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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Infinite loop when setting value in record's `on_update` callback","articleBody":"There is an infinite loop possible when using a record with `always_update=True` and `blocking=True`. Run the following IOC:\n\n```\n# Import the basic framework components.\nfrom softioc import softioc, builder, asyncio_dispatcher\n\n# Create an asyncio dispatcher, the event loop is now running\ndispatcher = asyncio_dispatcher.AsyncioDispatcher()\n\n# Set the record prefix\nbuilder.SetDeviceName(\"MY-DEVICE-PREFIX\")\n\n# Create some records\nasync def wrapped_method(value):\n    print(\"In callback\")\n    record.set(0, process=False)\n\nrecord = builder.Action(\n        \"TEST\",\n        on_update=wrapped_method,\n        blocking=True,\n        initial_value=0,\n        ZNAM=\"Idle\",\n        ONAM=\"Active\",\n    )\n\n# Boilerplate get the IOC started\nbuilder.LoadDatabase()\nsoftioc.iocInit(dispatcher)\n\n# Finally leave the IOC running with an interactive shell.\nsoftioc.interactive_ioc(globals())\n\n```\nand do `caput MY-DEVICE-PREFIX:TEST 1`. In the IOC output console you will see the `print` statement repeating forever, despite `process=False`.\n\nThe reason for this is that when `blocking=True` is specified, it changes how EPICS executes the `db_put_field` command present in the `record.set` method. It returns immediately, rather than running synchronously. This causes our logic that tracks the `process=False` request to break, and effectively put it back to the default value of `process=True`. \n\nIt is not immediately obvious what the fix is for this; we need to somehow associate the async callback with the initial synchronous `record.set`, but there is no way to pass context into the EPICS processing chain. \n\nThis issue originally found in https://github.com/DiamondLightSource/FastCS/issues/291.","author":{"url":"https://github.com/AlexanderWells-diamond","@type":"Person","name":"AlexanderWells-diamond"},"datePublished":"2026-02-02T10:06:11.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/201/pythonSoftIOC/issues/201"}