lxc: make container's init process session leader

This patch forces container's init process, to become a session leader,
that is its session ID is made the same as its process ID.
That might seem unnecessary in general, but if we want to checkpoint a
container with CRIU, which is needed for container migration,
we must ensure that the SID of each process inside the container points
to a process that lives in the same PID namespace as the container.
Therefore, we force that the session leader is the init.

Signed-off-by: Katerina Koukiou <k.koukiou@gmail.com>

src/lxc/lxc_container.c

@@ -2245,6 +2245,15 @@ static int lxcContainerChild(void *data)
                              argv->npassFDs, argv->passFDs) < 0)
         goto cleanup;
 
+    /* Make init process of the container the leader of the new session.
+     * That is needed when checkpointing container.
+     */
+    if (setsid() < 0) {
+        virReportSystemError(errno, "%s",
+                             _("Unable to become session leader"));
+        goto cleanup;
+    }
+
     ret = 0;
  cleanup:
     VIR_FREE(ttyPath);