Go SDK: answer TaskHandlerParseRequest with registered handlers

[jason810496]

Stack (bottom to top), on native #74037 (stack #74170): #73973, #73974, #73975, #74317, #73976, #74067, #74135, #74140, #74141, #73971, #74030, #74031, #74032, #74136, #74137, #74138, #74139

A Go bundle now answers #73974's probe with every handler registered through airflow.TaskHandler and how its params bind, and ExecutableCoordinator can find and start the binary a stub task would run. Until now a Go bundle exited on a TaskHandlerParseRequest as an unknown first frame. Nothing calls the probe yet: #74135 does.

type LoadInput struct {
    Table string `arg:"table_name"`
    Limit int
}

bundle.Register(
    airflow.TaskHandler("etl", "extract", func(actx airflow.Context, region string, day *time.Time) error { ... }),
    airflow.TaskHandler("etl", "load", func(actx airflow.Context, in LoadInput) error { ... }),
)

answers:

TaskHandlerParsingResult(
    fileloc="/bundles/go/etl",
    task_handlers={"etl": [
        TaskHandlerDeclaration(task_id="extract", binding="positional", params=[
            TaskHandlerParam(name=None, value_schema={"type": "string"}),
            TaskHandlerParam(name=None, value_schema={
                "anyOf": [{"type": "string", "format": "date-time"}, {"type": "null"}]}),
        ]),
        TaskHandlerDeclaration(task_id="load", binding="named", params=[
            TaskHandlerParam(name="table_name", exact_name=True, value_schema={"type": "string"}),
            TaskHandlerParam(name="Limit", value_schema={"type": "integer", "format": "int64"}),
        ]),
    ]},
)

• Only airflow.TaskHandler registrations are declared. Tasks of a Dag built with airflow.Dag are never task handlers, so the answer leaves them out.
• Binding follows how Go decodes arguments. Flat params are positional and nameless, because Go reflection has no parameter names. A lone struct is named: arg: tags match exactly, other fields ignoring case and _. A lone struct with no bindable fields, such as time.Time, is named with params=None, so only the handler's presence is checked.
• value_schema states only what decoding into the Go type enforces, in the vocabulary build_arg_bindings emits for Python annotations. Pointers, slices and maps accept null. any, custom json.Unmarshaler types, json.Number and []byte get no schema, because they accept more than one JSON shape.
• Serve waits for the parent to acknowledge the answer, so the parent has the result before the process exits.
• The probe checks the binary a worker would run: _find_task_handler_artifact is the worker's own scan, and _build_parse_task_handler_command runs the same trailer and hash check a task gets and returns an absolute path, so exec never searches PATH.
• The executable bundle scan now walks the Dag bundle in sorted path order, as the TypeScript scan does, so the Dag processor and every worker pick the same binary. A worker's pick changes only when two binaries list the same Dag: the first usable one in sorted order now wins.
• The language SDK spec gains the task handler parse lifecycle, and its version stays 1.0.

Compatibility: a bundle packed from main's Go SDK before this change reports 2026-10-30 but cannot answer, so its probe fails and #74135 logs a warning. Repacking fixes it. Bundles from the released betas report 2026-06-16 and are not probed.

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