Whenever the "exec" task driver is being used, nomad runs a plug in that in time runs the task on a container under the hood. If by any circumstance the executor is killed, the task is reparented to the init service and wont be stopped by Nomad in case of a job updated or stop.
This commit introduces two mechanisms to avoid this behaviour:
* Adds signal catching and handling to the executor, so in case of a SIGTERM, the signal will also be passed on to the task.
* Adds a pre start clean up of the processes in the container, ensuring only the ones the executor runs are present at any given time.
No functional changes, just cleaning up deprecated usages that are
removed in v2 and replace one call of .Slice with .ForEach to avoid
making the intermediate copy.
* client: refactor cpuset partitioning
This PR updates the way Nomad client manages the split between tasks
that make use of resources.cpus vs. resources.cores.
Previously, each task was explicitly assigned which CPU cores they were
able to run on. Every time a task was started or destroyed, all other
tasks' cpusets would need to be updated. This was inefficient and would
crush the Linux kernel when a client would try to run ~400 or so tasks.
Now, we make use of cgroup heirarchy and cpuset inheritence to efficiently
manage cpusets.
* cr: tweaks for feedback
