experiment(vegas): vegas_scroll.prefetch_gate runs the prefetch only while the render thread waits on vsync

The render thread spends most of each refresh in SwapOnVSync with the GIL
released, then needs it back the moment the swap returns. With plugin
rendering on the prefetch thread, it often has to wait for it -- behind
bytecode for up to the switch interval, behind a GIL-holding C call for as
long as that takes -- and hdpi's late frames of 2-5 refreshes went up.

src/common/render_gate.py opens a window around each swap, up to just
before the refresh the swap will return on, and a profile hook on the
prefetch thread parks it outside that window. It is never parked holding a
lock the render thread also takes (the Vegas buffer, cache and state
locks, logging, threading, importlib, the cache), never when no frame has
been swapped for 50ms, and never for more than 50ms at a time.

Off by default and ignored on a binding that keeps the GIL in
SwapOnVSync, where the window would never let the prefetch run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_render_gate.py

@@ -0,0 +1,369 @@
+"""The render gate (src/common/render_gate.py) and its wiring into Vegas."""
+
+import os
+import sys
+import threading
+import time
+from pathlib import Path
+
+os.environ.setdefault("EMULATOR", "true")
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from src.common import render_gate  # noqa: E402
+from src.common.render_gate import RenderGate  # noqa: E402
+
+PERIOD = 0.010
+
+
+class Clock:
+    def __init__(self):
+        self.now = 100.0
+
+    def __call__(self):
+        return self.now
+
+
+def _running(gate, clock, swaps=render_gate.MIN_SAMPLES + 1, hold=1):
+    """Drive ``swaps`` on-time swaps through the gate, a refresh apart."""
+    for _ in range(swaps):
+        gate.before_swap(hold)
+        clock.now += hold * PERIOD
+        gate.after_swap(hold)
+
+
+class TestWindow:
+    def test_no_window_until_the_period_is_known(self):
+        clock = Clock()
+        gate = RenderGate(clock)
+        _running(gate, clock, swaps=3)
+        assert gate.refresh_period() is None
+        gate.before_swap(1)
+        assert gate._open_until == 0.0
+        # ...and nothing parks meanwhile.
+        assert not gate._should_park(sys._getframe(), clock.now + 0.009)
+
+    def test_the_period_is_the_low_end_of_the_swap_gaps(self):
+        clock = Clock()
+        gate = RenderGate(clock)
+        _running(gate, clock, swaps=20)
+        gate.before_swap(1)
+        clock.now += 0.030            # one late frame lengthens its gap only
+        gate.after_swap(1)
+        assert gate.refresh_period() == pytest.approx(PERIOD)
+
+    def test_opens_until_just_before_the_refresh_the_swap_returns_on(self):
+        clock = Clock()
+        gate = RenderGate(clock)
+        _running(gate, clock)
+        last = clock.now
+        clock.now += 0.003            # the render thread's own work
+        gate.before_swap(1)
+        assert gate._open_until == pytest.approx(
+            last + PERIOD - render_gate.MARGIN_SECONDS)
+
+    def test_a_held_frame_opens_for_the_whole_hold(self):
+        clock = Clock()
+        gate = RenderGate(clock)
+        _running(gate, clock, hold=2)
+        last = clock.now
+        clock.now += 0.003
+        gate.before_swap(2)
+        assert gate._open_until == pytest.approx(
+            last + 2 * PERIOD - render_gate.MARGIN_SECONDS)
+
+    def test_a_late_frame_opens_until_the_next_boundary(self):
+        clock = Clock()
+        gate = RenderGate(clock)
+        _running(gate, clock)
+        last = clock.now
+        clock.now += 0.013            # missed its refresh: returns on the next
+        gate.before_swap(1)
+        assert gate._open_until == pytest.approx(
+            last + 2 * PERIOD - render_gate.MARGIN_SECONDS)
+
+    def test_closed_the_moment_the_swap_returns(self):
+        clock = Clock()
+        gate = RenderGate(clock)
+        _running(gate, clock)
+        gate.before_swap(1)
+        clock.now += PERIOD
+        gate.after_swap(1)
+        assert gate._open_until == 0.0
+
+
+class TestWhenToPark:
+    @pytest.fixture
+    def gate(self):
+        clock = Clock()
+        gate = RenderGate(clock)
+        _running(gate, clock)
+        gate.before_swap(1)
+        return gate
+
+    def test_runs_inside_the_window(self, gate):
+        assert not gate._should_park(sys._getframe(), gate._open_until - 0.001)
+
+    def test_parks_once_it_has_closed(self, gate):
+        assert gate._should_park(sys._getframe(), gate._open_until + 0.0001)
+
+    def test_never_with_no_render_loop_to_protect(self, gate):
+        stale = gate._last_return + render_gate.STALE_SECONDS + 0.001
+        assert not gate._should_park(sys._getframe(), stale)
+
+    def test_never_holding_a_guarded_lock(self, gate):
