From: Oliver Peng <oliver.peng@...>
Date: 2008-03-04T05:54:10+09:00
Subject: Re: SystemStackError: stack level too deep

Robert Klemme wrote:
> 2008/3/3, Oliver Peng <oliver.peng@skywave.com>:
>> Robert Klemme wrote:
> 
>>  as the sample code in book 'Programming ruby'.
> There is no trace of Process.waitall in the signal handler sample code.
> 
>>  When a child process dies, a signal, SIGCHLD (or SIGCLD) is sent to its
>>  parent process. So when I run Process.wait, I always should get the
>>  response at once. And this code can work. The only problem is sometimes
>>  SystemStackError was raised.
>>
>>  Can you please use sample code to show me where is the problem?
> 
> The problem is in waitall.  The sample code otherwise works ok.  Try 
> this
> 
> trap "CLD" do
>   puts "#{Process.wait} exited"
>   # alternative
>   p Process.wait2
> end
> 
> 5.times do |i|
>   pid = fork do
>     puts "proc #{i}"
>   end and puts "#{pid} created"
> end
> 
> puts "all children created"
> 
> sleep 10
> 
> Here's another approach
> 
> require 'pp'
> 
> 5.times do |i|
>   pid = fork do
>     puts "proc #{i}"
>   end and puts "#{pid} created"
> end
> 
> puts "all children created"
> 
> pp Process.waitall
> 
> 
> Kind regards
> 
> robert

Ok. I see. You mean that waitall shouldn't be used in trap 'CLD' because 
it would wait until all process exit.

But I use wait at first and get the SystemStackError exception. Then I 
try to use waitall later. So I still don't know why the SystemStackError 
exception is raised.

I also try to write test code and please have a look:

----------------------------------------------------
Server code:
require 'socket'

trap('CLD') do
  pid = Process.wait
  puts "pid #{pid} exited"
end

class TestServer

def initialize
  @tcpServer = TCPServer.new('0.0.0.0', 7777)
end

def run
  while tcp = @tcpServer.accept
    fork do
      begin
  while str = tcp.gets()
    tcp.puts("hello")
  end
      ensure
  tcp.close_read
  tcp.close_write
      end
    end
    tcp.close
  end
end
end

server = TestServer.new

server.run

------------------------------------------------
Client code:

require 'socket'

threads = []
100.times do
  threads << Thread.new() do
    while true
      socket = TCPSocket.new('localhost', 7777)
      socket.puts("hello")
      sleep 1
      socket.gets
      socket.close
      sleep 10
    end
  end
end

threads.each {|thr| thr.join}

From the server output, I can see that CLD signal has been traped and 
server puts the process exit. But from top I can see that zombie process 
are getting more and more. Is there some problem in the code logic?

Thanks.
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