Wait for a random interval before starting the scheduler's tasks

This comes from an idea from @djmb, so that, when we have multiple schedulers,
they don't start all at the same time after a deploy, enqueuing All The Things
and causing a thundering herd.

lib/solid_queue/interruptible.rb

@@ -17,7 +17,7 @@ def interrupt
  end
 
  def interruptible_sleep(time)
- if self_pipe[:reader].wait_readable(time)
+ if time > 0 && self_pipe[:reader].wait_readable(time)
  loop { self_pipe[:reader].read_nonblock(SELF_PIPE_BLOCK_SIZE) }
  end
  rescue Errno::EAGAIN, Errno::EINTR

lib/solid_queue/runner.rb

@@ -11,6 +11,7 @@ module Runner
 
  def start(mode: :supervised)
  boot_in mode
+ observe_starting_delay
 
  run_callbacks(:start) do
  if mode == :async
@@ -44,6 +45,10 @@ def boot_in(mode)
  SolidQueue.logger.info("[SolidQueue] Starting #{self}")
  end
 
+ def observe_starting_delay
+ interruptible_sleep(initial_jitter)
+ end
+
  def register_signal_handlers
  %w[ INT TERM ].each do |signal|
  trap(signal) do
@@ -81,6 +86,10 @@ def shutdown
  procline "shutting down"
  end
 
+ def initial_jitter
+ 0
+ end
+
  def stopping?
  @stopping
  end

lib/solid_queue/scheduler.rb

@@ -58,6 +58,10 @@ def unblock_blocked_executions
  BlockedExecution.unblock(batch_size)
  end
 
+ def initial_jitter
+ Kernel.rand(0...polling_interval)
+ end
+
  def metadata
  super.merge(batch_size: batch_size, polling_interval: polling_interval)
  end