manage hostname setting better (LP: #596993)

On EBS instances, a shutdown and later start would end up with a
different IP address.  

In the case where the user has not modified /etc/hostname from its
original value (seeded by metadata's 'local-hostname'), then cloud-init
will again set the hostname and update /etc/hostname.

In the case where the user *has* modified /etc/hostname, it will remain
user managed.

Additionally, if /etc/cloud/cloud.cfg contains 'preserve_hostname' value
set to a True value, then /etc/hostname will not ever be touched.

LP: #596993

1 files changed, 75 insertions, 8 deletions

diff --git a/cloud-init.py b/cloud-init.py
index af09a66c..ff70017f 100755
--- a/cloud-init.py
+++ b/cloud-init.py
@@ -24,6 +24,7 @@ import cloudinit
 import cloudinit.util as util
 import time
 import logging
+import errno
 
 def warn(str):
     sys.stderr.write(str)
@@ -70,9 +71,14 @@ def main():
         raise
 
     try:
-        hostname = cloud.get_hostname()
-        cloud.sem_and_run("set_hostname", "once-per-instance",
-            set_hostname, [ hostname ], False)
+        if util.get_cfg_option_bool(cloud.cfg,"preserve_hostname",False):
+            log.debug("preserve_hostname is set. not managing hostname")
+        else:
+            hostname = cloud.get_hostname()
+            cloud.sem_and_run("set_hostname", "once-per-instance",
+                set_hostname, [ hostname, log ], False)
+            cloud.sem_and_run("update_hostname", "always",
+                update_hostname, [ hostname, log ], False)
     except:
         warn("failed to set hostname\n")
 
@@ -100,11 +106,72 @@ def apply_locale(locale):
     util.render_to_file('default-locale', '/etc/default/locale', \
         { 'locale' : locale })
 
-def set_hostname(hostname):
-    subprocess.Popen(['hostname', hostname]).communicate()
-    f=open("/etc/hostname","wb")
-    f.write("%s\n" % hostname)
-    f.close()
+# read hostname from a 'hostname' file
+# allow for comments and stripping line endings.
+# if file doesn't exist, or no contents, return default
+def read_hostname(filename, default=None):
+    try:
+        fp = open(filename,"r")
+        lines = fp.readlines()
+        fp.close()
+        for line in lines:
+            hpos = line.find("#")
+            if hpos != -1:
+                line = line[0:hpos]
+            line = line.rstrip()
+            if line:
+                return line
+    except IOError, e:
+        if e.errno == errno.ENOENT: pass
+    return default
+    
+def set_hostname(hostname, log):
+    try:
+        subprocess.Popen(['hostname', hostname]).communicate()
+        util.write_file("/etc/hostname","%s\n" % hostname, 0644)
+        log.debug("populated /etc/hostname with %s on first boot", hostname)
+    except:
+        log.error("failed to set_hostname")
+
+def update_hostname(hostname, log):
+    prev_file="%s/%s" % (cloudinit.datadir,"previous-hostname")
+    etc_file = "/etc/hostname"
+
+    hostname_prev = None
+    hostname_in_etc = None
+
+    try:
+        hostname_prev = read_hostname(prev_file)
+    except:
+        log.warn("Failed to open %s" % prev_file)
+    
+    try:
+        hostname_in_etc = read_hostname(etc_file)
+    except:
+        log.warn("Failed to open %s" % etc_file)
+
+    update_files = []
+    if not hostname_prev or hostname_prev != hostname:
+        update_files.append(prev_file)
+
+    if (not hostname_in_etc or 
+        (hostname_in_etc == hostname_prev and hostname_in_etc != hostname)):
+        update_files.append(etc_file)
+
+    try:
+        for fname in update_files:
+            util.write_file(fname,"%s\n" % hostname, 0644)
+            log.debug("wrote %s to %s" % (hostname,fname))
+    except:
+        log.warn("failed to write hostname to %s" % fname)
+
+    if hostname_in_etc and hostname_prev and hostname_in_etc != hostname_prev:
+        log.debug("%s differs from %s. assuming user maintained" %
+                  (prev_file,etc_file))
+
+    if etc_file in update_files:
+        log.debug("setting hostname to %s" % hostname)
+        subprocess.Popen(['hostname', hostname]).communicate()
 
 if __name__ == '__main__':
     main()