UX: Add a rake task to monitor progress for long rebakes (#27517)

* UX: Add a rake task to monitor progress for long rebakes

When doing mass rebaking, this task will print progress, speed and
expected time to completion (ETC) on a loop. It is meant for rebakes
that take several hours or days.

It will calculate a 10m moving average over the past 6 hours, and print
ETC accordingly.

It also shows Sidekiq stats in case Sidekiq jobs for rebaking were
enqueued separately, instead of using the rebake_posts tasks in this
file (which are 100% synchronous and do not touch Sidekiq at all).

NOTE: only the currently unbaked count at task start time is considered;
this is useful in live communities with lots of traffic, where new posts
might otherwise change the goal posts continuously.

* Satisfy stree

lib/tasks/posts.rake

@@ -207,6 +207,61 @@ def remap_posts(find, type, ignore_case, replace = "")
   i
 end
 
+desc "monitor rebaking progress for the current unbaked post count; Ctrl-C to exit"
+task "posts:monitor_rebaking_progress", [:csv] => [:environment] do |_, args|
+  if args[:csv]
+    puts "utc_time_now,remaining_to_bake,baked_in_last_period,etc_in_days,sidekiq_enqueued,sidekiq_scheduled"
+  end
+
+  # remember last ID right now so the goal post isn't constantly moved by new posts being created
+  last_id_as_of_now = Post.where(baked_version: nil).order("id desc").first&.id
+  if last_id_as_of_now.nil?
+    warn "no posts to bake; all done"
+    exit
+  end
+
+  report_time_in_mins = 10
+  window_size_in_hs = 6
+
+  deltas = []
+  last = nil
+
+  while true
+    now = Post.where("id <= ? and baked_version is null", last_id_as_of_now).count
+
+    if last
+      delta_now = last - now
+      deltas.unshift delta_now
+
+      deltas = deltas.take((window_size_in_hs * 60) / report_time_in_mins)
+      average = deltas.reduce(:+).to_f / deltas.length.to_f / report_time_in_mins.to_f
+      etc_days = sprintf("%.2f", (now.to_f / average) / 60.0 / 24.0)
+    else
+      last = now
+      etc_days = 999 # fake initial value so that the column is 100% valid floats
+    end
+
+    s = Sidekiq::Stats.new
+
+    if args[:csv]
+      puts [Time.now.utc.iso8601, now, last - now, etc_days, s.enqueued, s.scheduled_size].join(",")
+    else
+      puts [
+             Time.now.utc.iso8601,
+             "unbaked old posts remaining: #{now}",
+             "baked in last period: #{last - now}",
+             "ETC based on #{window_size_in_hs}h avg: #{etc_days} days",
+             "SK enqueued: #{s.enqueued}",
+             "SK scheduled: #{s.scheduled_size}",
+             "waiting #{report_time_in_mins}min",
+           ].join(" - ")
+    end
+
+    last = now
+    sleep report_time_in_mins * 60
+  end
+end
+
 desc "Remap all posts matching specific string"
 task "posts:remap", %i[find replace type ignore_case] => [:environment] do |_, args|
   require "highline/import"