LEDMatrix/docs/IPC_CONTROL_SOCKET.md

Control socket (web → display)

The display process serves a Unix socket that the web interface uses to send it commands and get an answer back. It replaces the cache-file "mailboxes" on the SD card one command at a time. Stage 1 carries on-demand start, stop and status. Stage 2 makes those commands land within a frame on every kind of screen, and adds brightness.set and plugin.reload. The file mailbox stays as a fallback for one release.

Socket	/run/ledmatrix/control.sock (tmpfs)
Served by	the display process (src/ipc/server.py), started by DisplayController.run()
Used by	the web interface (src/ipc/client.py): POST /api/v3/display/on-demand/start and /stop, POST /api/v3/plugins/update (reload), POST /api/v3/config/main (brightness)
Contract	src/ipc/contract.py: messages, versions, framing and the socket path; both sides import it
Override	LEDMATRIX_CONTROL_SOCKET=/some/path.sock for both processes, or =off to disable it

Why

Before the socket, the web interface sent commands by writing a cache key (display_on_demand_request) that the display read every 0.25 s.

• No acknowledgement. The route answered "success" once the file was written, whether or not a display was running to read it.
• Lost requests. The display had to read the request and then delete it. A request written between those two steps could be thrown away (see _consume_on_demand_request). The cache has no atomic claim to prevent it.
• Fragile. Each channel repeated its own permission, atomic-write, staleness and in-memory-cache rules. Two of them caused bugs: a memory_ttl bug ignored every on-demand request after the first for an hour, and a stopped display was still reported as "active" for two minutes.

The socket answers every command, carries one request per message (so nothing can overwrite it), and belongs to the display process. If the display is not running, the socket does not exist, and the web interface knows right away.

Protocol (version 1)

Framing. One JSON object per line (newline-delimited JSON), UTF-8, at most 64 KiB per line (MAX_MESSAGE_BYTES). Senders encode with ensure_ascii, so a newline never appears inside a message. A connection can carry several requests. Each request gets exactly one response, in order.

Request

{"v": 1, "id": "5f0c…", "cmd": "on_demand.start",
 "args": {"plugin_id": "clock", "mode": null, "duration": 30, "pinned": false}}

• v is the protocol version.
• id is a printable string of 1-128 characters. It is echoed back in the response, and for on-demand commands it is also the on-demand request_id.
• cmd is a command name.
• args is an object. It may be omitted when a command takes no arguments.

Response

{"v": 1, "id": "5f0c…", "ok": true,  "result": {"accepted": true, "request_id": "5f0c…", "queued": 1}}
{"v": 1, "id": "5f0c…", "ok": false, "error": {"code": "busy", "message": "…"}}

id is null only when the request could not be parsed far enough to have one. Clients branch on error.code, never on the message text.

Commands

cmd	args	result	Kind
hello	{versions: [int], client?: str}	{version, versions, commands, max_message_bytes, server}	answered directly
ping	—	{pong: true}	answered directly
on_demand.start	{plugin_id?, mode?, duration?, pinned?} (at least one of plugin_id and mode)	ack	queued
on_demand.stop	—	ack	queued
on_demand.status	—	{on_demand: {...}, current_mode, display_active}	answered directly
brightness.set	{brightness: int 0-100}	{brightness, panel_brightness, dimmed, display_active}	queued, awaited (2 s)
plugin.reload	{plugin_id}	{plugin_id, reloaded: true, version, modes}	queued, awaited (10 s)

duration is a number of seconds, or a numeric string. 0, null or "" mean "until stopped". pinned must be a real boolean: the REST route has already converted strings like "false" before it sends the command. The on_demand object in on_demand.status is the same dict the display publishes to display_on_demand_state.

brightness.set sets the panel's normal brightness. It is transient: it writes nothing to config.json, and the next config the display's watcher loads (or a restart) puts the configured value back. The web interface sends it after it has saved the setting, so the two agree. The dim schedule still applies on top, so panel_brightness is the dim level while the schedule dims. While the schedule has the display off, the new level is kept for when it comes back on.

plugin.reload loads a plugin the display is running again from disk, manifest included: the steps of disabling it live and enabling it again, with its modes kept in their place in the rotation. Only a running plugin can be reloaded (not_loaded otherwise), so the id never makes the display import anything new. A plugin loaded only for an on-demand session gets busy. A new version that fails to load gets failed and stays out of the rotation, as it would after a restart. The load runs off the render thread, so the panel keeps scrolling while it happens (see below).

