[njoubert-cleverage]

Description

Fixes #373.

ProcessManager now finalizes the tasks (FinalizableTaskInterface::finalize()) when the process fails too, so that a long-running worker (Messenger, UI bundle) no longer keeps the resources of a failed process open (files of the CSV / JSON stream / XML tasks, cursor of doctrine_database_reader).

• Only the tasks that have been initialized are finalized (not a task whose initialize() failed, nor the tasks not reached), each one only once (a failure after the normal finalization does not finalize again).
• The process history is flagged as failed before the finalization: finalize() can check $state->getProcessHistory()->isFailed() to skip footers / publishing on failure.
• A finalization failure is logged as critical with its task (as in the normal finalization), the next tasks are still finalized and the original exception is rethrown unchanged; the failing task is kept for the logs of the failure (ProcessManager: the task configuration used by the logs is not restored after a child task or a sub-process #333).
• StatCounterTask logs its count when the process fails too.
• UPGRADE.md: new v5.6 section.

Requirements

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

Breaking changes

None for working processes: the success path is unchanged. Contract change of FinalizableTaskInterface::finalize(), now also called when the process fails (see UPGRADE.md): a custom finalize() must not assume the data are complete.

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