Save & Load the task queue

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Users might like to save the current task queue to a file and append tasks from a file to the current queue.

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The queue is a list of Tasks. Each Task is a dictionary.

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On loading, how should failures be handled?

• don't load any tasks if one of the new ones fail
• load only the accepted tasks
• stop loading on first task that fails to load

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First job is to write the queue to a file, then reload from it.

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Related code:

Example (from 4y ago) of the task queue: https://github.com/BCDA-APS/bdp_controls/blob/320bb4a5c7acdd6e89de30b02c186025034c2151/qserver/game_client/examples.py#L7-L50

Example (command line) loading one task: https://github.com/BCDA-APS/bdp_controls/blob/320bb4a5c7acdd6e89de30b02c186025034c2151/qserver/game_client/examples.py#L70-L75

Overview: https://github.com/BCDA-APS/bdp_controls/blob/320bb4a5c7acdd6e89de30b02c186025034c2151/qserver/game_client/examples.py#L70-L75

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Example working with the queue from Python client:

    RM.queue_clear()
    RM.item_add(BPlan("prime_hdf_plugin"))
    RM.item_add(BPlan("move_coarse_positioner", x=0, y=0))
    RM.item_add(BPlan("move_fine_positioner", x=0, y=0))
    RM.item_add(BPlan("new_sample", coarse_gain=0, fine_gain=1, jitter=1.5))
    RM.item_add(BPlan("add_metadata", game_key="starting image"))
    RM.item_add(BPlan("add_metadata", history_file=str(hfile)))
    RM.item_add(BPlan("open_shutter"))
    RM.item_add(BPlan("sleep", 1))
    RM.item_add(BPlan("take_image", IMAGE_TIME, compression="zlib"))
    RM.item_add(BPlan("close_shutter"))
    RM.item_add(BPlan("remove_metadata_key", "game_key"))
    RM.queue_start()
    RM.wait_for_idle()

1. Clears all tasks from the queue.
2. Adds several plans to the queue, some with kwargs.
3. Runs the queue.
4. Waits for the queue to finish.

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See content in duplicate #40