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Restructuring DisplayController.run()

run() in src/display_controller.py decides what the panel shows and runs it. This document is the plan for turning it from one long loop into three parts with clear jobs: an Arbiter that decides, a ScreenRunner that runs one screen, and Sources that each know about one kind of content. It covers the target design, the stages that get there, and how each stage is checked.

The goal is to change how the control flow is organised, not to move code into more files. Each stage ships as its own PR, and none of them changes what the panel shows unless that PR says so and updates the golden traces on purpose.

Why

  • The priority order is written in branch order, twice. It is Follower, on-demand, WiFi notice, live priority, Vegas, rotation. In run() that order exists only as the order of if blocks. Vegas repeats part of it in its interrupt callback (_check_vegas_interrupt).
  • Preemption is found by re-checking. A screen ends early when something else changed current_display_mode or is_display_active underneath it. run() notices with five separate current_display_mode != active_mode checks: after an empty pass, in each of the two frame loops, after the frame loops, and before rotating.
  • Most recent fixes were ordering bugs between these branches (#618, #644, #649, #652): a lost mode switch, rotating past an on-demand request, spinning when every mode is empty.
  • It could not be tested without threads, real sleeps and stopping the loop by raising from a patched method.

What run() does today

Each pass, in order:

  1. loop_pass() (watchdog). Apply a pending plugin enable/disable, then any plugin reloads the control socket asked for (_apply_pending_plugin_reloads; a pending reload ends the screen before it, like a WiFi notice, as Source.RELOAD at the runner's service points). The static screen's frame sleep and the dwell wait on the socket's queue instead of sleeping (_wait_frame_interval, _sleep_with_plugin_updates); without a socket, as in the golden traces, they are the plain sleeps.
  2. With no modes: dwell 1 s, next pass.
  3. Poll on-demand requests and expiry, release plugins loaded only for on-demand, tick plugin updates, drop an expired WiFi notice, evaluate the schedule (an on-demand session overrides scheduled-off), apply the brightness target. Then gather the Arbiter's inputs (_arbiter_inputs) and call Arbiter.decide().
  4. Scheduled off: blank, dwell up to 60 s. _blank_while_scheduled_off
  5. Follower: render one frame from the leader. _run_follower_frame
  6. WiFi notice (unless on-demand): draw it, dwell 0.5 s. _show_wifi_notice. It also ends a running screen within about a second (the runner's service points), and a screen cut short resumes after it.
  7. The Sources below the notice: read whether Vegas is on and make the live-priority scan (_arbiter_inputs_below_wifi, where run() always read them), and call decide() again. It answers OnDemand (the session's current mode), Live (the next live mode, round-robin; a game that goes live during a screen takes over at the next service point, at most once a second), Vegas (LEGACY) or Rotation. _take_plan applies the answer: a live claim or the resume when live priority ends, the on-demand index.
  8. Vegas (_run_vegas_iteration): one iteration of up to max_cycle_duration. A completed iteration ends the pass, and so does one that yielded for the schedule, a reload or a WiFi notice. Any other interrupted one asks decide() once more (vegas_yielded): a game that stopped the ticker, or an on-demand session that started, shows next.
  9. One screen: pick the plugin (_plugin_for_mode) and hand the plan to the ScreenRunner (src/screen_runner.py). It draws the first frame through the executor (_dispatch_first_frame), has the controller fill in the plugin's durations, dynamic flag and frame policy (_complete_plan), runs the 125 Hz or 1 Hz frame loop with a service point after each frame, makes up the minimum duration, and returns an Outcome. On PREEMPTED the pass ends without advancing. On no content, rotate at once (_note_empty_pass, _skip_failed_plugin_modes). Otherwise ArbiterState.after() picks the next mode (_advance_after_screen).

Design

def run(self):
    while True:
        inputs = self._arbiter_inputs()                     # schedule, follower, notice
        plan = Arbiter.decide(self._arbiter_state(), inputs, now)
        ...                                                 # off / follower / notice
        plan = self._take_plan(Arbiter.decide(state, self._arbiter_inputs_below_wifi(inputs), now))
        if plan.source is Source.LEGACY:                    # Vegas, until stage 4
            plan = self._run_vegas_iteration(...)
        outcome = runner.run(plan, plugin)                  # ExitReason + elapsed
        if outcome.exit_reason is not ExitReason.PREEMPTED:
            self._advance_after_screen(plan, outcome)       # ArbiterState.after

The controller's attributes (current_display_mode, current_mode_index, on_demand_*, _live_resume_index) stay the record that the web UI, the control socket and the on-demand cache read. _arbiter_state() snapshots them into a frozen ArbiterState; the transitions are pure methods on it, and the controller writes their result back (_adopt_state).