Acknowledgements. A queued on-demand command is accepted, not done. {"accepted": true, "request_id": …} means the command is waiting in the render thread's queue. Since stage 2 the render thread waits on that queue instead of sleeping, so it applies the command within one frame on every kind of screen (see below). Any outcome is published as before (display_on_demand_state, and status/error for a bad plugin or mode), and it can be read with on_demand.status.

Awaited commands. brightness.set and plugin.reload are answered only once the render thread has applied them, with their result or their error. The connection thread waits for that (2 s and 10 s, AWAIT_SECONDS in the contract); the render thread never waits for a client. When the render thread has not got to the command in time, the answer is pending: the command stays queued and is still applied, so a client treats pending as "not known to be done", not as a refusal. The client's own timeout is one second longer than the display's wait, so pending arrives before the client gives up.

Versions. Every request carries v. For any command except hello, a v the display does not speak gets unsupported_version. hello is checked by its versions list instead, and its result names the highest version both sides share, so a client can find out what a display supports before it relies on anything newer. The client sends v: 1 and falls back to the mailbox when the display refuses it. It does not send hello first, which saves a round trip.

New commands are added within a version, so stage 2 is still version 1. A display that does not know a command answers unknown_command, which the web interface treats like any other socket failure and falls back from, and hello lists the commands a display knows. The version changes only when the envelope or the meaning of an existing command changes.

Error codes: bad_json, bad_request, message_too_large, unsupported_version, unknown_command, invalid_args, busy (queue full, or too many connections), forbidden (peer credentials refused), internal. Stage 2 adds pending (accepted, not applied in time, still queued), not_loaded (plugin.reload of a plugin the display is not running) and failed (the render thread tried, and it did not work).

Try it on a device:

python3 - <<'EOF'
from src.ipc import client          # run from the project directory
print(client.on_demand_status())
print(client.brightness_set(60))
EOF

How the display applies a command

The server's threads never touch rendering. A connection thread parses the request, validates it against the contract, and then does one of two things:

• For a command that changes the panel, it puts a QueuedCommand on a bounded queue (16 entries) and answers with the ack, or, for an awaited command, with the outcome the render thread reports back through the command's CommandOutcome.
• For a query, it answers from a status snapshot the display provides (DisplayController._control_status). The snapshot only reads attributes.

The render thread drains the queue in _poll_on_demand_requests(), the same place it reads the mailbox:

• An on-demand command goes to _handle_on_demand_request(), which is the mailbox's own handler. The two paths share all of their code: activation, the processed-id guard, error publishing, and resuming the rotation afterwards.
• brightness.set is applied there and then (_apply_control_brightness), and the current frame is pushed again so the panel shows it.
• plugin.reload starts at the top of the next loop pass, the place where plugins are enabled and disabled live, because there no display() and no Vegas iteration is on the stack (_apply_pending_plugin_reloads). Until then the current screen ends early, as it does for a WiFi notice: the frame loops, the dwell and Vegas's interrupt check all treat a pending reload as a reason to stop (_screen_preempted).
• Only the quick half of the reload runs on the render thread (_start_plugin_reload): the plugin's modes leave the rotation, its config subscription is dropped, and PluginManager.detach_plugin takes the instance out of plugins. After that nothing new calls the old instance: no update(), and no Vegas fetch. The rotation then advances (Vegas resumes its strip), and frames keep coming.
• The slow half runs on a plugin-reload-<id> thread (_PluginReloadJob). It waits for the plugin's lock, then tears the old instance down (unload_detached_plugin) and loads the new one (reload_plugin). The lock can be held for seconds by a Vegas render of the old instance. On ledpi the render thread used to wait for it here, and a football reload froze the panel for 3.0 s.
• The new instance joins the rotation between two frames (_finish_plugin_reloads, from _service_pending_changes or the top of the loop). Its modes go back to their old places, Vegas is told to fetch it again, and the command is answered.
• While the plugin reloads, it is out of the rotation. Vegas scrolls what its strip already holds of it. An on-demand request for it gets plugin-reloading. A config reconcile neither loads it a second time nor unloads it mid-load; a disable saved meanwhile is applied once the reload is done. A second reload of the same plugin runs after the first.

The 0.25 s floor on the mailbox read does not apply to the queue, because draining it costs no disk read. A queued command also lets _service_pending_changes() skip its own floor.

