Try to wait for workers a bit longer

...in case we have none when starting the jobs.
We may still start the jobs with just one worker, but that's
better than being unable to handle slow startup at all.
Too bad no issue # exists for this, but fixed!

Author: TheAMM

src/thumbnailer_server.lua

@@ -241,7 +241,7 @@ mp.register_script_message("mpv_thumbnail_script-job", do_worker_job)
 
 -- Register this worker with the master script
 local register_timer = nil
-local register_timeout = mp.get_time() + 3
+local register_timeout = mp.get_time() + 1.5
 
 local register_function = function()
     if mp.get_time() > register_timeout and register_timer then

src/thumbnailer_shared.lua

@@ -25,6 +25,10 @@ local Thumbnailer = {
         -- Extra options for the workers
         worker_extra = {},
     },
+    -- Set in register_client
+    worker_register_timeout = nil,
+    -- A timer used to wait for more workers in case we have none
+    worker_wait_timer = nil,
     workers = {}
 }
 
@@ -253,6 +257,8 @@ function Thumbnailer:get_thumbnail_path(time_position)
 end
 
 function Thumbnailer:register_client()
+    self.worker_register_timeout = mp.get_time() + 2
+
     mp.register_script_message("mpv_thumbnail_script-ready", function(index, path)
         self:on_thumb_ready(tonumber(index), path)
     end)
@@ -285,8 +291,8 @@ function Thumbnailer:register_client()
     local thumb_script_key = not thumbnailer_options.disable_keybinds and "T" or nil
     mp.add_key_binding(thumb_script_key, "generate-thumbnails", function()
         if self.state.available then
-            self:start_worker_jobs()
             mp.osd_message("Started thumbnailer jobs")
+            self:start_worker_jobs()
         else
             mp.osd_message("Thumbnailing unavailabe")
         end
@@ -353,8 +359,6 @@ function Thumbnailer:prepare_source_path()
 end
 
 function Thumbnailer:start_worker_jobs()
-    self.state.enabled = true
-
     -- Create directory for the thumbnails, if needed
     local l, err = utils.readdir(self.state.thumbnail_directory)
     if err then
@@ -372,12 +376,33 @@ function Thumbnailer:start_worker_jobs()
 
     local worker_count = #worker_list
 
+    -- In case we have a worker timer created already, clear it
+    -- (For example, if the video-dec-params change in quick succession or the user pressed T, etc)
+    if self.worker_wait_timer then
+        self.worker_wait_timer:stop()
+    end
+
     if worker_count == 0 then
-        local err = "No thumbnail workers found. Make sure you are not missing a script!"
-        msg.error(err)
-        mp.osd_message(err, 3)
+        local now = mp.get_time()
+        if mp.get_time() > self.worker_register_timeout then
+            -- Workers have had their time to register but we have none!
+            local err = "No thumbnail workers found. Make sure you are not missing a script!"
+            msg.error(err)
+            mp.osd_message(err, 3)
+
+        else
+            -- We may be too early. Delay the work start a bit to try again.
+            msg.warn("No workers found. Waiting a bit more for them.")
+            -- Wait at least half a second
+            local wait_time = math.max(self.worker_register_timeout - now, 0.5)
+            self.worker_wait_timer = mp.add_timeout(wait_time, function() self:start_worker_jobs() end)
+        end
 
     else
+        -- We have at least one worker. This may not be all of them, but they have had
+        -- their time to register; we've done our best waiting for them.
+        self.state.enabled = true
+
         msg.debug( ("Splitting %d thumbnails amongst %d worker(s)"):format(self.state.thumbnail_count, worker_count) )
 
         local frame_job_order = self:_create_thumbnail_job_order()