Fixes https://github.com/hashicorp/nomad/issues/5587
When a nomad 0.9 client is handling an alloc generated by a nomad 0.8
server, we should check the alloc.TaskResources for networking details
rather than task.Resources.
We check alloc.TaskResources for networking for other tasks in the task
group [1], so it's a bit odd that we used the task.Resources struct
here. TaskRunner also uses `alloc.TaskResources`[2].
The task.Resources struct in 0.8 was sparsly populated, resulting to
storing of 0 in port mapping env vars:
```
vagrant@nomad-server-01:~$ nomad version
Nomad v0.8.7 (21a2d93eecf018ad2209a5eab6aae6c359267933+CHANGES)
vagrant@nomad-server-01:~$ nomad server members
Name Address Port Status Leader Protocol Build Datacenter Region
nomad-server-01.global 10.199.0.11 4648 alive true 2 0.8.7 dc1 global
vagrant@nomad-server-01:~$ nomad alloc status -json 5b34649b | jq '.Job.TaskGroups[0].Tasks[0].Resources.Networks'
[
{
"CIDR": "",
"Device": "",
"DynamicPorts": [
{
"Label": "db",
"Value": 0
}
],
"IP": "",
"MBits": 10,
"ReservedPorts": null
}
]
vagrant@nomad-server-01:~$ nomad alloc status -json 5b34649b | jq '.TaskResources'
{
"redis": {
"CPU": 500,
"DiskMB": 0,
"IOPS": 0,
"MemoryMB": 256,
"Networks": [
{
"CIDR": "",
"Device": "eth1",
"DynamicPorts": [
{
"Label": "db",
"Value": 21722
}
],
"IP": "10.199.0.21",
"MBits": 10,
"ReservedPorts": null
}
]
}
}
```
Also, updated the test values to mimic how Nomad 0.8 structs are
represented, and made its result match the non compact values in
`TestEnvironment_AsList`.
[1] 24e9040b18/client/taskenv/env.go (L624-L639)
[2] https://github.com/hashicorp/nomad/blob/master/client/allocrunner/taskrunner/task_runner.go#L287-L303
**The Bug:**
You may have seen log lines like this when running 0.9.0-dev:
```
... client.alloc_runner.task_runner: some environment variables not available for rendering: ... keys="attr.driver.docker.volumes.enabled, attr.driver.docker.version, attr.driver.docker.bridge_ip, attr.driver.qemu.version"
```
Not only should we not be erroring on builtin driver attributes, but the
results were nondeterministic due to map iteration order!
The root cause is that we have an old root attribute for all drivers
like:
```
attr.driver.docker = "1"
```
When attributes were opaque variable names it was fine to also have
"nested" attributes like:
```
attr.driver.docker.version = "1.2.3"
```
However in the HCLv2 world the variable names are no longer opaque: they
form an object tree. The `docker` object can no longer both hold a value
(`"1"`) *and* nested attributes (`version = "1.2.3"`).
**The Fix:**
Since the old `attr.driver.<name> = "1"` attribues are useless for task
config interpolation, create a new precedence rule for creating the task
config evaluation context:
*Maps take precedence over primitives.*
This means `attr.driver.docker.version` will always take precedence over
`attr.driver.docker`. The results are determinstic and give users access
to the more useful metadata.
I made this a general precedence rule instead of special-casing driver
attrs because it seemed like better default behavior than spamming
WARNings to logs that were likely unactionable by users.
