net: Wait for dhclient to daemonize before reading lease file

cloud-init uses dhclient to fetch the DHCP lease so it can extract
DHCP options.  dhclient creates the leasefile, then writes to it;
simply waiting for the leasefile to appear creates a race between
dhclient and cloud-init. Instead, wait for dhclient to be parented by
init. At that point, we know it has written to the leasefile, so it's
safe to copy the file and kill the process.

cloud-init creates a temporary directory in which to execute dhclient,
and deletes that directory after it has killed the process. If
cloud-init abandons waiting for dhclient to daemonize, it will still
attempt to delete the temporary directory, but will not report an
exception should that attempt fail.

LP: #1794399

1 files changed, 16 insertions, 0 deletions

diff --git a/cloudinit/util.py b/cloudinit/util.py
index 7800f7bc..a8a232b6 100644
--- a/cloudinit/util.py
+++ b/cloudinit/util.py
@@ -2876,4 +2876,20 @@ def udevadm_settle(exists=None, timeout=None):
     return subp(settle_cmd)
 
 
+def get_proc_ppid(pid):
+    """
+    Return the parent pid of a process.
+    """
+    ppid = 0
+    try:
+        contents = load_file("/proc/%s/stat" % pid, quiet=True)
+    except IOError as e:
+        LOG.warning('Failed to load /proc/%s/stat. %s', pid, e)
+    if contents:
+        parts = contents.split(" ", 4)
+        # man proc says
+        #  ppid %d     (4) The PID of the parent.
+        ppid = int(parts[3])
+    return ppid
+
 # vi: ts=4 expandtab