Sources

Each kind of content is a Source. A Source looks at the state and the inputs and either offers a screen or passes. The Arbiter asks them in this order:

Order Source Offers a screen when Code
gate ScheduledOff the schedule is off and no on-demand session overrides it decide
1 Follower a sync leader is driving this panel decide
2 OnDemand a session is active (its mode list, index, expiry and pin) _on_demand_plan
3 Wifi a status message is pending and on-demand is not active decide
4 Live a live-priority plugin has live content (round-robin across several) live_pick
5 Vegas Vegas is enabled and nothing above wants the panel LEGACY, run by _run_vegas_iteration
6 Rotation always: the rotation's current mode rotation_plan

ScheduledOff is a gate in front of the Sources because that is how it works today: a scheduled-off panel stays blank even for a follower, and only an on-demand session overrides it.

The Rotation answers state.current_mode, not available_modes[current_mode_index]: the two agree except where something moved the panel off the list and the rotation carries on from there (a live mode no rotation entry names, or None after a session ended with no enabled mode to resume to), and run() always showed current_display_mode.

Arbiter

Arbiter.decide(state, inputs, now, running=None) -> ScreenPlan

decide is a pure function: it does no I/O, takes no locks and does not sleep. It can be tested with plain tables of (state, inputs, now) mapped to an expected plan.

  • ArbiterState: the current mode; the rotation and its index; the on-demand session's modes, index, expiry and pin; the live resume point; whether a mid-screen takeover has not shown yet. Transitions: next_on_demand, showing, claim_live, release_live, after.
  • ArbiterInputs: whether the schedule has the panel on, an on-demand session, a follower, the WiFi notice, the live modes (None where no scan was made), whether Vegas is on and keeps live content in its ticker, whether this pass's Vegas iteration has yielded, and (mid-screen) whether a plugin reload is waiting.
  • ScreenPlan:
Field Meaning
source which Source won
mode, plugin what to draw (None for a blank or follower plan); the plugin id once resolved
min_duration, max_duration from _resolve_durations and the on-demand bound (on_demand_bound), filled in after the first frame; an on-demand plan's max_duration is what is left of the session at now
dynamic run until the plugin's cycle completes, between min and max
frame_policy HIGH_FPS or STATIC, today _needs_high_fps; see stage 5
preemptible_by the Sources allowed to interrupt this plan mid-screen
notice, deadline, ends_live the WiFi notice; the on-demand expiry for the bound; "live priority just ended, resume the rotation first"

decide cannot ask a plugin anything, so the fields a plugin answers are filled in by the controller after the first frame, where they were always read (_complete_plan).

With running, decide answers the mid-screen question instead: running itself while the screen holds, else the plan that ends it (_hold_or_preempt), in the order the frame loops always checked:

  1. Live: a game went live while a non-live screen runs. It is the one preemption that changes the state (the rotation moves to the live mode and remembers where it was), and it is claimed even when a WiFi notice is also pending; the next pass shows the notice, then the game.
  2. The panel's mode moved under the screen (on-demand started, ended or changed mode; the rotation was rebuilt).
  3. The schedule turned the panel off.
  4. A WiFi notice (unless on-demand outranks it), compared with its expiry.
  5. A plugin reload is waiting (between frames only).

Every screen is preemptible by the gate, OnDemand, Wifi, Live, Rotation and a reload (SCREEN_PREEMPTERS), except that a live screen leaves Live out (LIVE_PREEMPTERS): live games take turns between screens. A follower and Vegas are looked at only between screens.

ScreenRunner

ScreenRunner(clock: FrameClock, host: ScreenHost).run(plan, plugin) -> Outcome

The ScreenRunner draws the first frame (_dispatch_first_frame), runs the frame loop that the plan's frame policy selects, services pending changes between frames, and returns one ExitReason:

ExitReason Golden-trace exit
DURATION target duration reached (duration)
CYCLE_COMPLETE dynamic plugin finished after its minimum (cycle-complete)
EMPTY first frame returned False, or no plugin (empty; raised when display() raised inside the executor; no-plugin, breaker)
ERROR the dispatch itself raised (error)
DISPLAY_FALSE a later frame returned False (display-false)
PREEMPTED another Source took the panel (on-demand-*, schedule-off, live, wifi, ...)
RELOAD a plugin reload is waiting: the screen ends early but counts as shown, and the rotation advances

PREEMPTED replaces the five current_display_mode != active_mode checks. The runner asks its host at named service points (Checkpoint): FRAME after each frame (and when a socket command wakes the 1 Hz wait), AFTER_LOOP / AFTER_COMPLETED_LOOP when the frame loop ends, after_dwell after the make-up dwell, and FINAL before the rotation advances. Each is one decide(..., running=plan) call (DisplayController._screen_check). The checkpoint says whether a pending reload counts there and when the WiFi notice file is read (NoticeRead): the read is throttled to once a second and deletes an expired file, so it happens exactly where the loop always read it.

