How could I use Redis calls in trap context in Ruby?

Question

My script gets elements from the end of the redis database, doing some work with it. I have to be sure that if the script ends with ^ C or some other signal, the element is returned to the database. I'm trying to do it

require "redis" class Test attr_reader :quit def initialize @redis = Redis.new end def trap_signal trap("INT") { puts "get ready to exit" @redis.rpush "TestQueue", @elem # I need to be sure that @emelent always puts back in the database @quit = true} end def run! trap_signal @elem = "test string" @redis.rpush "TestQueue", @elem while !quit @redis.blpop "TestQueue", @elem # Do some work whith @element sleep 1 # And put it back in the database @redis.rpush "TestQueue", @elem end end end Test.new.run! 

but get this error

 ^Cget ready to exit /usr/lib/ruby/2.1.0/monitor.rb:185:in `lock': can't be called from trap context (ThreadError) from /usr/lib/ruby/2.1.0/monitor.rb:185:in `mon_enter' from /usr/lib/ruby/2.1.0/monitor.rb:209:in `mon_synchronize' from /home/kusayu/.gem/ruby/2.1.0/gems/redis-3.2.0/lib/redis.rb:37:in `synchronize' from /home/kusayu/.gem/ruby/2.1.0/gems/redis-3.2.0/lib/redis.rb:991:in `rpush' from test.rb:13:in `block in trap_signal' from test.rb:24:in `call' from test.rb:24:in `sleep' from test.rb:24:in `run!' from test.rb:32:in `<main>' 

Answer 1

Your code is already working correctly, just remove @redis.rpush from the signal handler.

You should not run “heavy” operations in the signal handler (and you will get an exception because of this anyway). It is much better to use a variable like @quit = true to signal to the main loop that it is time to complete, and then let the main loop handle the correct cleanup.

So, if you simply remove @redis.rpush from the INT signal handler, you will make sure that the element is returned back to the database, because the main loop will be completed only once @quit is true .