Attempt to provision an unconnected MongoDB results in obscure null-pointer/undefined error message.

Problem

Imagine the following, I have an IoT Agent, but my database is not present:

version: "3.1"

volumes:
    mongodb: ~

services:
    iot-agent:
        image: fiware/iotagent-json
        hostname: iot-agent
        container_name: fiware-iot-agent
        expose:
            - "4041"
            - "7896"
        ports:
            - "4041:4041"
            - "7896:7896"
        environment:
            - "IOTA_CB_HOST=orion"
            - "IOTA_CB_PORT=1026"
            - "IOTA_NORTH_PORT=4041"
            - "IOTA_REGISTRY_TYPE=mongodb"
            - "IOTA_MONGO_HOST=mongodb"
            - "IOTA_MONGO_PORT=27017"
            - "IOTA_MONGO_DB=iotagent-json"
            - "IOTA_HTTP_PORT=7896"
            - "IOTA_PROVIDER_URL=http://iot-agent:4041"

At the moment the system will attempt to connect n times and throw appropriate errors on startup:

1937c3092b43 | op=IoTAgentNGSI.DbConn | srv=n/a | subsrv=n/a | msg=Attempting to connect to MongoDB instance. Attempt 5 | comp=IoTAgent
fiware-iot-agent | time=2019-09-19T08:06:51.684Z | lvl=ERROR | corr=28407af0-2871-46d8-b9b0-1937c3092b43 | trans=28407af0-2871-46d8-b9b0-1937c3092b43 | op=IoTAgentNGSI.DbConn | srv=n/a | subsrv=n/a | msg=MONGODB-001: Error trying to connect to MongoDB: MongoError: failed to connect to server [mongodb:27017] on first connect [MongoError: getaddrinfo ENOTFOUND mongodb mongodb:27017] | comp=IoTAgent
fiware-iot-agent | time=2019-09-19T08:06:51.685Z | lvl=ERROR | corr=28407af0-2871-46d8-b9b0-1937c3092b43 | trans=28407af0-2871-46d8-b9b0

However after the Nth failure, startup will continue regardless. I can then happily query the /iot/about (using it as a heartbeat) and assume everything is fine ... until I attempt to actually do anything with the database e.g. provision a service and the request unhelpfully returns the following:

{
    "name": "TypeError",
    "message": "Cannot read property 'findOne' of undefined"
}

and the log states:

fiware-iot-agent | time=2019-09-19T08:08:24.415Z | lvl=DEBUG | corr=5f66b80f-7035-4348-a058-024f88b75d52 | trans=5f66b80f-7035-4348-a058-024f88b75d52 | op=IoTAgentNGSI.GenericMiddlewares | srv=openiot | subsrv=/ | msg=Error [TypeError] handling request: Cannot read property 'findOne' of undefined | comp=IoTAgent

The reason for this failure is quite simple, mongoose cannot create the schema model since it can't connect to a non-existent database.

Solution

Since the IoT Agent is unusable, I guess the start-up should fail completely and exit if the database cannot connect.

The alternative would be to add an undefined check on each invocation of .model.findOne() and throw a more appropriate error message.

If we're going to fail, we might as well fail in an understandable manner.

Begin with the startup MongoDB connection and retry handling, then trace the provisioning path that invokes .model.findOne(). Reproduce the state after the final connection failure using /iot/about and a provisioning request. Done means startup fails clearly when MongoDB is unavailable, or the provisioning request returns an understandable database error instead of an undefined-property TypeError.