configuration of task execution environment variables

[jesteria]

As a configurer of Fate, I should be able to specify the environment variables of a scheduled task's execution process.

In detail:

• Environment variables should be configurable at the level of the individual task and globally in the defaults file.

• Environment should be inheritable and this inheritance should be itself configurable. Currently, tasks inherit the environment variables of the scheduler process; and, this should be maintained as the default. Configuration should permit tasks to be isolated from environment inheritance as well.

Further:

• Environment variables might be specified as mappings, under a task key named one of (or perhaps any of): env, environ and/or environment.
• Environment keys and values may include interpolations.
• Inheritance might be specified by task key isolation with string enumeration values:
  • none: (the default) specifies that the task environment merges the scheduler's environment variables, any defaults and any in the task configuration.
  • system: specifies that the task environment merges any defaults with any task configuration – system environment variables are not included.
  • all: specifies that the task environment includes only variables configured for that task.

Note:

• The plumbum env is a frozen copy of the scheduler's env and could be suitable for use. plumbum offers command methods setenv and with_env [doc] (which are thread-safe). Nonetheless, might go thru run_bg() keyword env=… (see configuration of task execution user #7), at least in so far as this is more straightforward/compact. (Regardless, getting a cleared/non-inheriting env might require that the scheduler constructs its own, or its module's own, LocalMachine (subclass!) to avoid clearing the plumbum.local default/global machine env. …Or, it might be even worse than that.)