DEV: Log Unicorn worker timeout backtraces to Rails.logger (#27257)

This commit introduces the following changes:

1. Introduce the `SignalTrapLogger` singleton which starts a single
   thread that polls a queue to log messages with the specified logger.
   This thread is necessary becasue most loggers cannot be used inside
   the `Signal.trap` context as they rely on mutexes which are not
   allowed within the context.

2. Moves the monkey patch in `freedom_patches/unicorn_http_server_patch.rb` to
   `config/unicorn.config.rb` which is already monkey patching
   `Unicorn::HttpServer`.

3. `Unicorn::HttpServer` will now automatically send a `USR2` signal to
   a unicorn worker 2 seconds before the worker is timed out by the
   Unicorn master.

4. When a Unicorn worker receives a `USR2` signal, it will now log only
   the main thread's backtraces to `Rails.logger`. Previously, it was
   `put`ing the backtraces to `STDOUT` which most people wouldn't read.
   Logging it via `Rails.logger` will make the backtraces easily
   accessible via `/logs`.

lib/demon/base.rb

@@ -9,9 +9,9 @@ class Demon::Base
     @demons
   end
 
-  def self.start(count = 1, verbose: false)
+  def self.start(count = 1, verbose: false, logger: nil)
     @demons ||= {}
-    count.times { |i| (@demons["#{prefix}_#{i}"] ||= new(i, verbose: verbose)).start }
+    count.times { |i| (@demons["#{prefix}_#{i}"] ||= new(i, verbose:, logger:)).start }
   end
 
   def self.stop
@@ -39,7 +39,7 @@ class Demon::Base
   attr_reader :pid, :parent_pid, :started, :index
   attr_accessor :stop_timeout
 
-  def initialize(index, rails_root: nil, parent_pid: nil, verbose: false)
+  def initialize(index, rails_root: nil, parent_pid: nil, verbose: false, logger: nil)
     @index = index
     @pid = nil
     @parent_pid = parent_pid || Process.pid
@@ -47,6 +47,11 @@ class Demon::Base
     @stop_timeout = 10
     @rails_root = rails_root || Rails.root
     @verbose = verbose
+    @logger = logger || Logger.new(STDERR)
+  end
+
+  def log(message, level: :info)
+    @logger.public_send(level, message)
   end
 
   def pid_file
@@ -72,6 +77,7 @@ class Demon::Base
 
   def stop
     @started = false
+
     if @pid
       Process.kill(stop_signal, @pid)
 
@@ -99,7 +105,7 @@ class Demon::Base
       wait_for_stop.call
 
       if alive?
-        STDERR.puts "Process would not terminate cleanly, force quitting. pid: #{@pid} #{self.class}"
+        log("Process would not terminate cleanly, force quitting. pid: #{@pid} #{self.class}")
         Process.kill("KILL", @pid)
       end
 
@@ -125,8 +131,9 @@ class Demon::Base
       rescue StandardError
         -1
       end
+
     if dead
-      STDERR.puts "Detected dead worker #{@pid}, restarting..."
+      log("Detected dead worker #{@pid}, restarting...")
       @pid = nil
       @started = false
       start
@@ -138,7 +145,7 @@ class Demon::Base
 
     if existing = already_running?
       # should not happen ... so kill violently
-      STDERR.puts "Attempting to kill pid #{existing}"
+      log("Attempting to kill pid #{existing}")
       Process.kill("TERM", existing)
     end
 
@@ -199,7 +206,7 @@ class Demon::Base
             Process.kill "KILL", Process.pid
           end
         rescue => e
-          STDERR.puts "URGENT monitoring thread had an exception #{e}"
+          log("URGENT monitoring thread had an exception #{e}")
         end
         sleep 1
       end