panic parsing JSON config file

[The-Loeki]

Nomad version

0.3.2
0.4.0-rc1

Operating system and Environment details

CentOS 7 64-bit
Ubuntu 16.04 64-bit

Issue

When a config file:

• is a JSON file
• has any section in it
  • with only one key
  • which is a dict

Nomad agent panics with an index-out-of-range.

Reproduction steps

root@n01:~# cat /etc/nomad/doeswork.hcl
client {
    options {
        driver.whitelist = "docker"
    }
}
root@n01:~# cat /etc/nomad/borking-config.json
{
  "client": {
    "options": {
      "driver.whitelist": "docker"
    }
  }
}
root@n01:~# /usr/local/bin/nomad agent -log-level=DEBUG -config /etc/nomad/doeswork.hcl 
root@n01:~# /usr/local/bin/nomad agent -log-level=DEBUG -config /etc/nomad/borking-config.json 

Nomad Client logs

root@n01:~# /usr/local/bin/nomad agent -version
Nomad v0.4.0-dev ('bc09a0444722617a3a0ee0daa28d24b93d9d3e5b+CHANGES'
root@n01:~# /usr/local/bin/nomad agent -log-level=DEBUG -config /etc/nomad/doeswork.hcl 
==> Must specify either server, client or dev mode for the agent.
root@n01:~# /usr/local/bin/nomad agent -log-level=DEBUG -config /etc/nomad/borking-config.json 
panic: runtime error: index out of range

goroutine 1 [running]:
panic(0x11b6100, 0xc820010070)
    /opt/go/src/runtime/panic.go:481 +0x3e6
github.com/hashicorp/nomad/command/agent.parseClient(0xc82000de80, 0xc8201e20c0, 0x0, 0x0)
    /opt/gopath/src/github.com/hashicorp/nomad/command/agent/config_parse.go:298 +0xbc7
github.com/hashicorp/nomad/command/agent.parseConfig(0xc82000de00, 0xc820149a40, 0x0, 0x0)
    /opt/gopath/src/github.com/hashicorp/nomad/command/agent/config_parse.go:145 +0x8e7
github.com/hashicorp/nomad/command/agent.ParseConfig(0x7f66f0d97258, 0xc820026150, 0xc820026150, 0x0, 0x0)
    /opt/gopath/src/github.com/hashicorp/nomad/command/agent/config_parse.go:63 +0x33d
github.com/hashicorp/nomad/command/agent.ParseConfigFile(0x7fff4d5fe859, 0x1e, 0x0, 0x0, 0x0)
    /opt/gopath/src/github.com/hashicorp/nomad/command/agent/config_parse.go:30 +0x151
github.com/hashicorp/nomad/command/agent.LoadConfig(0x7fff4d5fe859, 0x1e, 0x3, 0x0, 0x0)
    /opt/gopath/src/github.com/hashicorp/nomad/command/agent/config.go:813 +0x135
github.com/hashicorp/nomad/command/agent.(*Command).readConfig(0xc82014c640, 0x1231520)
    /opt/gopath/src/github.com/hashicorp/nomad/command/agent/command.go:141 +0x119f
github.com/hashicorp/nomad/command/agent.(*Command).Run(0xc82014c640, 0xc82000a070, 0x3, 0x3, 0x0)
    /opt/gopath/src/github.com/hashicorp/nomad/command/agent/command.go:380 +0x1a3
github.com/hashicorp/nomad/vendor/github.com/mitchellh/cli.(*CLI).Run(0xc8200c2480, 0xc8200c2480, 0x0, 0x0)
    /opt/gopath/src/github.com/hashicorp/nomad/vendor/github.com/mitchellh/cli/cli.go:153 +0x5ee
main.RunCustom(0xc82000a060, 0x4, 0x4, 0xc82010f050, 0xc8200160b8)
    /opt/gopath/src/github.com/hashicorp/nomad/main.go:55 +0x4d5
main.Run(0xc82000a060, 0x4, 0x4, 0x1cf4350)
    /opt/gopath/src/github.com/hashicorp/nomad/main.go:20 +0x4c
main.main()
    /opt/gopath/src/github.com/hashicorp/nomad/main.go:16 +0x60

RCA

I'm no go expert, but looking at
https://github.com/hashicorp/nomad/blob/v0.4.0-rc1/command/agent/config_parse.go#L144
I'd say the client section gets detected & passed on to https://github.com/hashicorp/nomad/blob/v0.4.0-rc1/command/agent/config_parse.go#L298 which is the source of the panic.

So, considering o := list.Filter("any-section"); len(o.Items) > 0 and o.Elem().Items[0] eventually does not exist, but only for JSON, I'm pretty stumped.

The weird thing is, analysing further it only happens if there's a single dict in the section, this will bork:

{
"server": {
  "not-even-valid": {}
}
}

and this will work (as in config error of course):

{
"server": {
  "not-even-valid": true
}
}

[bsphere]

the exact thing happens with the following json after adding the client key -
{"bind_addr": "0.0.0.0", "advertise": {"http": "127.0.0.1:4646", "rpc": "172.31.51.51:4647", "serf": "172.31.51.51:4648"}, "client": {"options": {"docker.privileged.enabled": "true"}}}

[amfern]

same happens for me {"client": {"options": {}}}

[github-actions]

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