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Stream.cycle/1 infinite loop on halted enumerable #15343
Description
Elixir and Erlang/OTP versions
Erlang/OTP 28 [erts-16.2] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]
Elixir 1.20.0-rc.4 (46ebf7c) (compiled with Erlang/OTP 28)
Operating system
any
Current behavior
Repro:
Construct a stateful stream resource that yields one element and pass it to cycle
{:ok, agent} = Agent.start_link(fn -> 0 end)
stream =
Stream.resource(
fn -> Agent.get_and_update(agent, fn n -> {n, n + 1} end) end,
fn
0 -> {[:a], :done}
_ -> {:halt, :ok}
end,
fn _ -> :ok end
)
task = Task.async(fn -> Stream.cycle(stream) |> Enum.take(3) end)
case Task.yield(task, 2_000) || Task.shutdown(task, :brutal_kill) do
{:ok, result} -> IO.inspect(result, label: "ok")
nil -> IO.puts("HANG: Stream.cycle entered an unbounded loop with no progress")
end
Result:
HANG: Stream.cycle entered an unbounded loop with no progress
Expected behavior
ArgumentError
Activity
josevalim commentedon May 6, 2026
Closing in favor of the PR!