[njoubert-cleverage]

Description

Fixes #371.

Executing a process while it is already running, by itself or through its sub-processes (A → B → A with process_executor), resets the states and task instances of the running process (one configuration per process code): the running process then returns the output of the deepest execution, or the recursion ends with an out of memory error.

A recursion whose last step executes the process again (e.g. pagination) works today apart from its return value, so it is deprecated rather than rejected:

• ProcessManager keeps the stack of the processes being executed (during doExecute() only: a listener of the start / end / fail events executing the process again is not concerned) and triggers a deprecation since 5.6 naming the chain (A -> B -> A); the execution goes on as before.
• The exception in 6.0 is Executing a process while it is already running throws an exception #380.
• The same sub-process executed several times in a row is not concerned; process_launcher runs a separate PHP process.
• UPGRADE.md: new v5.6 section with the alternatives (iterable task, process_launcher).

Requirements

• Documentation updates
  • Reference
  • Cookbooks
  • Changelog
• Unit tests

Breaking changes

None: deprecation only.

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