Use persistent paths for mkswap (1%)

Differential Revision: D64398147

fbshipit-source-id: f49896d0a6234544807c1c94306e7ce3d1c4d082

cookbooks/fb_swap/README.md

@@ -24,6 +24,7 @@ Attributes
 * node['fb_swap']['filesystem']
 * node['fb_swap']['strict']
 * node['fb_swap']['allow_unmanaged']
+* node['fb_swap']['use_persistent_paths']
 
 Usage
 -----
@@ -97,3 +98,7 @@ You can disable the sanity check which looks for unmanaged swap devices via:
 ```ruby
 node.default['fb_swap']['allow_unmanaged'] = true
 ```
+
+### use_persistent_paths
+When set to true, we will persist drive identifiers that can survive a reboot
+based on `/dev/disk/by-id`

cookbooks/fb_swap/attributes/default.rb

@@ -24,4 +24,5 @@
   'swapoff_allowed_because' => nil,
   'strict' => true,
   'allow_unmanaged' => false,
+  'use_persistent_paths' => false,
 }

cookbooks/fb_swap/libraries/default.rb

@@ -18,6 +18,8 @@
 
 module FB
   module FbSwap
+    DEV_ID_DIR = '/dev/disk/by-id'.freeze
+
     def self._validate(node)
       device = self._device(node)
       file = self._file(node)
@@ -264,6 +266,36 @@ def self._device(node)
       end
     end
 
+    def self._device_id_map
+      # Create an ID map from device names to identifiers
+      # Sort resolvers to ensure idempotency, as there can be
+      # multiple symlinks pointing to the same raw device
+      id_map = {}
+
+      Dir.open(DEV_ID_DIR).sort.each do |entry|
+        next if %w{. ..}.include?(entry)
+
+        p = "#{DEV_ID_DIR}/#{entry}"
+        id_map[File.basename(File.readlink(p))] = entry
+      end
+      return id_map
+    end
+
+    # Not using persistent paths is sensitive to
+    # changes in device enumeration. The kernel
+    # provides no guarantees here
+    def self._get_persistent_device_path(node)
+      device = self._device(node)
+      if node['fb_swap']['use_persistent_paths']
+        swap_dev = device.delete_prefix('/dev/')
+        device_id_map = self._device_id_map
+        device_id = device_id_map[swap_dev]
+        return "#{DEV_ID_DIR}/#{device_id}"
+      else
+        return device
+      end
+    end
+
     def self._file(node)
       "#{self._filesystem(node)}swapvol/swapfile"
     end

cookbooks/fb_swap/templates/default/manage-swap-device.service.erb

@@ -1,15 +1,8 @@
-[Unit]
-Description=Format size of swap device
-DefaultDependencies=no
-Requires=-.mount
-After=-.mount
-
-[Service]
-Type=oneshot
-RemainAfterExit=yes
 <%
 require 'shellwords'
+
 device = FB::FbSwap._device(node)
+device_path = FB::FbSwap._get_persistent_device_path(node)
 # if there's an existing device, we should round-trip the existing label and
 # uuid. This way if this code stops being used, and we fall back on the entry
 # in fstab, we'll still find the device using the same predicate as before.
@@ -20,9 +13,21 @@ uuid = info['uuid']
 # still very close to correct.
 options = label ? "--label #{Shellwords.escape(label)} " : ''
 options = "#{options}--uuid #{Shellwords.escape(uuid)} " if uuid
+requires = FB::Systemd.path_to_unit(device_path, 'device')
 -%>
+
+[Unit]
+Description=Format size of swap device
+DefaultDependencies=no
+Requires=<%= requires %>
+After=<%= requires %>
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+
 <% if node['fb_swap']['enabled'] -%>
-ExecStart=/usr/sbin/mkswap <%= options %><%= device %> <%= node['fb_swap']['_calculated']['device_size_bytes'] / 1024 %>
+ExecStart=/usr/sbin/mkswap <%= options %><%= device_path %> <%= node['fb_swap']['_calculated']['device_size_bytes'] / 1024 %>
 <% else -%>
 ExecStart=/usr/bin/echo "Not formatting swap when disabled"
 <% end -%>