review: simplify how mail templates are compiled

This is nominally more code, but the code in NewCluster() repeats itself
less.

internal/core/cluster.go

@@ -22,7 +22,6 @@ package core
 import (
 	"context"
 	"slices"
-	"text/template"
 	"time"
 
 	"github.com/gophercloud/gophercloud/v2"
@@ -81,16 +80,14 @@ func NewCluster(config ClusterConfiguration) (c *Cluster, errs errext.ErrorSet)
 	}
 
 	// Create mail templates
-	confirmTPl, err := template.New("confirm").Parse(config.MailTemplates.ConfirmedCommitments.Body)
+	err := c.Config.MailTemplates.ConfirmedCommitments.Compile()
 	if err != nil {
 		errs.Addf("could not parse confirmation mail template: %w", err)
 	}
-	c.Config.MailTemplates.ConfirmedCommitments.Compiled = confirmTPl
-	expireTpl, err := template.New("expire").Parse(config.MailTemplates.ExpiringCommitments.Body)
+	err = c.Config.MailTemplates.ExpiringCommitments.Compile()
 	if err != nil {
 		errs.Addf("could not parse expiration mail template: %w", err)
 	}
-	c.Config.MailTemplates.ExpiringCommitments.Compiled = expireTpl
 
 	return c, errs
 }

internal/core/config.go

@@ -148,6 +148,7 @@ type AutogrowQuotaDistributionConfiguration struct {
 	UsageDataRetentionPeriod                  util.MarshalableTimeDuration `yaml:"usage_data_retention_period"`
 }
 
+// MailTemplateConfiguration appears in type Configuration.
 type MailTemplateConfiguration struct {
 	ConfirmedCommitments MailTemplate `yaml:"confirmed_commitments"`
 	ExpiringCommitments  MailTemplate `yaml:"expiring_commitments"`

internal/core/mail.go

@@ -52,12 +52,22 @@ type CommitmentNotification struct {
 	Resource   AZResourceLocation
 }
 
+// MailTemplate is a template for notification mails generated by Limes.
+// It appears in type MailTemplateConfiguration.
 type MailTemplate struct {
 	Subject  string             `yaml:"subject"`
 	Body     string             `yaml:"body"`
 	Compiled *template.Template `yaml:"-"` // filled during Config.Validate()
 }
 
+// Compile compiles the provided mail body template.
+// This needs to be run once before any call to Render().
+func (t *MailTemplate) Compile() (err error) {
+	t.Compiled, err = template.New("mail-body").Parse(t.Body)
+	return err
+}
+
+// Render generates a mail notification for a completed commitment workflow.
 func (t MailTemplate) Render(m CommitmentGroupNotification, projectID db.ProjectID, now time.Time) (db.MailNotification, error) {
 	if len(m.Commitments) == 0 {
 		return db.MailNotification{}, fmt.Errorf("mail: no commitments provided for projectID: %v", projectID)