Reinstate hostname hook for GCE

This DHCP exit hook to shorten the system hostname on GCE was previously
installed by build-debian-cloud and bootstrap-vz, but seems to have been
inadvertently removed in commit c81045cc6e
as part of a broad cross-cloud cleanup. Again, this was caught by our
validation tests, and we might be done with the fixes at this point.

In this commit, I'm reinstating the hook with a name change and an
explanatory comment, to reduce the risk of this vanishing accidentally
in the future.

Change-Id: I4e7268f8b9ab3b2a7fc8b510898c6fbdd685aa53

bootstrapvz/providers/gce/__init__.py

@@ -36,7 +36,7 @@ def resolve_tasks(taskset, manifest):
 	                tasks.configuration.GatherReleaseInformation,
 
 	                tasks.host.DisableIPv6,
-	                tasks.host.SetHostname,
+	                tasks.host.InstallHostnameHook,
 	                tasks.boot.ConfigureGrub,
 	                initd.InstallInitScripts,
 	                ssh.AddSSHKeyGeneration,

bootstrapvz/providers/gce/tasks/host.py

@@ -17,12 +17,25 @@ class DisableIPv6(Task):
 			print >>config_file, "net.ipv6.conf.all.disable_ipv6 = 1"
 
 
-class SetHostname(Task):
-	description = "Setting hostname"
+class InstallHostnameHook(Task):
+	description = "Installing hostname hook"
 	phase = phases.system_modification
 
 	@classmethod
 	def run(cls, info):
+		# There's a surprising amount of software out there which doesn't react well to the system
+		# hostname being set to a potentially long the fully qualified domain name, including Java 7
+		# and lower, quite relevant to a lot of cloud use cases such as Hadoop. Since Google Compute
+		# Engine's out-of-the-box domain names are long but predictable based on project name, we
+		# install this hook to set the hostname to the short hostname but add a suitable /etc/hosts
+		# entry.
+		#
+		# Since not all operating systems which Google supports on Compute Engine work with the
+		# /etc/dhcp/dhclient-exit-hooks.d directory, Google's internally-built packaging uses the
+		# consistent install path of /usr/share/google/set-hostname, and OS-specific build steps are
+		# used to activate the DHCP hook. In any future Debian-maintained distro-specific packaging,
+		# the updated deb could handle installing the below symlink or the script itself into
+		# /etc/dhcp/dhclient-exit-hooks.d.
 		log_check_call(['chroot', info.root, 'ln', '-s',
 		                '/usr/share/google/set-hostname',
 		                '/etc/dhcp/dhclient-exit-hooks.d/set-hostname'])