[njoubert-cleverage]

Description

Fixes #367.

Regression of 5.5 (#343): the JSON buffer of process_launcher (json_buffering) is created before the sub-process is launched, so SubprocessInstance::getResult() returned the path of an empty cdm_buffer_*.json-stream file when the sub-process wrote nothing (no end_point, or an output that is not an array), and the buffers were never removed.

• SubprocessInstance::getResult() removes an empty buffer and returns null: the task outputs nothing again, as in 5.4.1 and as documented.
• New SubprocessInstance::removeBuffer(), called by ProcessLauncherTask for a failed sub-process and for the still running sub-processes stopped because of it (their buffers are never output). An already terminated sub-process keeps its buffer and is handled normally (output on the next call with error_strategy: skip).
• The buffers that are output are still not removed by the task (a downstream task may read them later): remove them once read, as documented.

Requirements

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

Breaking changes

None: the path of a non-empty buffer is still output and kept; only empty buffers (which were not output before 5.5) and the buffers of failed / stopped sub-processes are removed.

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