Isolation: wired up cgroup support to the config system.

This hooks the cgroup support up to the config system so it can actually
be used.

To make use of this in unit a new "cgroup" section has been added to the
isolation configuration.

e.g

  "applications": {
      "python": {
          "type": "python",
          "processes": 5,
          "path": "/opt/unit/unit-cgroup-test/",
          "module": "app",

          "isolation": {
              "cgroup": {
                  "path": "app/python"
              }
          }
      }
  }

Now there are two ways to specify the path, relative, like the above
(without a leading '/') and absolute (with a leading '/').

In the above case the "python" application is placed into its own cgroup
under CGROUP_ROOT/<main unit process cgroup>/app/python. Whereas if you
specified say

  "path": "/unit/app/python"

Then the python application would be placed under
CGROUP_ROOT/unit/app/python

The first option allows you to easily take advantage of any resource
limits that have already been configured for unit.

With the second method (absolute pathname) if you know of an already
existing cgroup where you'd like to place it, you can, e.g

  "path": "/system.slice/unit/python"

Where system.slice has already been created by systemd and may already
have some overall system limits applied which would also apply to unit.
Limits apply down the hierarchy and lower groups can't exceed the
previous group limits.

So what does this actually look like? Lets take the unit-calculator
application[0] and have each of its applications placed into their own
cgroup. If we give each application a new section like

  "isolation": {
      "cgroup": {
          "path": "/unit/unit-calculator/add"
      }
  }

changing the path for each one, we can visualise the result with the
systemd-cgls command, e.g

  │   └─session-5.scope (#4561)
  │     ├─  6667 sshd: andrew [priv]
  │     ├─  6684 sshd: andrew@pts/0
  │     ├─  6685 -bash
  │     ├─ 12632 unit: main v1.28.0 [/opt/unit/sbin/unitd --control 127.0.0.1:808>
  │     ├─ 12634 unit: controller
  │     ├─ 12635 unit: router
  │     ├─ 13550 systemd-cgls
  │     └─ 13551 less
  ├─unit (#4759)
  │ └─unit-calculator (#5037)
  │   ├─subtract (#5069)
  │   │ ├─ 12650 unit: "subtract" prototype
  │   │ └─ 12651 unit: "subtract" application
  │   ├─multiply (#5085)
  │   │ ├─ 12653 unit: "multiply" prototype
  │   │ └─ 12654 unit: "multiply" application
  │   ├─divide (#5101)
  │   │ ├─ 12671 unit: "divide" prototype
  │   │ └─ 12672 node divide.js
  │   ├─sqroot (#5117)
  │   │ ├─ 12679 unit: "sqroot" prototype
  │   │ └─ 12680 /home/andrew/src/unit-calculator/sqroot/sqroot
  │   └─add (#5053)
  │     ├─ 12648 unit: "add" prototype
  │     └─ 12649 unit: "add" application

We used an absolute path so the cgroups will be created relative to the
main cgroupfs mount, e.g /sys/fs/cgroup

We can see that the main unit processes are in the same cgroup as the
shell from where they were started, by default child process are placed
into the same cgroup as the parent.

Then we can see that each application has been placed into its own
cgroup under /sys/fs/cgroup

Taking another example of a simple 5 process python application, with

  "isolation": {
      "cgroup": {
          "path": "app/python"
      }
  }

Here we have specified a relative path and thus the python application
will be placed below the existing cgroup that contains the main unit
process. E.g

  │   │ │ ├─app-glib-cinnamon\x2dcustom\x2dlauncher\x2d3-43951.scope (#90951)
  │   │ │ │ ├─   988 unit: main v1.28.0 [/opt/unit/sbin/unitd --no-daemon]
  │   │ │ │ ├─   990 unit: controller
  │   │ │ │ ├─   991 unit: router
  │   │ │ │ ├─ 43951 xterm -bg rgb:20/20/20 -fg white -fa DejaVu Sans Mono
  │   │ │ │ ├─ 43956 bash
  │   │ │ │ ├─ 58828 sudo -i
  │   │ │ │ ├─ 58831 -bash
  │   │ │ │ └─app (#107351)
  │   │ │ │   └─python (#107367)
  │   │ │ │     ├─ 992 unit: "python" prototype
  │   │ │ │     ├─ 993 unit: "python" application
  │   │ │ │     ├─ 994 unit: "python" application
  │   │ │ │     ├─ 995 unit: "python" application
  │   │ │ │     ├─ 996 unit: "python" application
  │   │ │ │     └─ 997 unit: "python" application

[0]: <https://github.com/lcrilly/unit-calculator>

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

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