DEV: fix flaky spec (#29972)

Spec was flaky cause work could still be in pipeline after the defer
length is 0. Our length denotes the backlog, not the in progress
count.

This adds a mechanism for gracefully stopping the queue and avoids
wait_for callse

lib/scheduler/defer.rb

@@ -24,6 +24,7 @@ module Scheduler
       @reactor = nil
       @timeout = DEFAULT_TIMEOUT
       @stats = LruRedux::ThreadSafeCache.new(STATS_CACHE_SIZE)
+      @finish = false
     end
 
     def timeout=(t)
@@ -72,7 +73,11 @@ module Scheduler
       end
     end
 
-    def stop!
+    def stop!(finish_work: false)
+      if finish_work
+        @finish = true
+        @thread&.join
+      end
       @thread.kill if @thread&.alive?
       @thread = nil
       @reactor&.stop
@@ -96,7 +101,7 @@ module Scheduler
         @thread =
           Thread.new do
             @thread.abort_on_exception = true if Rails.env.test?
-            do_work while true
+            do_work while (!@finish || !@queue.empty?)
           end if !@thread&.alive?
       end
     end

spec/lib/scheduler/defer_spec.rb

@@ -28,7 +28,7 @@ RSpec.describe Scheduler::Defer do
     @defer.later("second") {}
     @defer.later("bad") { raise "boom" }
 
-    wait_for(200) { @defer.length == 0 }
+    @defer.stop!(finish_work: true)
 
     stats = Hash[@defer.stats]
 
@@ -105,11 +105,8 @@ RSpec.describe Scheduler::Defer do
 
   it "can queue jobs properly" do
     s = nil
-
     @defer.later { s = "good" }
-
+    @defer.stop!(finish_work: true)
-    wait_for(1000) { s == "good" }
-
     expect(s).to eq("good")
   end