Document that Go SDK registration closes at Serve

airflow-core/docs/authoring-and-scheduling/language-sdks/go.rst

@@ -163,9 +163,14 @@ Dag, and the ``task_id`` must match a ``@task.stub`` function in that Dag. Neith
 function name, so a handler can be named whatever reads best in Go.
 
 ``TaskHandler`` also checks the signature of the function it is given and panics if the check fails -- for
-instance when the function does not take an ``airflow.Context`` first, or does not return an ``error``.
-Because ``main`` registers every handler before ``Serve``, a mistake stops the executable as soon as it
-starts rather than when the task first runs.
+instance when the function does not take an ``airflow.Context`` first, does not return an ``error``, or
+declares a variadic ``...`` parameter, which no stub argument can fill. Because ``main`` registers every
+handler before ``Serve``, a mistake stops the executable as soon as it starts rather than when the task
+first runs.
+
+``Serve`` closes registration: a ``Register`` left below it in ``main`` panics rather than adding a handler
+to the map the runtime is already answering from, so what a bundle can run never depends on how far
+``main`` has got.
 
 A package that defines task handlers of its own can export them as a ``[]airflow.Registerable`` for ``main``
 to pass on with ``bundle.Register(reports.Handlers()...)``.

go-sdk/README.md

@@ -94,7 +94,11 @@ func main() {
 
 `TaskHandler` names the `dag_id` and the `task_id` explicitly; neither is derived from the Go function
 name, so a handler can be called whatever reads best in Go. It checks the signature of the function and
-panics if the check fails, which stops the executable as it starts rather than when the task first runs.
+panics if the check fails -- a variadic `...` parameter, which no stub argument can fill, is one such
+failure -- so the executable stops as it starts rather than when the task first runs. `Serve` then closes
+registration, so a `Register` left below it in `main` panics instead of changing what the running bundle
+answers for.
+
 A package that defines handlers of its own can export them as a `[]airflow.Registerable` for `main` to
 pass on with `bundle.Register(reports.Handlers()...)`.