From 7a5ff6dd78c74177ea05fec303128380719d0c5b Mon Sep 17 00:00:00 2001
From: Eric Norris <enorris@etsy.com>
Date: Fri, 8 Nov 2024 15:55:21 -0500
Subject: [PATCH] feat: lazy timer commands

Before this commit, it was not possible to lazily declare an fb_timers
entry that utilized a node attribute inside the command.

After this commit, specifying a Proc for `command` or entries in
`commands` will cause fb_timers to evaluate the proc when rendering the
service template.
---
 cookbooks/fb_timers/README.md                     |  3 ++-
 cookbooks/fb_timers/spec/default_spec.rb          |  4 ++++
 .../fb_timers/spec/fixtures/default/lazy.service  | 12 ++++++++++++
 .../fb_timers/spec/fixtures/default/lazy.timer    | 15 +++++++++++++++
 cookbooks/fb_timers/templates/default/service.erb |  2 +-
 5 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 cookbooks/fb_timers/spec/fixtures/default/lazy.service
 create mode 100644 cookbooks/fb_timers/spec/fixtures/default/lazy.timer

diff --git a/cookbooks/fb_timers/README.md b/cookbooks/fb_timers/README.md
index 5c363f962..6274a0b07 100644
--- a/cookbooks/fb_timers/README.md
+++ b/cookbooks/fb_timers/README.md
@@ -65,7 +65,8 @@ Required fields:
   when you want your job to run. Corresponds to the `OnCalendar` field of the
   systemd timer. See below for helpers to generate common calendar patterns.
 * `command`: The command to run. Corresponds to the `ExecStart` field of the
-  systemd service.
+  systemd service. Specify a Proc to lazily evaluate the command string, useful
+  for an attribute-driven command.
 * `commands`: The commands to run. Will generate several `ExecStart` lines.
   Useful if you want to run multiple commands in sequence, without forking to
   bash. Mixing `commands` and `command` will produce a warning, but the
diff --git a/cookbooks/fb_timers/spec/default_spec.rb b/cookbooks/fb_timers/spec/default_spec.rb
index a28f578cf..360745dae 100644
--- a/cookbooks/fb_timers/spec/default_spec.rb
+++ b/cookbooks/fb_timers/spec/default_spec.rb
@@ -156,6 +156,10 @@
             'command' => '/usr/local/bin/foobar.sh',
             'timer_options' => { 'OnBootSec' => '1s' },
           },
+          'lazy' => {
+            'calendar' => '*:0/15:0',
+            'command' => proc { '/usr/local/bin/foobar.sh' },
+          },
         }
       end
     end
diff --git a/cookbooks/fb_timers/spec/fixtures/default/lazy.service b/cookbooks/fb_timers/spec/fixtures/default/lazy.service
new file mode 100644
index 000000000..d5e8fd060
--- /dev/null
+++ b/cookbooks/fb_timers/spec/fixtures/default/lazy.service
@@ -0,0 +1,12 @@
+# This file managed by chef.
+# Local changes to this file will be overwritten.
+
+[Unit]
+Description=Run scheduled task lazy
+After=network.target
+
+[Service]
+Type=oneshot
+Slice=system-timers-lazy.slice
+ExecStart=/usr/local/bin/foobar.sh
+TimeoutStopSec=90s
diff --git a/cookbooks/fb_timers/spec/fixtures/default/lazy.timer b/cookbooks/fb_timers/spec/fixtures/default/lazy.timer
new file mode 100644
index 000000000..46ddb6dbe
--- /dev/null
+++ b/cookbooks/fb_timers/spec/fixtures/default/lazy.timer
@@ -0,0 +1,15 @@
+# This file managed by chef.
+# Local changes to this file will be overwritten.
+
+[Unit]
+Description=Run scheduled task lazy
+
+[Install]
+WantedBy=timers.target
+
+[Timer]
+OnCalendar=*:0/15:0
+AccuracySec=1s
+Persistent=false
+RandomizedDelaySec=0s
+Unit=lazy.service
diff --git a/cookbooks/fb_timers/templates/default/service.erb b/cookbooks/fb_timers/templates/default/service.erb
index 717de165f..0e6c38bb0 100644
--- a/cookbooks/fb_timers/templates/default/service.erb
+++ b/cookbooks/fb_timers/templates/default/service.erb
@@ -26,7 +26,7 @@ EnvironmentFile=<%= @conf['envfile'] %>
 Slice=system-timers-<%= @conf['name'] %>.slice
 <% end %>
 <% @conf['commands'].each do |command| %>
-ExecStart=<%= command %>
+ExecStart=<%= if command.instance_of?(Proc) then command.call() else command end %>
 <% end %>
 <% if @conf['timeout'] %>
 TimeoutStartSec=<%= @conf['timeout'] %>