DEV: Print out backtraces of all threads when spec times out on CI (#25356)

Why this change?

We have been seeing specs time out on GitHub CI but the problem is that
we are unable to debug those timeouts due to a lack of information. This
change seeks to print out the backtraces of all threads right before a
spec times out on CI.

What does this change do?

1. Starts a thread on CI which will wait for a spec to start running.
1. Once a spec starts running, the thread will sleep for
   `PER_SPEC_TIMEOUT_SECONDS -1` seconds.
1. After sleeping, the thread checks if the spec is still running and
   prints out the backtraces of all threads if it is. Otherwise, the
thread does nothing and runs the next loop.
1. At the end of each spec run, we ensure that the thread is in a
   waiting state for the next spec run to start.

Note that there is no need for us to teardown or cleanup the thread
since the process terminates after running all the tests.

spec/rails_helper.rb

@@ -458,14 +458,55 @@ RSpec.configure do |config|
     class SpecTimeoutError < StandardError
     end
 
+    mutex = Mutex.new
+    condition_variable = ConditionVariable.new
+    test_running = false
+    is_waiting = false
+
+    backtrace_logger =
+      Thread.new do
+        loop do
+          mutex.synchronize do
+            is_waiting = true
+            condition_variable.wait(mutex)
+            is_waiting = false
+          end
+
+          sleep PER_SPEC_TIMEOUT_SECONDS - 1
+
+          if mutex.synchronize { test_running }
+            puts "::group::[#{Process.pid}] Threads backtraces 1 second before timeout"
+
+            Thread.list.each do |thread|
+              puts "\n"
+              thread.backtrace.each { |line| puts line }
+              puts "\n"
+            end
+
+            puts "::endgroup::"
+          end
+        rescue StandardError => e
+          puts "Error in backtrace logger: #{e}"
+        end
+      end
+
     config.around do |example|
       Timeout.timeout(
         PER_SPEC_TIMEOUT_SECONDS,
         SpecTimeoutError,
         "Spec timed out after #{PER_SPEC_TIMEOUT_SECONDS} seconds",
       ) do
+        mutex.synchronize do
+          test_running = true
+          condition_variable.signal
+        end
+
         example.run
       rescue SpecTimeoutError
+      ensure
+        mutex.synchronize { test_running = false }
+        backtrace_logger.wakeup
+        sleep 0.01 while !mutex.synchronize { is_waiting }
       end
     end
   end