osc: use property notifications and a timer instead of "tick" events

Traditionally, the OSC used mpv's "tick" event, which was approximately
sent once per video frame. It didn't try to track any other state, and
just updated everything.

This is sort of a problem in many corner cases and non-corner cases. For
example, it would eat CPU in the paused state (probably to some degree
also the mpv core's fault), or would waste power or even throw errors
("event queue overflows") on high FPS video.

Change this to not using the tick event. Instead, react to a number of
property change events. Rate-limit actual redrawing with a timer; the
next update cannot happen sooner than the hardcoded 30ms OSC frame
duration. This has also the effect that multiple successive updates are
(mostly) coalesced.

This means the OSC won't eat your CPU when the player is fucking paused.
(It'll still update if e.g. the cache is growing, though.) There is some
potential for bugs whenever it uses properties that are not explicitly
observed. (In theory we could easily change this to a reactive concept
to avoid such things, but whatever.)

player/lua/osc.lua

@@ -126,8 +126,9 @@ local state = {
     message_text,
     message_timeout,
     fullscreen = false,
-    timer = nil,
-    cache_idle = false,
+    tick_timer = nil,
+    tick_last_time = 0,                     -- when the last tick() was run
+    cache_state = nil,
     idle = false,
     enabled = true,
     input_enabled = true,
@@ -139,7 +140,7 @@ local state = {
 }
 
 local window_control_box_width = 80
-
+local tick_delay = 0.03
 
 --
 -- Helperfunctions
@@ -1900,7 +1901,7 @@ function osc_init()
         if user_opts.seekrangestyle == "none" then
             return nil
         end
-        local cache_state = mp.get_property_native("demuxer-cache-state", nil)
+        local cache_state = state.cache_state
         if not cache_state then
             return nil
         end
@@ -1909,14 +1910,17 @@ function osc_init()
             return nil
         end
         local ranges = cache_state["seekable-ranges"]
-        for _, range in pairs(ranges) do
-            range["start"] = 100 * range["start"] / duration
-            range["end"] = 100 * range["end"] / duration
-        end
         if #ranges == 0 then
             return nil
         end
-        return ranges
+        local nranges = {}
+        for _, range in pairs(ranges) do
+            nranges[#nranges + 1] = {
+                ["start"] = 100 * range["start"] / duration,
+                ["end"] = 100 * range["end"] / duration,
+            }
+        end
+        return nranges
     end
     ne.eventresponder["mouse_move"] = --keyframe seeking when mouse is dragged
         function (element)
@@ -1980,8 +1984,8 @@ function osc_init()
     ne = new_element("cache", "button")
 
     ne.content = function ()
-        local cache_state = mp.get_property_native("demuxer-cache-state", {})
-        if not (cache_state["seekable-ranges"] and
+        local cache_state = state.cache_state
+        if not (cache_state and cache_state["seekable-ranges"] and
             #cache_state["seekable-ranges"] > 0) then
             -- probably not a network stream
             return ""
@@ -2119,7 +2123,7 @@ function hide_osc()
     elseif (user_opts.fadeduration > 0) then
         if not(state.osc_visible == false) then
             state.anitype = "out"
-            control_timer()
+            request_tick()
         end
     else
         osc_visible(false)
@@ -2128,61 +2132,39 @@ end
 
 function osc_visible(visible)
     state.osc_visible = visible
-    control_timer()
+    request_tick()
     update_margins()
 end
 
 function pause_state(name, enabled)
     state.paused = enabled
-    control_timer()
+    request_tick()
 end
 
-function cache_state(name, idle)
-    state.cache_idle = idle
-    control_timer()
+function cache_state(name, st)
+    state.cache_state = st
+    request_tick()
 end
 
-function control_timer()
-    if (state.paused) and (state.osc_visible) and
-        ( not(state.cache_idle) or not (state.anitype == nil) ) then
-
-        timer_start()
-    else
-        timer_stop()
+-- Request that tick() is called (which typically re-renders the OSC).
+-- The tick is then either executed immediately, or rate-limited if it was
+-- called a small time ago.
+function request_tick()
+    if state.tick_timer == nil then
+        state.tick_timer = mp.add_timeout(0, tick)
     end
-end
 
-function timer_start()
-    if not (state.timer_active) then
-        msg.trace("timer start")
-
-        if (state.timer == nil) then
-            -- create new timer
-            state.timer = mp.add_periodic_timer(0.03, tick)
-        else
-            -- resume existing one
-            state.timer:resume()
+    if not state.tick_timer:is_enabled() then
+        local now = mp.get_time()
+        local timeout = tick_delay - (now - state.tick_last_time)
+        if timeout < 0 then
+            timeout = 0
         end
-
-        state.timer_active = true
+        state.tick_timer.timeout = timeout
+        state.tick_timer:resume()
     end
 end
 
-function timer_stop()
-    if (state.timer_active) then
-        msg.trace("timer stop")
-
-        if not (state.timer == nil) then
-            -- kill timer
-            state.timer:kill()
-        end
-
-        state.timer_active = false
-    end
-end
-
-
-
 function mouse_leave()
     if user_opts.hidetimeout >= 0 then
         hide_osc()
@@ -2415,7 +2397,7 @@ function process_event(source, what)
         if element_has_action(elements[n], action) then
             elements[n].eventresponder[action](elements[n])
         end
-        tick()
+        request_tick()
     end
 end
 
@@ -2470,6 +2452,12 @@ function tick()
         -- Flush OSD
         mp.set_osd_ass(osc_param.playresy, osc_param.playresy, "")
     end
+
+    state.tick_last_time = mp.get_time()
+
+    if state.anitype ~= nil then
+        request_tick()
+    end
 end
 
 function do_enable_keybindings()
@@ -2539,17 +2527,16 @@ mp.observe_property("window-maximized", "bool",
 mp.observe_property("idle-active", "bool",
     function(name, val)
         state.idle = val
-        tick()
+        request_tick()
     end
 )
 mp.observe_property("pause", "bool", pause_state)
-mp.observe_property("cache-idle", "bool", cache_state)
+mp.observe_property("demuxer-cache-state", "native", cache_state)
 mp.observe_property("vo-configured", "bool", function(name, val)
-    if val then
-        mp.register_event("tick", tick)
-    else
-        mp.unregister_event(tick)
-    end
+    request_tick()
+end)
+mp.observe_property("playback-time", "number", function(name, val)
+    request_tick()
 end)
 
 -- mouse show/hide bindings