In the 125 Hz loop the live-priority scan is made before the frame's sleep (_screen_service), at the moments it always was, and weighed by the service point after the sleep, where the loop always decided to end the screen.

FrameClock provides time(), perf_counter() and sleep(), the shape of the time module. In production it is _ModuleClock, which looks up src.display_controller.time on each call, so the golden traces' fake clock drives the runner as it drove the inline loops. The runner's log lines use the controller's logger, so they keep their source in the journal.

Stages

Stage Change Behaviour change Verified by
1 Golden traces; extract helpers from run() none traces generated on main pass unchanged; mutation check
2 Arbiter with Follower and Wifi Sources none traces unchanged; Arbiter unit tables; ledpi smoke
3 ScreenRunner, FrameClock, ExitReason, PREEMPTED; OnDemand, Live, Rotation Sources none traces unchanged; ledpi frame soak A/B
4 Vegas as a Source driven by run_frame() none intended traces against the real coordinator; ledpi Vegas soak A/B
5 Plugins declare frame_policy DEBUG instead of INFO for the FPS line traces; soak on a static-heavy rotation

Stage 1 (#704)

  • test/_run_loop_harness.py builds a real DisplayController through __init__ on in-memory fakes (plugins, cache, config service, plugin manager, sync manager, display manager). It swaps the module's time and datetime for one fake clock and runs the real run() until a horizon. The first frame of each screen still goes through the real PluginExecutor and the per-plugin locks.
  • test/test_run_loop_golden.py has 15 scenarios, each compared with test/fixtures/run_loop_golden/<scenario>.json:
    • plain rotation (display_durations override, a high-FPS scroller, a plugin whose display() takes no display_mode)
    • empty modes and a mode with no plugin; an all-empty rotation (the 1 s pause)
    • plugin errors and the circuit breaker
    • dynamic duration (cycle complete, plugin cap, global cap)
    • live priority taking over and handing back; live round-robin
    • on-demand start/stop/expiry; pinned on-demand; a session resumed after a restart, and one that cannot resume (its plugin did not load); a request naming a live mode the plugin's live check would drop
    • schedule off and dim, with an on-demand override during downtime
    • WiFi notice; sync follower
    • Vegas, with and without live_in_ticker
  • Each trace row is [start, mode, duration, exit_reason, frames, force_clear]. The exit reason is the event that decided what came next.
  • All 18 tests run in under a second. The goldens were generated from main's run() before any code moved.
  • Vegas uses FakeVegas, which implements only the contract the controller depends on: run_iteration() returns True after its duration and False when the interrupt or live check asks it to yield, checking at the real coordinator's cadence. Running the real coordinator on the fake clock belongs to stage 4.
  • Twelve helpers were extracted from run() (listed under "What run() does today"). Breaking any one of them fails at least one golden trace.

Stage 2: Arbiter, starting with Follower and Wifi (done)

  1. Add ScreenPlan and an Arbiter with the ScheduledOff gate, Follower and Wifi. Every other case returns a LEGACY plan, which means "carry on with the existing code" (steps 7-9).
  2. run() calls decide() after the bookkeeping in step 3 and dispatches on plan.source: blank, _run_follower_frame(), the WiFi notice, or the existing path. Inputs that Sources read (follower active, the pending WiFi message, schedule state) are collected first, so decide() stays pure.
  3. Unit-test decide() with tables. The golden traces must not change. The Wifi Source must keep the mid-screen preemption described in step 6 of "What run() does today".

Follower and Wifi go first because each is one self-contained branch that ends the pass. They prove the plumbing without touching the frame loops.

