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, 31 insertions, 13 deletions

diff --git a/cloudinit/net/dhcp.py b/cloudinit/net/dhcp.py
index 0db991db..c98a97cd 100644
--- a/cloudinit/net/dhcp.py
+++ b/cloudinit/net/dhcp.py
@@ -9,6 +9,7 @@ import logging
 import os
 import re
 import signal
+import time
 
 from cloudinit.net import (
     EphemeralIPv4Network, find_fallback_nic, get_devicelist,
@@ -127,7 +128,9 @@ def maybe_perform_dhcp_discovery(nic=None):
     if not dhclient_path:
         LOG.debug('Skip dhclient configuration: No dhclient command found.')
         return []
-    with temp_utils.tempdir(prefix='cloud-init-dhcp-', needs_exe=True) as tdir:
+    with temp_utils.tempdir(rmtree_ignore_errors=True,
+                            prefix='cloud-init-dhcp-',
+                            needs_exe=True) as tdir:
         # Use /var/tmp because /run/cloud-init/tmp is mounted noexec
         return dhcp_discovery(dhclient_path, nic, tdir)
 
@@ -195,24 +198,39 @@ def dhcp_discovery(dhclient_cmd_path, interface, cleandir):
            '-pf', pid_file, interface, '-sf', '/bin/true']
     util.subp(cmd, capture=True)
 
-    # dhclient doesn't write a pid file until after it forks when it gets a
-    # proper lease response. Since cleandir is a temp directory that gets
-    # removed, we need to wait for that pidfile creation before the
-    # cleandir is removed, otherwise we get FileNotFound errors.
+    # Wait for pid file and lease file to appear, and for the process
+    # named by the pid file to daemonize (have pid 1 as its parent). If we
+    # try to read the lease file before daemonization happens, we might try
+    # to read it before the dhclient has actually written it. We also have
+    # to wait until the dhclient has become a daemon so we can be sure to
+    # kill the correct process, thus freeing cleandir to be deleted back
+    # up the callstack.
     missing = util.wait_for_files(
         [pid_file, lease_file], maxwait=5, naplen=0.01)
     if missing:
         LOG.warning("dhclient did not produce expected files: %s",
                     ', '.join(os.path.basename(f) for f in missing))
         return []
-    pid_content = util.load_file(pid_file).strip()
-    try:
-        pid = int(pid_content)
-    except ValueError:
-        LOG.debug(
-            "pid file contains non-integer content '%s'", pid_content)
-    else:
-        os.kill(pid, signal.SIGKILL)
+
+    ppid = 'unknown'
+    for _ in range(0, 1000):
+        pid_content = util.load_file(pid_file).strip()
+        try:
+            pid = int(pid_content)
+        except ValueError:
+            pass
+        else:
+            ppid = util.get_proc_ppid(pid)
+            if ppid == 1:
+                LOG.debug('killing dhclient with pid=%s', pid)
+                os.kill(pid, signal.SIGKILL)
+                return parse_dhcp_lease_file(lease_file)
+        time.sleep(0.01)
+
+    LOG.error(
+        'dhclient(pid=%s, parentpid=%s) failed to daemonize after %s seconds',
+        pid_content, ppid, 0.01 * 1000
+    )
     return parse_dhcp_lease_file(lease_file)