FerretDB are equal

Actual result

The responses are different:

2022-09-01T08:31:08.727+0200	DEBUG	// 127.0.0.1:62708 -> 127.0.0.1:27017 	clientconn/conn.go:438	Response message:
{
  "Checksum": 0,
  "FlagBits": 0,
  "Sections": [
    {
      "Document": {
        "$k": [
          "lastErrorObject",
          "value",
          "ok"
        ],
        "lastErrorObject": {
          "$k": [
            "n",
            "updatedExisting",
            "upserted"
          ],
          "n": 1,
          "updatedExisting": false,
          "upserted": 12345
        },
        "value": {
          "$k": [
            "_id",
            "v"
          ],
          "_id": 12345,
          "v": 43
        },
        "ok": {
          "$f": 1
        }
      },
      "Kind": 0
    }
  ]
}



2022-09-01T08:31:08.730+0200	DEBUG	// 127.0.0.1:62708 -> 127.0.0.1:27017 	clientconn/conn.go:437	Proxy response header: length:   106, id: 78919, response_to:   45, opcode: OP_MSG
2022-09-01T08:31:08.731+0200	DEBUG	// 127.0.0.1:62708 -> 127.0.0.1:27017 	clientconn/conn.go:438	Proxy response message:
{
  "Checksum": 0,
  "FlagBits": 0,
  "Sections": [
    {
      "Document": {
        "$k": [
          "lastErrorObject",
          "value",
          "ok"
        ],
        "lastErrorObject": {
          "$k": [
            "n",
            "updatedExisting",
            "upserted"
          ],
          "n": 1,
          "updatedExisting": false,
          "upserted": 12345
        },
        "value": null,
        "ok": {
          "$f": 1
        }
      },
      "Kind": 0
    }
  ]
}



2022-09-01T08:31:08.732+0200	DEBUG	// 127.0.0.1:62708 -> 127.0.0.1:27017 	clientconn/conn.go:223	Header diff:
--- res header
+++ proxy header
@@ -1 +1 @@
-length:   127, id:    7, response_to:   45, opcode: OP_MSG
+length:   106, id: 78919, response_to:   45, opcode: OP_MSG

Body diff:
--- res body
+++ proxy body
@@ -21,10 +21,3 @@
         },
-        "value": {
-          "$k": [
-            "_id",
-            "v"
-          ],
-          "_id": 12345,
-          "v": 43
-        },
+        "value": null,
         "ok": {

TODO:

• Find the current issue number mentioned in the compat tests for findAndModify, remove "skip", fix the problem and make tests pass

[rumyantseva]

TODO: check that the fix fixed the problem mentioned in the issue

[chilagrow]

This is covered by https://github.com/FerretDB/FerretDB/blob/main/integration/findandmodify_compat_test.go#L249 and no issue number found in the code.