What shipped:

  • src/display_arbiter.py (on the mypy ratchet) holds Source (SCHEDULED_OFF, FOLLOWER, WIFI, LEGACY), ArbiterInputs, ArbiterState, WifiNotice, ScreenPlan and Arbiter.decide. ScreenPlan has only the fields stage 2 uses: source, max_duration (60 s for the blank, 0.5 s for the notice, the constants run() used to hard-code) and notice. mode, plugin, the other durations, frame_policy and preemptible_by arrive with the Sources that need them.
  • ArbiterState is empty: no stage-2 Source remembers anything between passes. now is passed but not read, because the top-of-pass WiFi check never compared the expiry and must not start (the table pins this).
  • ArbiterInputs holds schedule_on, on_demand_active, follower_active and wifi_notice. _arbiter_inputs derives schedule_on as is_display_active and not on_demand_schedule_override, so the gate (blank when the schedule is off and no on-demand session overrides it) blanks exactly when is_display_active is False, as before, including #714's on-demand ending in off hours. It reads the WiFi notice only when the notice could win, because _check_wifi_status_message has side effects (its 1 Hz throttle, deleting an expired file) that those passes never had.
  • The mid-screen rule is wifi_notice_preempts(notice, on_demand, now), which _wifi_notice_pending calls; it does compare the expiry.
  • run() still calls _publish_current_mode_state_if_changed, _apply_pending_vegas_init and process_deferred_updates at the same points relative to the branches, so the order of side effects in a pass is unchanged.
  • test/test_display_arbiter.py: the 16-row table (every combination of the four inputs, written out), the mid-screen table, purity checks (no clock reads, nothing mutated, no I/O imports), and the controller's snapshot through an on-demand session that overrides the schedule and ends. A mutation run broke 23 pieces once each (the gate, the order, each Source, the dwells, the expiry comparison, the snapshot's reads, each dispatch in run()); every one failed a test.

Stage 3: ScreenRunner and PREEMPTED (done; awaiting the ledpi soak)

The plan, from where stage 2 left off:

  1. ArbiterState gains the rotation index, the on-demand mode list, index, expiry and pin, and the live resume point. ArbiterInputs gains the live modes and whether Vegas is enabled and keeps live content in the ticker.
  2. OnDemand returns its current mode with the session's bound, reading now for the expiry. Live returns the next live mode (round-robin). Rotation returns the rotation's mode. ScreenPlan gains mode, plugin, min_duration, max_duration, dynamic, frame_policy and preemptible_by.
  3. ScreenRunner.run(plan) returns an ExitReason; state.after(outcome) replaces _advance_after_screen's step and the live-resume bookkeeping. Each mid-screen check asks decide() whether a Source in plan.preemptible_by now wins.
  4. The control socket (_drain_control_commands, _wait_for_control) and state publishing stay where they are; the runner calls them at its service points.

What shipped, one commit each: the runner; then the OnDemand, Live and Rotation Sources; then one decide() call at the service points.

  • src/screen_runner.py (on the mypy ratchet): ScreenRunner, FrameClock, ExitReason, Outcome, Checkpoint, NoticeRead, Screen and the ScreenHost protocol, which DisplayController implements through _ScreenHost (one-line forwards to its own methods). The two frame loops, the make-up dwell and the dynamic-duration exit moved in unchanged, pacing included.
  • src/display_arbiter.py: Source gains ON_DEMAND, LIVE, ROTATION and RELOAD; LEGACY means only Vegas. FramePolicy. ArbiterState and ArbiterInputs as listed under "Arbiter". The pure helpers on_demand_bound (_clamp_to_on_demand), live_pick (_check_live_priority's pick), live_takeover (the mid-screen claim) and rotation_plan.
  • A pass asks decide() twice: once with the inputs every pass reads, and once, only when nothing above the notice took the panel, with the Vegas check and the live scan, read where run() always read them (the scan asks every live-priority plugin, and the Vegas check applies queued Vegas config, so reading them earlier, on a follower or notice pass, would be a change). A Vegas iteration that yields asks a third time.
  • _resolve_active_mode, _clamp_to_on_demand and _screen_preempted are gone. _apply_live_priority, _check_live_priority, _check_live_takeover and _wifi_notice_pending remain (Vegas, the dwell sleep and the tests call them), built on the same pure rules.
  • _sleep_with_plugin_updates keeps its own break rules. It also serves the blank, the notice and the idle wait, which are not screens, and its rules are edge-triggered (an on-demand session starting on the mode already showing ends a dwell but not a frame loop); folding them into decide() would change behaviour.

