From: Ludwigi Beethoven <aix_tech@...>
Date: 2003-07-27T00:27:42+09:00
Subject: Re: Confused about locking a file via file.flock(File::LOCK_EX)


--- Brian Candler <B.Candler@pobox.com> wrote:
> On Sat, Jul 26, 2003 at 08:57:00PM +0900, Ludwigi
> Beethoven wrote:
> > I guess I would have to grab the file and update
> it
> > rather quickly, and hope that nobody else is
> updating
> > it at the same time via some other way.
> 
> The requirement was to prevent another user editing
> the same file with 'vi',
> right?
> 
> On my system (FreeBSD), if I edit a file with vi,
> then try to edit the same
> file in vi in another window, I get:
> 
> /etc/motd already locked, session is read-only.
> /etc/motd: unmodified, readonly: line 1
> 
> This implies that there is some sort of locking
> going on.
> 
> Now, if I run this test program:
> 
> def testlock(filename)
>   f = File.open(filename)
>   f.flock(File::LOCK_EX)
>   sleep 30
>   f.close
> end
> testlock("/etc/motd")
> 
> then vi /etc/motd also reports that the file is
> locked. Is that not the case
> for you?
> 
> If not, then the "update it rather quickly and hope"
> is most safely done by:
> - creating a copy of the file with a different name
> and modified contents
> - doing a 'rename' operation to replace the original
> file with it
> 
> This is an atomic operation - it will either succeed
> or fail. Of course
> someone else who has it open in a buffer could
> overwrite it again, but at
> least it will always be a complete and valid file,
> not corrupted by two
> people writing to it simultaneously.
> 
> Regards,
> 
> Brian.
> 

Well, I am using AIX version 4. I tried a similar
test and I was able to change and save /etc/hosts.
Perhaps there is a difference right there between 
AIX and FreeBSD. As a matter of facts, I will 
attach below a copy of the program that I am writing.
Perhaps someone can make suggestions.

The program will provide remote printing support to a
Windows/98 ws with a printer attached to it, managed
by a lpd server. The user comes in via a VPN that
provides a different IP address with every connection.
That's the reason to update the /etc/hosts table.

If anyone knows an off the shelv software that will do
this then I will recommend that to the client. Althoug
it is more fun to write it myself as I learn ruby.

Of course I am not providing ALL the details, such us
the AIX printer config, which I think is irrelevant at
this point. For the UDP part I used a piece of code
from "The Ruby Way".

Thank you

Here is the ruby code:

require "socket"

log="/tmp/udpsrv.log"
begin
   out = File.new(log, "a")
rescue => e
   puts "Problem with I/O operation: #{e}"
   exit 1
else
   time = Time.now
   time1 = time.strftime("%a%b%d%Y %H:%M:%S")
   out.puts time1 + " " + $0 + " Started Executing..."
   PORT = 12321
   time = Time.now
   time1 = time.strftime("%a%b%d%Y %H:%M:%S")
   out.puts time1 + " Listening on port: " + PORT.to_s

   server = UDPSocket.open       # Using UDP here...
   server.bind("cwsa", PORT)
   out.close

   loop do
     puts "Waiting for requests"
     out = File.new(log, "a")
     time = Time.now
     time1 = time.strftime("%a%b%d%Y %H:%M:%S")
     out.puts time1 + " Wait for requests."

     text, sender = server.recvfrom(64)

     time = Time.now
     time1 = time.strftime("%a%b%d%Y %H:%M:%S")
 
     out.puts time1 + " Client domain: " +
sender[0].to_s
     out.puts time1 + " Client port:   " +
sender[1].to_s
     out.puts time1 + " Client host:   " +
sender[2].to_s
     out.puts time1 + " Client IP adddress: " +
sender[3].to_s
     out.puts time1 + " Client requests: " + text.to_s


     system("rm /tmp/newhosts 1>/dev/null 2>&1")    #
Remove old file
     function, printer = text.chomp.split
     if function.include?("Enable")
        # Replace old definition from /etc/hosts table
entry for this printer
        fileArr = IO.readlines("/tmp/hosts")
        f = File.new("/tmp/newhosts", "w+")
	index = 0   
	found = false
        fileArr.each do |line|
	   if line.include?("VPNTEST")
	     fileArr[index] = sender[3].to_s + "    " + 
"VPNTEST"
 	     found = true
  	     break
           end
	   index += 1
        end # End of do loop

        if !found    # Add to end of hosts file, if it
was not found
	   f.close   # Firts close file
	   entry = sender[3].to_s + "     " + "VPNTEST"   
	   #system("echo #{entry} >> /tmp/hosts")
           system("mv /tmp/hosts /etc/hosts")  #
UNCOMMENT
	else # Re-write hosts file back with the change IP
address
           f.puts fileArr 
	   f.close
	   #system("mv /tmp/newhosts /tmp/hosts")
           system("mv /tmp/newhosts /etc/hosts")
        end

        system("enable VPNTEST >>/tmp/dpsrv.log
2>>/tmp/udpsrv.log")
     else
	if function.include?("Disable")
	   system("disable VPNTEST >>/tmp/dpsrv.log
2>>/tmp/udpsrv.log")
	else
	   out.puts time1 + " Request " + text.to_s + "
ignored."
        end
     end

     out.close
   end
end




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