Resetting index when capping connection pool

[dschenkelman]

Context

After migrating to mongo driver 2.x we got this error (including here for full context):

TypeError: Cannot read property 'isConnected' of undefined
    at executeSingleOperation (/usr/local/lib/node_modules/auth0-api2/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:702:17)
    at Server.command (/usr/local/lib/node_modules/auth0-api2/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:830:12)
    at /usr/local/lib/node_modules/auth0-api2/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:364:12
    at /usr/local/lib/node_modules/auth0-api2/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:353:18
    at ScramSHA1.reauthenticate (/usr/local/lib/node_modules/auth0-api2/node_modules/mongodb/node_modules/mongodb-core/lib/auth/scram.js:303:25)
    at applyAuthentications (/usr/local/lib/node_modules/auth0-api2/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:349:36)
    at null.<anonymous> (/usr/local/lib/node_modules/auth0-api2/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:361:5)
    at g (events.js:257:16)
    at emitOne (events.js:77:13)
    at emit (events.js:166:7)
    at null.<anonymous> (/usr/local/lib/node_modules/auth0-api2/node_modules/mongodb/node_modules/mongodb-core/lib/connection/pool.js:125:12)
    at emitOne (events.js:77:13)
    at emit (events.js:166:7)
    at Socket.<anonymous> (/usr/local/lib/node_modules/auth0-api2/node_modules/mongodb/node_modules/mongodb-core/lib/connection/connection.js:365:12)
    at emitNone (events.js:72:20)
    at Socket.emit (events.js:163:7)

Issue

Based on the stack trace, the only way to get undefined for connection in this line is if the pool's index is pointing to a wrong place in the connections array.

After going through the code in pool.js the only place where I could find that the index invariant is broken is in capConnections.

Fix

To fix the issue I'm resetting the index when capping the connections if the current index value is invalid. The reason for choosing 0 is that it maintains the round-robin approach as the next valid connection would have been at index 0.

I also added a check to verify that the connection actually exists in case there are any other issues with the connections pool, as otherwise an exception would crash apps.

Let me know if you have any feedback.