Behaviour, checked three ways:

  • Golden traces: unchanged, no regeneration.
  • Every harness run in the suite (67: the goldens plus the live-takeover, WiFi+live, socket-wake, plugin-reload, schedule and tick tests) was captured with every sleep, display() call, WiFi read, live scan, publish, dwell and scroll-state call logged, and diffed against origin/main. Identical, except:
    • a Vegas pass used to scan the live plugins twice at the same instant (step 7, then step 8's "is anything live?"); it scans once;
    • _apply_live_priority(None) calls that changed nothing are not made;
    • throttled WiFi reads that returned the cached answer (no side effect) after a notice had already ended the screen are not made;
    • in the 125 Hz loop the live scan still runs before the frame's sleep, but the claim is made by the service point after it, so the "live" state change happens 8 ms later. The screen ends at the same frame as before.
  • Tables: test/test_display_arbiter.py (OnDemand, Live, Vegas/Rotation, after, the 24-row mid-screen table, live_takeover) and test/test_screen_runner.py (the runner on a scripted host and fake clock; the controller's service point and the reads it makes). A mutation run broke each moved or new piece once; see the PR.

This stage touches frame pacing (the 8 ms deadline sleep, the 1 ms yield), so it needs a frame soak on ledpi, A/B against main, before it merges. Coordinate with whoever owns scroll performance (docs/SCROLL_PERFORMANCE.md).

Stage 4: Vegas as a Source

The controller calls coordinator.run_frame() once per frame from the ScreenRunner instead of handing over to run_iteration() for up to max_cycle_duration. The interrupt callback and the second copy of the priority order go away, because preemption becomes PREEMPTED. The vegas-plugin-tick thread that is spawned every 4 s becomes the controller's normal update tick. Extend the harness to drive the real coordinator on the fake clock, which means patching its time and running its prefetch inline. Verify with a Vegas soak on ledpi, A/B.

Stage 5: frame_policy

Plugins declare frame_policy (STATIC, PERIODIC(hz), ANIMATED(fps), SCROLL). _needs_high_fps becomes the mapping for legacy plugins (needs_high_fps, the static-image special case, enable_scrolling), and its per-screen INFO line drops to DEBUG. It is already read twice per screen: once quietly before the first frame, so _dispatch_first_frame can end the previous scroll for a screen that runs the 1 Hz loop (_start_screen_handover), and once after it to pick the loop. A declared policy answers both.

How each stage is verified

  • Golden traces. Run python -m pytest test/test_run_loop_golden.py; it takes about a second. A refactoring stage must leave every trace unchanged. A deliberate behaviour change regenerates them with LEDMATRIX_REGEN_GOLDEN=1 in its own commit, and the commit message explains each changed row. A new scenario's golden is generated against main's run() first, then checked against the branch.
  • Mutation check. Break each moved or new piece once, for example take max of the caps instead of min, or skip the live hold. At least one trace must fail each time. Stage 1 did this for all twelve helpers.
  • Full suite. Diff the FAILED/ERROR ids against a baseline run of main in a separate worktree. The Windows host has a stable set of pre-existing failures, so never compare against zero.
  • ledpi soak (stages 2-5). With the service running the branch: python3 scripts/frame_soak.py --preview for 10 minutes on a scrolling rotation, and on Vegas for stages 3-4. Alternate which build goes first. Compare late-frame rate and freezes with main. Also check by hand that on-demand start, stop and expiry, a live game taking over and handing back, and the schedule turning the panel off and on all behave as before.

Behaviour the traces pin down that may be wrong

Stage 1 recorded six behaviours as they were, each to be fixed in its own PR that updates the affected trace and explains why. All six are fixed:

  • A WiFi notice was only checked between screens, and Vegas yielded to one and then showed a rotation screen instead. Notices now preempt within about a second, and Vegas yields straight to them (#712; wifi_notice, vegas).
  • A live game only took over between screens, and Vegas yielded to one and then showed a rotation screen first. Games now take over within about a second, and Vegas yields straight to them (#713; live_priority, vegas).
  • An on-demand session that ended during scheduled-off kept the panel on until the next minute, and a schedule window's end minute counted as on only sometimes. Windows are now half-open [start, end), and the panel blanks as soon as on-demand ends in off hours (#714; schedule).

A new one found later goes the same way: record it here with the trace that shows it, then fix it in its own PR, not inside a restructure stage.

Open:

  • Vegas stops for a sync follower (its interrupt check includes is_follower_active), but the yield path never looks at a follower, so a full rotation screen (20 s in the test) runs before the next pass hands the panel to the leader. Found by stage 3's mutation run; test_screen_runner.py::TestThroughRun::test_vegas_yielding_to_a_follower_shows_a_rotation_screen_first pins it. Stage 4, which drops the interrupt callback, is the natural place to fix it.