Mongo engine creates 3 connections to db server per incoming connection.

[greedy52]

Description

What happened:
While investigating #10869, the mongo client is found to create 3 connections per mongo.Connect.

Once our mongo engine receives these 3 connections, then it creates 3 connections to the db server per incoming connection so 3 x 3 = 9 connections are established to db server in total

[mongo client] ---x3---> [teleport] ---x9---> [mongo server]

What you expected to happen:
Teleport should NOT multiply number of connections to the server

Reproduction Steps

As minimally and precisely as possible, describe step-by-step how to reproduce the problem.

1. Add some debug logs for TestDatabaseAccessMongoConnectionCount in integration test

Server Details

• Teleport version (run teleport version): v9

Client Details

• Tsh version (tsh version): n/a

Debug Logs

For a single test client, that does one find, test server receives

2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60959, command isMaster name:root-mongo mongodb/test.go:172
2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60959, command isMaster name:root-mongo mongodb/test.go:172

2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60960, command isMaster name:root-mongo mongodb/test.go:172

2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60961, command authenticate name:root-mongo mongodb/test.go:172
2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60961, command isMaster name:root-mongo mongodb/test.go:172
2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60961, command isMaster name:root-mongo mongodb/test.go:172

2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60962, command isMaster name:root-mongo mongodb/test.go:172
2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60962, command isMaster name:root-mongo mongodb/test.go:172

2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60963, command isMaster name:root-mongo mongodb/test.go:172

2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60964, command authenticate name:root-mongo mongodb/test.go:172
2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60964, command isMaster name:root-mongo mongodb/test.go:172
2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60964, command isMaster name:root-mongo mongodb/test.go:172

2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60966, command isMaster name:root-mongo mongodb/test.go:172

2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60967, command isMaster name:root-mongo mongodb/test.go:172

2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60968, command authenticate name:root-mongo mongodb/test.go:172
2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60968, command isMaster name:root-mongo mongodb/test.go:172
2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60968, command ping name:root-mongo mongodb/test.go:172
2022-03-08T08:28:28-05:00 DEBU [MONGODB]   TestServer.handleConnection, connection 127.0.0.1:60968, command find name:root-mongo mongodb/test.go:172

Possible solutions

We may have to modify the mongo driver to disable some background connections that are not necessary for our proxy.

[greedy52]

Tested with

• Teleport v14
• mongosh 2.0.1
• Mongo Atlas 6.0.10

Per tsh db connect

• Teleport Database agent sees 6 sessions
• Mongo Atlas Primary shard sees 18 connections, Secondary sees 12 connections

Seems number of connections can be reduced by setting maxPoolSize=1