feat: add whyoolw/sharednd — auto DND while screen sharing (niri)

Headless service that follows niri msg -j event-stream and enables
notification Do Not Disturb via the host IPC (notification-dnd-set)
while at least one screencast is active, restoring it afterwards.

Side effects: spawns a persistent shell retry loop around
`niri msg -j event-stream`, runs `niri msg -j casts` /
`niri msg version` and `noctalia msg notification-dnd-*` on demand.
No network access, no filesystem writes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

sharednd/service.luau

@@ -0,0 +1,146 @@
+-- ShareDND service: watches niri screencasts and toggles notification
+-- Do Not Disturb while the screen is being shared.
+--
+-- Detection is event-driven: a persistent `niri msg -j event-stream` runs
+-- under runStream, wrapped in a shell retry loop because the stream API has
+-- no exit notification. Any Cast* event triggers a re-query of
+-- `niri msg -j casts`, which is the authoritative state, so no per-event
+-- bookkeeping is needed and reconnects re-sync for free (the stream sends
+-- the full current state, including CastsChanged, on connect).
+
+local function cfg(key)
+  return noctalia.getConfig(key)
+end
+
+-- true while at least one screencast is believed to be running
+local sharingActive = false
+-- true when this plugin enabled DND (and therefore owns turning it off)
+local dndSetByUs = false
+
+local queryInFlight = false
+local queryDirty = false
+
+local function countCasts(json)
+  local needle = cfg("only_active") and '"is_active":true' or '"session_id":'
+  local n = 0
+  local pos = 1
+  while true do
+    local s, e = json:find(needle, pos, true)
+    if not s then
+      break
+    end
+    n = n + 1
+    pos = e + 1
+  end
+  return n
+end
+
+local function setDnd(on, cb)
+  noctalia.runAsync("noctalia msg notification-dnd-set " .. (on and "on" or "off"), function(res)
+    local ok = res ~= nil and res.exitCode == 0
+    if not ok then
+      noctalia.log("sharednd: notification-dnd-set failed")
+    end
+    if cb then
+      cb(ok)
+    end
+  end)
+end
+
+local function onSharingStarted()
+  noctalia.runAsync("noctalia msg notification-dnd-status", function(res)
+    if not sharingActive then
+      return -- sharing already ended while the status query was in flight
+    end
+    if res == nil or res.exitCode ~= 0 then
+      noctalia.log("sharednd: notification-dnd-status failed")
+      return
+    end
+    if (res.stdout or ""):match("^%s*on") then
+      -- DND was already on (enabled manually) — don't take ownership.
+      dndSetByUs = false
+      noctalia.log("sharednd: sharing started, DND already on")
+    else
+      setDnd(true, function(ok)
+        dndSetByUs = ok
+        if ok then
+          noctalia.log("sharednd: sharing started, DND enabled")
+          if not sharingActive then
+            -- sharing ended while the set was in flight — undo
+            setDnd(false)
+            dndSetByUs = false
+          end
+        end
+      end)
+    end
+  end)
+end
+
+local function onSharingStopped()
+  if dndSetByUs or cfg("always_off_after") then
+    setDnd(false)
+    noctalia.log("sharednd: sharing stopped, DND disabled")
+  else
+    noctalia.log("sharednd: sharing stopped, DND left as-is")
+  end
+  dndSetByUs = false
+end
+
+local function applyState(activeCount)
+  local active = activeCount > 0
+  if active == sharingActive then
+    return
+  end
+  sharingActive = active
+  if active then
+    onSharingStarted()
+  else
+    onSharingStopped()
+  end
+end
+
+local function queryCasts()
+  if queryInFlight then
+    queryDirty = true
+    return
+  end
+  queryInFlight = true
+  noctalia.runAsync("niri msg -j casts", function(res)
+    queryInFlight = false
+    if res ~= nil and res.exitCode == 0 and res.stdout ~= nil then
+      applyState(countCasts(res.stdout))
+    end
+    if queryDirty then
+      queryDirty = false
+      queryCasts()
+    end
+  end)
+end
+
+local function onEventLine(line)
+  -- Matches CastsChanged / CastStartedOrChanged / CastStopped. The casts
+  -- query is authoritative, so a rare false positive (a window title
+  -- containing '"Cast') only costs one extra query.
+  if line:find('"Cast', 1, true) then
+    queryCasts()
+  end
+end
+
+-- Boot. The niri check is a user-facing warning only; the stream loop below
+-- retries forever anyway, which also covers niri's socket coming up after
+-- noctalia during session start.
+noctalia.runAsync("niri msg version", function(res)
+  if res == nil or res.exitCode ~= 0 then
+    noctalia.notifyError(noctalia.tr("notify.no_niri_title"), noctalia.tr("notify.no_niri_body"))
+    noctalia.log("sharednd: niri IPC not reachable at startup")
+  end
+end)
+
+-- The process group is killed on plugin disable, taking the shell loop and
+-- the stream down with it.
+if not noctalia.runStream("while :; do niri msg -j event-stream 2>/dev/null; sleep 3; done", onEventLine) then
+  noctalia.log("sharednd: failed to start niri event-stream")
+end
+
+-- Safety net in case the initial CastsChanged is ever missed.
+queryCasts()