Waking the render thread (stage 2)

Stage 1 made the socket answer, but not land sooner: a queued command waited for the same polls the mailbox does. Measured on ledpi (Pi 4, 24 fps Vegas), a start took 1.02 s on a static screen and about 0.4 s in Vegas either way. Now the queue wakes the render thread:

• The waits. The server sets a threading.Event whenever it queues a command. The render thread waits on it (ControlServer.wait_for_command) where it used to sleep: the static screen's 1 s frame sleep (_wait_frame_interval) and the dwell's 0.25 s ticks (_sleep_with_plugin_updates, which also covers scheduled-off and the empty-rotation pause). On a wake it applies the command at once. A command that does not end the screen, such as a brightness, does not cut the frame short: the wait carries on to the end of the interval, so the plugin is still drawn once a second.
• Vegas. The coordinator still runs its interrupt check every 10 frames, and now also at any frame where urgent() is true. The display passes "a control socket command is queued", which is one Event.is_set() per frame.
• Scrolling screens already service pending changes every frame.

So a command lands within a millisecond or so on a static screen and in a dwell, and within one frame in Vegas and on a scrolling screen. The mailbox keeps its old delays. Commands still run only on the render thread: the connection threads only queue them and set the event. The one exception is the slow half of plugin.reload (tearing down and loading the plugin), which runs on its own thread. Every change to the display's state still happens on the render thread.

The waits are timed Event.wait() calls: no polling, and no more wake-ups than the sleeps they replace when nothing arrives. Measured under WSL (Python 3.12, 20 s runs in the order before, after, after, before, with the socket's accept thread up), the idle process used 0.015–0.018% of a core before and 0.019–0.021% after on a static screen, and 0.035–0.037% before and 0.047% after in a dwell: about 25 µs more per wait, from Event.wait's own bookkeeping. A client's send to the render thread waking took 0.72 ms median (1.04 ms max), and a whole brightness.set round trip 0.64 ms median.

Without a socket (Windows, LEDMATRIX_CONTROL_SOCKET=off) the waits are the plain sleeps they were.

Exactly once. A command and a mailbox write for the same request share one request_id. If the client times out after the display queued the command and then also writes the mailbox, the display processes the request once. The existing on_demand_request_id and processed-id checks drop the second copy.

Robustness

All of this runs inside the display process, so nothing a client does may block the render loop or crash it:

• Bounded connections. Each connection gets its own daemon thread, with at most 8 at once. One more is answered busy and closed.
• Timeouts. Each read and write times out after 2 s. A message must arrive whole within 5 s of its first byte. An idle connection is closed after 10 s. A slow or stuck client costs one thread for a few seconds.
• Malformed input. A line that is not JSON gets bad_json, and the connection carries on. A line longer than 64 KiB gets message_too_large, and the connection is closed, because the next message boundary cannot be found. A client that disconnects mid-message is dropped silently. No exception from a handler leaves the connection thread.
• Full queue. When the queue is full, the client gets busy and falls back to the mailbox. A full queue means the render thread is stuck, and the systemd watchdog deals with that.
• Awaited commands. The wait for an awaited command's outcome happens on its connection thread and is bounded (AWAIT_SECONDS), so a stuck render thread costs that client pending and one connection slot for at most 10 s. The render thread settles an outcome without blocking; one nobody is waiting for any more is simply dropped.
• Startup. The server binds under a temporary name, sets the mode and the group, then renames the socket into place, so it never appears with the umask's permissions. It removes a stale socket (a file that nothing is listening on). It never removes a live socket or a file that is not a socket. close() removes the socket only if it is still the one this process created.
• Never fatal. If the server cannot start (Windows, no AF_UNIX, a bind failure, LEDMATRIX_CONTROL_SOCKET=off), it logs that and the display runs as before. The web interface then uses the mailbox.

Security model

The display runs as root and the web interface as the installing user (see PERMISSIONS.md). The socket admits exactly those two, plus anything else in the group they share:

1. The directory. /run/ledmatrix is created by RuntimeDirectory=ledmatrix in ledmatrix.service (#687): root-owned, 0755, on tmpfs, and removed when the display stops. Under an older unit, the display creates the directory itself as root, as it does for the heartbeat. No installer change is needed.
2. The socket file. The file is root:<shared group> with mode 0660, and the kernel refuses connect() to anyone without write permission on it. The shared group is the cache directory's group whenever that directory is group-writable. That is ledmatrix on an installed device (/var/cache/ledmatrix is root:ledmatrix 2775), and it is the same rule DiskCache uses for every file the two services share. Otherwise the group is the project directory's (get_shared_group_gid(), which config files use). With neither, the mode is 0600 and only root can connect.
3. Peer credentials. Where the kernel reports them (SO_PEERCRED, on Linux), the server checks every connection again. It accepts root, the display's own user, or a member of the shared group: the peer's primary gid, or a supplementary group read from /proc/<pid>/status. If /proc is unreadable, it uses the group database. Any other peer gets forbidden and is disconnected. This covers a socket mode that someone loosened by hand.

