name: testing-message-queue description: "Write tests for the bocpy message queue — the lock-free tag-based MPSC ring buffer. Use when: testing send/receive, FIFO ordering, selective receive, timeouts, cross-thread messaging, multi-producer scenarios, tag capacity limits, set_tags behavior, or payload round-trip fidelity."

Testing the Message Queue

This skill describes how to write tests for the bocpy message queue — the lock-free, tag-based MPSC ring buffer implemented in C that underlies send()/receive().

Architecture Overview

Test File Location

Message queue tests live in test/test_message_queue.py.

Fixture: Resetting Queue State

Every test must start with a clean slate. Use an autouse fixture that calls set_tags([]) before and after each test. This drains all queues and returns them to auto-assign mode — no set_tags call is needed inside individual tests unless the test is specifically exercising set_tags behavior.

MAX_QUEUES = 16

@pytest.fixture(autouse=True)
def reset_queues():
    set_tags([])
    yield
    set_tags([])

Why set_tags([]) instead of set_tags([...16 tags...])?

Calling set_tags is optional. The system auto-assigns tags to queues on first use. The purpose of the fixture is to clear stale state, not to pre-allocate tags. Using set_tags([]) resets all 16 queues to UNASSIGNED so every test starts fresh, and most tests can exercise the auto-assignment path naturally.

Pattern 1 — Basic Send / Receive

The simplest test: send a message, receive it, assert on the result. No set_tags needed — the tag auto-assigns.

def test_basic():
    send("my_tag", "hello")
    tag, val = receive("my_tag", 1)
    assert tag == "my_tag"
    assert val == "hello"

Always pass a timeout to receive so tests fail fast rather than hanging.

Pattern 2 — Verifying FIFO Order

Messages on the same tag are delivered in FIFO order. Send a sequence, then receive and compare element-by-element.

def test_fifo():
    for i in range(10):
        send("fifo", i)
    for i in range(10):
        _, val = receive("fifo", 1)
        assert val == i

Pattern 3 — Selective Receive Across Multiple Tags

receive accepts a list or tuple of tags and returns the first available match. Messages on non-matching tags remain in their queues.

def test_selective():
    send("skip", "no")
    send("want", "yes")
    _, val = receive("want", 1)
    assert val == "yes"
    # "skip" message is still there
    _, val = receive("skip", 1)
    assert val == "no"

def test_multi_tag_receive():
    send("b", "found")
    tag, val = receive(["a", "b", "c"], 1)
    assert tag == "b"
    assert val == "found"

Pattern 4 — Timeout Behavior

The optional after callback fires only on timeout and replaces the default (TIMEOUT, None) return value.

def test_zero_timeout():
    tag, val = receive("empty", 0)
    assert tag == TIMEOUT
    assert val is None

def test_after_callback():
    tag, val = receive("miss", 0.05, lambda: ("fallback", 99))
    assert tag == "fallback"
    assert val == 99

def test_after_not_called_on_success():
    called = []
    send("ok", "payload")
    tag, val = receive(
        "ok", 1,
        lambda: (called.append(True), ("fb", -1))[-1],
    )
    assert tag == "ok"
    assert called == []

Pattern 5 — Cross-Thread Messaging

send and receive are thread-safe. Use threading.Thread to test cross-thread delivery.

def test_cross_thread():
    def sender():
        send("ct", "from_thread")

    t = threading.Thread(target=sender)
    t.start()
    tag, val = receive("ct", 5)
    t.join()
    assert tag == "ct"
    assert val == "from_thread"

def test_bidirectional():
    def echo():
        _, val = receive("ping", 5)
        send("pong", val * 2)

    t = threading.Thread(target=echo)
    t.start()
    send("ping", 21)
    _, val = receive("pong", 5)
    t.join()
    assert val == 42

Pattern 6 — Multiple Concurrent Producers

Multiple threads sending to the same tag exercises the lock-free MPSC enqueue. Collect all messages and verify completeness. Per-producer FIFO ordering is guaranteed (messages from a single thread arrive in send order).

def test_multi_producer():
    n_producers = 8
    msgs_per = 200

    def producer(pid):
        for i in range(msgs_per):
            send("mp", (pid, i))

    threads = [threading.Thread(target=producer, args=(p,))
               for p in range(n_producers)]
    for t in threads:
        t.start()

    received = []
    for _ in range(n_producers * msgs_per):
        _, val = receive("mp", 10)
        received.append(val)

    for t in threads:
        t.join()

    # Every (pid, seq) pair delivered exactly once
    assert sorted(received) == sorted(
        (p, i) for p in range(n_producers) for i in range(msgs_per)
    )

Pattern 7 — Tag Capacity Limits

There are exactly 16 queue slots. Exceeding them raises KeyError.

def test_capacity_exceeded():
    for i in range(MAX_QUEUES):
        send(f"tag{i}", "fill")
    with pytest.raises(KeyError, match="tag capacity exceeded"):
        send("one_too_many", "boom")

Pattern 8 — Testing set_tags Explicitly

Use set_tags tests to verify pre-assignment, clearing, overflow, and reset behavior. These tests call set_tags directly inside the test body.

def test_set_tags_clears():
    send("old", "stale")
    set_tags(["new"])
    # Old messages are gone; "new" starts empty.
    tag, val = receive("new", 0)
    assert tag == TIMEOUT

def test_set_tags_resets_to_auto():
    set_tags(["pre"])
    send("pre", "msg")
    set_tags([])  # back to auto-assign
    send("fresh", "works")
    tag, val = receive("fresh", 1)
    assert val == "works"

def test_set_tags_overflow():
    set_tags([f"t{i}" for i in range(MAX_QUEUES)])
    with pytest.raises(KeyError, match="tag capacity exceeded"):
        send("extra", "x")

def test_set_tags_rejects_too_many():
    with pytest.raises(IndexError):
        set_tags([f"t{i}" for i in range(MAX_QUEUES + 1)])

Pattern 9 — Payload Round-Trip Fidelity

Messages cross interpreter boundaries via cross-interpreter data (XIData) or pickle. Use @pytest.mark.parametrize to sweep payload types.

@pytest.mark.parametrize("payload", [
    0, -1, 2**31, 3.14, float("inf"),
    True, False, None,
    "", "hello",
    (), (1,), (1, (2, (3,))),
])
def test_roundtrip(payload):
    send("rt", payload)
    _, val = receive("rt", 1)
    assert val == payload

Error Handling Tests

Common Pitfalls