engine: resources, lang: Set resource fields more accurately

There were some bugs about setting resource fields that were structs
with various fields. This makes things more strict and correct. Now we
check for duplicate field names earlier (duplicates due to identical
aliases) and we also don't try and set private fields, or incorrectly
set partial structs.

Most interestingly, this also cleans up all of the resources and ensures
that each one has nicer docs and a clear struct tag for fields that we
want to use in mcl. These are mandatory now, and if you're missing the
tag, then we will ignore the field.

engine/resources/cron.go

@@ -75,6 +75,7 @@ func init() {
 }
 
 // CronRes is a systemd-timer cron resource.
+// TODO: If we want to have an actual `crond` resource, name it LegacyCron.
 type CronRes struct {
 	traits.Base
 	traits.Edgeable
@@ -86,46 +87,52 @@ type CronRes struct {
 	// Unit is the name of the systemd service unit. It is only necessary to
 	// set if you want to specify a service with a different name than the
 	// resource.
-	Unit string `yaml:"unit"`
+	Unit string `lang:"unit" yaml:"unit"`
+
 	// State must be 'exists' or 'absent'.
-	State string `yaml:"state"`
+	State string `lang:"state" yaml:"state"`
 
 	// Session, if true, creates the timer as the current user, rather than
-	// root. The service it points to must also be a user unit. It defaults to
-	// false.
-	Session bool `yaml:"session"`
+	// root. The service it points to must also be a user unit. It defaults
+	// to false.
+	Session bool `lang:"session" yaml:"session"`
 
 	// Trigger is the type of timer. Valid types are 'OnCalendar',
 	// 'OnActiveSec'. 'OnBootSec'. 'OnStartupSec'. 'OnUnitActiveSec', and
 	// 'OnUnitInactiveSec'. For more information see 'man systemd.timer'.
-	Trigger string `yaml:"trigger"`
+	Trigger string `lang:"trigger" yaml:"trigger"`
+
 	// Time must be used with all triggers. For 'OnCalendar', it must be in
 	// the format defined in 'man systemd-time' under the heading 'Calendar
 	// Events'. For all other triggers, time should be a valid time span as
 	// defined in 'man systemd-time'
-	Time string `yaml:"time"`
+	Time string `lang:"time" yaml:"time"`
 
 	// AccuracySec is the accuracy of the timer in systemd-time time span
 	// format. It defaults to one minute.
-	AccuracySec string `yaml:"accuracysec"`
+	AccuracySec string `lang:"accuracysec" yaml:"accuracysec"`
+
 	// RandomizedDelaySec delays the timer by a randomly selected, evenly
-	// distributed amount of time between 0 and the specified time value. The
-	// value must be a valid systemd-time time span.
-	RandomizedDelaySec string `yaml:"randomizeddelaysec"`
+	// distributed amount of time between 0 and the specified time value.
+	// The value must be a valid systemd-time time span.
+	RandomizedDelaySec string `lang:"randomizeddelaysec" yaml:"randomizeddelaysec"`
 
 	// Persistent, if true, means the time when the service unit was last
 	// triggered is stored on disk. When the timer is activated, the service
-	// unit is triggered immediately if it would have been triggered at least
-	// once during the time when the timer was inactive. It defaults to false.
-	Persistent bool `yaml:"persistent"`
+	// unit is triggered immediately if it would have been triggered at
+	// least once during the time when the timer was inactive. It defaults
+	// to false.
+	Persistent bool `lang:"persistent" yaml:"persistent"`
+
 	// WakeSystem, if true, will cause the system to resume from suspend,
-	// should it be suspended and if the system supports this. It defaults to
-	// false.
-	WakeSystem bool `yaml:"wakesystem"`
-	// RemainAfterElapse, if true, means an elapsed timer will stay loaded, and
-	// its state remains queriable. If false, an elapsed timer unit that cannot
-	// elapse anymore is unloaded. It defaults to true.
-	RemainAfterElapse bool `yaml:"remainafterelapse"`
+	// should it be suspended and if the system supports this. It defaults
+	// to false.
+	WakeSystem bool `lang:"wakesystem" yaml:"wakesystem"`
+
+	// RemainAfterElapse, if true, means an elapsed timer will stay loaded,
+	// and its state remains queriable. If false, an elapsed timer unit that
+	// cannot elapse anymore is unloaded. It defaults to true.
+	RemainAfterElapse bool `lang:"remainafterelapse" yaml:"remainafterelapse"`
 
 	file       *FileRes             // nested file resource
 	recWatcher *recwatch.RecWatcher // recwatcher for nested file