The commands are deliberately narrow. They start or stop on-demand display, read its state, set the brightness, and reload a plugin the display is already running, all of which anyone who can reach the web UI can already do (the last by restarting the display). Nothing on the socket runs a shell, writes a file, or names a path, and plugin.reload cannot make the display import a plugin it was not running. Stage 2 changed none of the access rules above.

Development. A display that is not root and cannot write to /run/ledmatrix, such as python3 run.py -e from a checkout, serves the socket at $TMPDIR/ledmatrix-<uid>/control.sock. That directory is private (0700), and the server refuses it if another user owns it. The web interface, run by the same user, looks there after /run/ledmatrix. The test suite sets LEDMATRIX_CONTROL_SOCKET=off (test/conftest.py), so a run on a device never touches the live display.

Stage plan

1. On-demand, with acks (done, #706). Contract, server, client. on_demand.start/stop/status, hello, ping. The REST routes try the socket first and report transport: "socket" | "mailbox" (plus socket_error on fallback). The mailbox is unchanged, and the plugins that write it directly (birdnet-go, mqtt-notifications, on-air, pomodoro-timer) keep working.
2. Commands that were restarts or polls (done).
   • The render thread waits on the queue instead of sleeping, and Vegas checks it every frame, so a command lands within a frame on every kind of screen (see "Waking the render thread").
   • brightness.set, transient and with no config.json write. POST /api/v3/config/main sends it after saving a brightness and reports brightness_transport; without the socket the config watcher applies the saved value, as before.
   • plugin.reload, which replaces the restart_required answer from #688 for a store update of an enabled plugin. POST /api/v3/plugins/update answers restart_required: false, reloaded: true once the new code runs, and falls back to the restart banner (with reload_error) otherwise.
   • config.reload was left out. Its only gain over the config watcher would be skipping the watcher's 2 s mtime poll, and the one setting where those seconds show, brightness, now has its own command. Plugin settings already reach the running plugin through the watcher, and the "which sections changed" ack had no reader: the web interface knows what it saved. A reload from the socket thread would also run every config subscriber on a second thread beside the watcher's.
3. A state stream. A subscribe command that keeps the connection open and pushes events: mode changes, on-demand state (including the outcome of an acked on-demand command, which today is only published), plugin runtime state, the outcome of a reload that answered pending, and the heartbeat. It replaces the polled display_current_state, plugin_runtime_snapshot (#690) and display-heartbeat.json (#687) for readers that hold a connection. The web interface relays it to its existing SSE stream. The files remain for one release for older readers.
   • The server's per-connection threads (8 at most) do not suit long-lived subscribers. A subscriber needs its own bound and a writer that drops events for a slow reader rather than blocking the display.
   • Events are produced on the render thread, so publishing must be a non-blocking hand-off, like the queue in the other direction.
   • The store's install of an already-enabled plugin, and an uninstall that keeps its config, still answer restart_required. With the stream they can use a load/unload command and report the result the same way the update route does now.
4. Retire the mailboxes. After a release in which every device has had the socket, the web interface stops writing display_on_demand_request, and the display stops polling it, logging the plugins that still write it so they can move to an in-process request_display(). The other cache keys used as messages (plugin_error_clear_request and the remaining display_* keys) move to the socket or to tmpfs.

Checking it on a device

ls -l /run/ledmatrix/control.sock                 # srw-rw---- root ledmatrix
sudo journalctl -u ledmatrix | grep "Control socket"
curl -s -X POST localhost:5000/api/v3/display/on-demand/start \
  -H 'Content-Type: application/json' -d '{"plugin_id":"clock","duration":20}'
# ... "transport": "socket"

If the response says "transport": "mailbox", socket_error gives the reason. no_socket means the display is stopped or predates the socket. refused usually means the web user is not in the socket's group, which takes effect when the web service restarts after the user is added.

Brightness and a plugin reload:

curl -s -X POST localhost:5000/api/v3/config/main \
  -H 'Content-Type: application/json' -d '{"brightness":40}'
# ... "brightness_transport": "socket"
curl -s -X POST localhost:5000/api/v3/plugins/update \
  -H 'Content-Type: application/json' -d '{"plugin_id":"clock-simple"}'
# after a real update of an enabled plugin: "restart_required": false, "reloaded": true
sudo journalctl -u ledmatrix | grep -E "Brightness set|Reload(ing|ed) plugin"

unknown_command in brightness_socket_error or reload_error means the display runs a stage-1 build: restart it once to pick up this one.