Summary

Mongo with higher volume of connections may return connectionId as double during the handshake process. The C(1.23.2) & C++(3.7.x)drivers fail to do a type check for double type connectionId in hello/ismaster response and hence fails to connect to mongo.

This happens in clusters with a high volume of connections created at a higher rate and/or without any connection pooling mechanism.

db.isMaster() response from mongo shell:

    db.isMaster()
    {
        "ismaster" : true,
        "msg" : "isdbgrid",
      ...
      ...
      "connectionId" : 3177689371,
      ...
      ...
    }

db.isMaster() response from python driver:

{
   'ok': 1.0, 
   'msg': 'isdbgrid', 
   'ismaster': True, 
   ...
   ...  
   'connectionId': 3177722651.0, <==== double value
   ...
   ...
}

The C driver fails to check connectionId value, 3177722651.0 with BSON_ITER_HOLDS_INT

Environment

 C / CXX driver versions:

{"name":"mongoc / mongocxx","version":"1.23.1 / 3.7.0"}
{"name":"mongoc / mongocxx","version":"1.23.2 / 3.7.0"} 

 C / CXX compiler and version:

gcc (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609
c++11 & c++17

Mongo:

Sharded Cluster with v4.4.14

How to Reproduce

It's difficult to reproduce the issue. Need to create connections at a higher rate to overflow int32 value of connectionId.

Temporary Fix

Restarting the mongo service would reset the connectionId counter.