Question

For this answer I wrote code like:

def show_wait_spinner
  dirty = false
  spinner = Thread.new{
    loop{
      print "*"
      dirty = true
      sleep 0.1
      print "\b"  
      dirty = false
    }    
  }
  yield
  spinner.kill
  print "\b" if dirty
end

print "A"
show_wait_spinner{ sleep rand }
puts "B"

The goal is to ensure that the final output was "AB"—to print a final "\b" if it was not already printed by the thread. That code seems messy to me in Ruby where begin/rescue/ensure exists. So I tried some other implementations of show_wait_spinner; all of them fail to ensure that "AB" is always the output, and never "A*B" or "AB*".

Is there a cleaner, more Ruby-esque way to implement this logic?

Stop at end of loop via Mutex

def show_wait_spinner
  stop = false
  stopm = Mutex.new
  spinner = Thread.new{
    loop{
      print "*"
      sleep 0.1
      print "\b"
      stopm.synchronize{ break if stop }
    }    
  }
  yield
  stopm.synchronize{ stop = true }
  STDOUT.flush
end

…but my logic must be off, since this always results in “A*B”.

Stop at end of loop via Thread-local variable

This second attempt results in sometimes “A*B” being printed, sometimes “AB”:

def show_wait_spinner
  stop = false
  spinner = Thread.new{
    Thread.current[:stop] = false
    loop{
      print "*"
      sleep 0.1
      print "\b"
      stopm.synchronize{ break if Thread.current[:stop] }
    }    
  }
  yield
  spinner[:stop] = true
  STDOUT.flush
end

Kill and Ensure the Thread

def show_wait_spinner
  spinner = Thread.new{
    dirty = false
    begin
      loop{
        print "*"
        dirty = true
        sleep 0.1
        print "\b"
        dirty = false
      }    
    ensure
      print "\b" if dirty
    end
  }
  yield
  spinner.kill
  STDOUT.flush
end

Raise and Rescue the Thread

def show_wait_spinner
  spinner = Thread.new{
    dirty = false
    begin
      loop{
        print "*"
        dirty = true
        sleep 0.1
        print "\b"
        dirty = false
      }    
    rescue
      puts "YAY"
      print "\b" if dirty
    end
  }
  yield
  spinner.raise
  STDOUT.flush
end

Answer 1

In your mutex example, you need to wait for the Thread to finish before exiting the method. Currently you set stop to true, then exit the method, print B and end your script before the spinner thread is able to wake up and print the last backspace to delete the * character.

Also, the break in stopm.synchronize{ break if stop } only exits the inner block, not the loop, so you need to use catch/throw or something.

However, you don’t need the mutex. This works for me in 1.9.3:

def show_wait_spinner
  exit = false
  spinner = Thread.new{
    loop{
      print "*"
      sleep 0.1
      print "\b" 
      break if exit
    }    
  }
  yield
  exit = true
  spinner.join
end

Adding $stdout.sync = true at the top makes it work in 1.8.7.