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POHow can I handle (or prevent) SIGCHLD signals from ruby backquote invocations?
 

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  1. POHow can I handle (or prevent) SIGCHLD signals from ruby backquote invocations?
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    <p>I have a long-running process with some child processes that must be restarted if they exit. To handle clean restarts of these child processes, I trap the exit signal with</p> <pre><code>trap("CLD") do cpid = Process.wait ... handle cleanup ... end </code></pre> <p>The long-running process occasionally needs to invoke 'curl' using a backquote as in</p> <pre><code>`/usr/bin/curl -m 60 http://localhost/central/expire` </code></pre> <p>The problem is that the backquote invocation is causing me to get a SIGCHLD and making my trap fire. This then gets stuck in the CLD trap because Process.wait does not finish. If there happen to be no (non-backquote) child processes at that time, the Process.wait instead gives an Errno::ECHILD exception.</p> <p>I can circumvent this problem by wrapping the backquote call with this line before:</p> <pre><code>sig_handler = trap("CLD", "IGNORE") # Ignore child traps </code></pre> <p>and this line after the backquote invocation:</p> <pre><code>trap("CLD", sig_handler) # replace the handler </code></pre> <p>but this means that I may miss a signal from the (non-backquote) child processes during that window, so I'm not really happy with that.</p> <p>So, is there a better way to do this? (I am using ruby 1.9.1p243 on GNU/Linux 2.6.22.6 if it matters)</p> <p>Update: The code below illustrates the problem (and my current solution for it). There seems to be some strange timing issue here since I don't always get the ECHILD exception. But just once is enough to mess things up.</p> <pre><code>#!/usr/bin/env ruby require 'pp' trap("CLD") do cpid = nil begin puts "\nIn trap(CLD); about to call Process.wait" cpid = Process.wait puts "In trap(CLD); Noting that ssh Child pid #{cpid}: terminated" puts "Finished Child termination trap" rescue Errno::ECHILD puts "Got Errno::ECHILD" rescue Exception =&gt; excep puts "Exception in CLD trap for process [#{cpid}]" puts PP.pp(excep, '') puts excep.backtrace.join("\n") end end #Backtick problem shown (we get an ECHILD most of the time) puts "About to invoke backticked curl" `/usr/bin/curl -m 6 http://developer.yahooapis.com/TimeService/V1/getTime?appid=YahooDemo` sleep 2; sleep 2 # Need two sleeps because the 1st gets terminated early by the trap puts "Backticked curl returns" # Using spawn puts "About to invoke curl using spawn" cpid = spawn("/usr/bin/curl -m 6 http://developer.yahooapis.com/TimeService/V1/getTime?appid=YahooDemo") puts "spawned child pid is #{cpid} at #{Time.now}" </code></pre>
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