+        rlock, lock = threading.RLock(), threading.Lock()
+        gate.guard(rlock, lock, None)
+        closed = gate._open_until + 0.0001
+        with rlock:
+            assert not gate._should_park(sys._getframe(), closed)
+        with lock:
+            assert not gate._should_park(sys._getframe(), closed)
+        assert gate._should_park(sys._getframe(), closed)
+
+    def test_an_rlock_held_by_another_thread_is_not_this_ones(self, gate):
+        rlock = threading.RLock()
+        gate.guard(rlock)
+        taken, done = threading.Event(), threading.Event()
+
+        def hold():
+            with rlock:
+                taken.set()
+                done.wait(5)
+        other = threading.Thread(target=hold)
+        other.start()
+        try:
+            taken.wait(5)
+            assert gate._should_park(sys._getframe(), gate._open_until + 0.0001)
+        finally:
+            done.set()
+            other.join()
+
+    @pytest.mark.parametrize("filename", [
+        "/usr/lib/python3.11/logging/__init__.py",
+        "/usr/lib/python3.11/threading.py",
+        "<frozen importlib._bootstrap>",
+        "/home/pi/LEDMatrix/src/cache/disk_cache.py",
+        "/home/pi/LEDMatrix/src/cache_manager.py",
+    ])
+    def test_never_inside_code_that_takes_shared_locks(self, gate, filename):
+        namespace = {}
+        exec(compile("import sys\ndef here():\n    return sys._getframe()\n",
+                     filename, "exec"), namespace)
+        frame = namespace["here"]()
+        assert render_gate._unsafe(frame, None)
+        assert not gate._should_park(frame, gate._open_until + 0.0001)
+
+    def test_what_lies_below_the_yielding_block_does_not_count(self):
+        # A thread's stack always starts in threading.py; only frames above
+        # the one that entered yielding() matter.
+        namespace = {}
+        exec(compile("def bootstrap(fn):\n    return fn()\n",
+                     "/usr/lib/python3.11/threading.py", "exec"), namespace)
+
+        def entered():
+            base = sys._getframe()
+
+            def work():
+                top = sys._getframe()
+                return render_gate._unsafe(top, None), render_gate._unsafe(top, base)
+            return work()
+        assert namespace["bootstrap"](entered) == (True, False)
+
+
+class TestYielding:
+    """A real background thread, parked and released by the gate."""
+
+    def _worker(self, gate, stop):
+        count = [0]
+
+        def step():
+            count[0] += 1
+
+        def work():
+            with gate.yielding():
+                while not stop.is_set():
+                    step()
+        thread = threading.Thread(target=work, daemon=True)
+        return thread, count
+
+    def test_parks_while_closed_and_runs_while_open(self):
+        clock = Clock()
+        gate = RenderGate(clock)
+        _running(gate, clock)
+        gate.before_swap(1)
+        stop = threading.Event()
+        thread, count = self._worker(gate, stop)
+        try:
+            clock.now = gate._open_until + 0.001      # window closed
+            thread.start()
+            time.sleep(0.2)
+            parked_steps = count[0]
+            # At most one step per MAX_WAIT timeout while parked.
+            assert parked_steps < 0.2 / render_gate.MAX_WAIT_SECONDS + 5
+            assert gate.parks >= 1
+
+            with gate._cond:                          # the next swap opens it
+                gate._open_until = clock.now + 3600.0
+                gate._generation += 1
+                gate._cond.notify_all()
+            time.sleep(0.1)
+            assert count[0] > parked_steps + 1000
+        finally:
+            stop.set()
+            with gate._cond:
+                gate._open_until = float("inf")
+                gate._generation += 1
+                gate._cond.notify_all()
+            thread.join(2)
+        assert not thread.is_alive()
+
+    def test_runs_freely_once_the_render_loop_stops(self):
+        clock = Clock()
+        gate = RenderGate(clock)
+        _running(gate, clock)
+        stop = threading.Event()
+        thread, count = self._worker(gate, stop)
+        clock.now += render_gate.STALE_SECONDS + 0.01
+        thread.start()
+        time.sleep(0.1)
+        stop.set()
+        thread.join(2)
+        assert count[0] > 1000
+        assert gate.parks == 0
+
+    def test_the_hook_comes_off_when_the_block_ends(self):
+        gate = RenderGate()
+        before = sys.getprofile()
+        with gate.yielding():
+            assert sys.getprofile() == gate._hook
+        assert sys.getprofile() is before
+
+
+class TestDisplayManager:
+    @pytest.fixture
+    def dm(self):
+        from src.display_manager import DisplayManager
+        DisplayManager._instance = None
+        DisplayManager._initialized = False
