From: Robert Klemme <shortcutter@...>
Date: 2009-05-21T04:50:59+09:00
Subject: Re: mysql vs. fork

On 20.05.2009 15:14, Daniel DeLorme wrote:
> It's commonly known that database connections do not play well with
> Process.fork; I thought that using at_exit{exit!} was the correct way
> to solve this problem, but it appears imperfect. Take this pseudo-code:
> 
> 1  db1 = mysql_connect()
> 2  Process.fork do
> 3    at_exit{exit!}

That line above looks dangerous.  After all, what's the point in forcing 
an exit while we are exiting?

> 4    db2 = mysql_connect()
> 5    db2.query(...)
> 6  end
> 7  Process.wait
> 8  db1.query(...)
> 
> With line 3, only the at_exit handlers defined in the child process are
> run when the child exits, ensuring that db1 stays alive and line 8 keeps
> working.

Not sure whether that's true.  May well be that exit! interrupts the 
exit handler process...

> At least that's what I thought. But in the past 2 days I've been getting
> some "MySQL server has gone away" errors from our servers, indicating
> that at_exit{exit!} is not enough to protect the db1 connection. It's
> not consistently reproducible; I can't get the error when directly
> testing but the flow of errormails is unmistakable. So what exactly
> was wrong in my understanding of at_exit, sockets, mysql, and fork?

The problem with your approach is that your client inherits the DB 
connection.  That's bad because the server thinks he has one peer only. 
  It's not much of an issue if the child does not use the socket but it 
would be better to close it.  Unfortunately you cannot simply close it 
because you likely won't get access to it as it is buried somewhere in 
your DB connection class.  And if you use the connection's close method 
chances are that the server shuts down the socket and both clients suffer.

You can experiment a bit with these:

#!/usr/bin/env ruby19

require 'socket'

server = TCPServer.new 7887
client = server.accept

client.each do |line|
   puts line
end


#!/usr/bin/env ruby19

require 'socket'

server = TCPSocket.new '127.0.0.1', 7887

fork do
   server.puts "child"
   server.close
end

sleep 2
server.puts "parent"
server.close


There is a much simpler solution - just don't inherit the open connection:

2  Process.fork do
4    db2 = mysql_connect()
5    db2.query(...)
6  end

1  db1 = mysql_connect()
7  Process.wait
8  db1.query(...)

Btw, it may well be that mysql_connect supports a block so you might be 
able to write this which is usually safer:

fork do
   mysql_connect do |db|
     db.query
   end
end

mysql_connect do |db|
   db.query
   Process.wait
   db.query
end

Cheers

	robert



-- 
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