[lxc-devel] lxc_container->start fork keeping server sockets alive?

[John Keiser]

Hi all,

I am trying to use lxc_container->start() from within a process that
happens to have started a TCP server, and even when my program exits, the
server socket stays alive.  This can be tested by running this program
(which starts a server and then starts a container) twice--the second time
resulting in a bind() error:

#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <errno.h>
#include <strings.h>
#include <stdlib.h>
#include <lxc/lxccontainer.h>

int main(int argc, char**argv)
{
  struct lxc_container *c;
  int listenfd,connfd,n;
  struct sockaddr_in servaddr;
  int port;

  // Start server
  port = atoi(argv[2]);
  printf("Listening on port %d ...\n", port);
  listenfd=socket(AF_INET,SOCK_STREAM,0);
  bzero(&servaddr,sizeof(servaddr));
  servaddr.sin_family = AF_INET;
  servaddr.sin_addr.s_addr=htonl(INADDR_ANY);
  servaddr.sin_port=htons(port);
  if (bind(listenfd,(struct sockaddr *)&servaddr,sizeof(servaddr)))
  {
    perror("Error calling bind()");
    exit(1);
  }
  listen(listenfd,1024);

  // Start
  printf("Starting container ...\n");
  c = lxc_container_new(argv[1], NULL);
  c->want_daemonize(c, true);
  c->want_close_all_fds(c, true);
  c->start(c, 0, NULL);

  return 0;
}


*> gcc test.c -llxc -o test*
*> lxc-create -n blarghle -t ubuntu*

*> ./test blarghle 9050*
Listening on port 9050 ...
Starting container ...
*> ./test blarghle 9050*
Listening on port 9050 ...
Error calling bind(): Address already in use

As you can see, even though ./test finished the first time, the second
./test cannot bind to the port because it is still in use.

This appears to be because the lxc init process has a ghost "./test" as its
parent:

*> ps -ef | grep 30276*
root     30276     1  0 18:37 ?        00:00:00 ./test blarghle 9050
root     30285 30276  0 18:37 ?        00:00:00 /sbin/init
*> lsof | grep 9050*
test      30276              root    3u     IPv4             366649
 0t0        TCP *:9050 (LISTEN)

I am stymied on how to investigate further.  My best guess is it has to do
with the double forking in
lxccontainer.h<https://github.com/lxc/lxc/blob/master/src/lxc/lxccontainer.c#L608-L621>,
though I don't really understand what that is for yet :)  Any thoughts?

Thanks!

--John
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