+        manager = DisplayManager({"display": {
+            "hardware": {"rows": 32, "cols": 64, "chain_length": 1, "parallel": 1},
+            "runtime": {"gpio_slowdown": 0}}}, suppress_test_pattern=True)
+        yield manager
+        manager.render_gate = None
+        manager.set_scrolling_state(False)
+        DisplayManager._instance = None
+        DisplayManager._initialized = False
+
+    def test_opens_around_each_swap(self, dm):
+        calls = []
+
+        class Spy:
+            def before_swap(self, hold):
+                calls.append(("before", hold))
+
+            def after_swap(self, hold):
+                calls.append(("after", hold))
+
+        real_swap = dm.matrix.SwapOnVSync
+
+        def swap(canvas, *args, **kwargs):
+            calls.append(("swap",))
+            return real_swap(canvas, *args, **kwargs)
+        dm.matrix.SwapOnVSync = swap
+        dm.render_gate = Spy()
+        dm.set_scrolling_state(True, 2)
+        dm.draw.rectangle([0, 0, 3, 3], fill=(255, 0, 0))
+        dm.update_display()
+        assert calls == [("before", 2), ("swap",), ("after", 2)]
+
+    def test_off_screen_drawing_never_touches_it(self, dm):
+        class Boom:
+            def before_swap(self, hold):
+                raise AssertionError("an off-screen frame reached the gate")
+            after_swap = before_swap
+
+        dm.render_gate = Boom()
+        with dm.offscreen():
+            dm.draw.rectangle([0, 0, 3, 3], fill=(255, 0, 0))
+            dm.update_display()
+
+
+class TestVegasWiring:
+    def _coordinator(self, **config):
+        from src.vegas_mode.config import VegasModeConfig
+        from src.vegas_mode.coordinator import VegasModeCoordinator
+
+        class Holder:
+            def __init__(self):
+                self._buffer_lock = threading.RLock()
+                self._prefetch_lock = threading.Lock()
+                self._cache_lock = threading.Lock()
+
+        c = VegasModeCoordinator.__new__(VegasModeCoordinator)
+        c.vegas_config = VegasModeConfig(**config)
+        c._state_lock = threading.Lock()
+        c.stream_manager = Holder()
+        c.render_pipeline = Holder()
+        c.plugin_adapter = Holder()
+        c.display_manager = type("DM", (), {"render_gate": None})()
+        return c
+
+    def test_off_by_default(self, monkeypatch):
+        monkeypatch.setattr(render_gate, "swap_releases_gil", lambda: True)
+        c = self._coordinator()
+        c._install_render_gate()
+        assert c.display_manager.render_gate is None
+
+    def test_installed_for_the_run_and_removed_after(self, monkeypatch):
+        monkeypatch.setattr(render_gate, "swap_releases_gil", lambda: True)
+        c = self._coordinator(prefetch_gate=True)
+        c._install_render_gate()
+        gate = c.display_manager.render_gate
+        assert isinstance(gate, RenderGate)
+        assert c._state_lock in gate._guarded
+        assert c.stream_manager._buffer_lock in gate._guarded
+        assert c.plugin_adapter._cache_lock in gate._guarded
+        c._remove_render_gate()
+        assert c.display_manager.render_gate is None
+
+    @pytest.mark.parametrize("releases", [False, None])
+    def test_ignored_without_a_binding_that_releases_the_gil(self, monkeypatch, releases):
+        monkeypatch.setattr(render_gate, "swap_releases_gil", lambda: releases)
+        c = self._coordinator(prefetch_gate=True)
+        c._install_render_gate()
+        assert c.display_manager.render_gate is None
+
+    def test_read_from_config(self):
+        from src.vegas_mode.config import VegasModeConfig
+        on = VegasModeConfig.from_config(
+            {"display": {"vegas_scroll": {"prefetch_gate": True}}})
+        assert on.prefetch_gate is True
+        assert on.to_dict()["prefetch_gate"] is True
+        assert VegasModeConfig.from_config(
+            {"display": {"vegas_scroll": {}}}).prefetch_gate is False
+
+    def test_the_prefetch_runs_inside_the_gate(self):
+        from src.vegas_mode.render_pipeline import RenderPipeline
+
+        gate = RenderGate()
+        seen = []
+
+        class Stream:
+            def take_next_group(self, offscreen_only=False):
+                seen.append((sys.getprofile() == gate._hook, offscreen_only))
+                return ["segment"]
+
+        pipeline = RenderPipeline.__new__(RenderPipeline)
+        pipeline.config = type("C", (), {"continuous_scroll": True})()
+        pipeline._prefetch_lock = threading.Lock()
+        pipeline._prefetch_thread = None
+        pipeline._prepared_group = None
+        pipeline.stream_manager = Stream()
+        pipeline.display_manager = type("DM", (), {"render_gate": gate})()
+        pipeline.start_prefetch()
+        pipeline._prefetch_thread.join(5)
+        assert seen == [(True, True)]
+        assert pipeline._prepared_group == ["segment"]