Initial hotplug support (#936)

Adds a udev script which will invoke a hotplug hook script on all net
add events. The script will write some udev arguments to a systemd FIFO
socket (to ensure we have only instance of cloud-init running at a
time), which is then read by a new service that calls a new 'cloud-init
devel hotplug-hook' command to handle the new event.

This hotplug-hook command will:
- Fetch the pickled datsource
- Verify that the hotplug event is supported/enabled
- Update the metadata for the datasource
- Ensure the hotplugged device exists within the datasource
- Apply the config change on the datasource metadata
- Bring up the new interface (or apply global network configuration)
- Save the updated metadata back to the pickle cache

Also scattered in some unrelated typing where helpful

3 files changed, 35 insertions, 0 deletions

diff --git a/systemd/cloud-init-generator.tmpl b/systemd/cloud-init-generator.tmpl
index 0713db16..0713db16 100755..100644
--- a/systemd/cloud-init-generator.tmpl
+++ b/systemd/cloud-init-generator.tmpl
diff --git a/systemd/cloud-init-hotplugd.service b/systemd/cloud-init-hotplugd.service
new file mode 100644
index 00000000..b64632ef
--- /dev/null
+++ b/systemd/cloud-init-hotplugd.service
@@ -0,0 +1,22 @@
+# Paired with cloud-init-hotplugd.socket to read from the FIFO
+# /run/cloud-init/hook-hotplug-cmd which is created during a udev network
+# add or remove event as processed by 10-cloud-init-hook-hotplug.rules.
+
+# On start, read args from the FIFO, process and provide structured arguments
+# to `cloud-init devel hotplug-hook` which will setup or teardown network
+# devices as configured by user-data.
+
+# Known bug with an enforcing SELinux policy: LP: #1936229
+# cloud-init-hotplud.service will read args from file descriptor 3
+
+[Unit]
+Description=cloud-init hotplug hook daemon
+After=cloud-init-hotplugd.socket
+
+[Service]
+Type=simple
+ExecStart=/bin/bash -c 'read args <&3; echo "args=$args"; \
+                        exec /usr/bin/cloud-init devel hotplug-hook $args; \
+                        exit 0'
+SyslogIdentifier=cloud-init-hotplugd
+TimeoutStopSec=5
diff --git a/systemd/cloud-init-hotplugd.socket b/systemd/cloud-init-hotplugd.socket
new file mode 100644
index 00000000..aa093016
--- /dev/null
+++ b/systemd/cloud-init-hotplugd.socket
@@ -0,0 +1,13 @@
+# cloud-init-hotplugd.socket listens on the FIFO file
+# /run/cloud-init/hook-hotplug-cmd which is created during a udev network
+# add or remove event as processed by 10-cloud-init-hook-hotplug.rules.
+
+# Known bug with an enforcing SELinux policy: LP: #1936229
+[Unit]
+Description=cloud-init hotplug hook socket
+
+[Socket]
+ListenFIFO=/run/cloud-init/hook-hotplug-cmd
+
+[Install]
+WantedBy=cloud-init.target