From 267368fd1968774d47b9553d113f9438eeeeed0c Mon Sep 17 00:00:00 2001
From: Matthew Jacobson <matthew.jacobson@grafana.com>
Date: Fri, 10 Jan 2025 17:06:13 -0500
Subject: [PATCH] Add function that returns all default builtin template
 definitions (#260)

---
 templates/default_template_test.go | 32 ++++++++++++++++---
 templates/template_data.go         | 50 ++++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+), 4 deletions(-)

diff --git a/templates/default_template_test.go b/templates/default_template_test.go
index f6719b72..dc45a67c 100644
--- a/templates/default_template_test.go
+++ b/templates/default_template_test.go
@@ -3,12 +3,14 @@ package templates
 import (
 	"context"
 	"net/url"
+	"strings"
 	"testing"
 	"time"
 
+	"github.com/stretchr/testify/require"
+
 	"github.com/prometheus/alertmanager/types"
 	"github.com/prometheus/common/model"
-	"github.com/stretchr/testify/require"
 
 	"github.com/grafana/alerting/logging"
 )
@@ -127,6 +129,19 @@ func TestDefaultTemplateString(t *testing.T) {
 	l := &logging.FakeLogger{}
 	expand, _ := TmplText(context.Background(), tmpl, alerts, l, &tmplErr)
 
+	tmplDef, err := DefaultTemplate()
+	require.NoError(t, err)
+
+	tmplFromDefinition, err := newTemplate()
+	require.NoError(t, err)
+	// Parse default template string.
+	err = tmplFromDefinition.Parse(strings.NewReader(tmplDef.Template))
+	require.NoError(t, err)
+	tmplFromDefinition.ExternalURL = externalURL
+
+	var tmplDefErr error
+	expandFromDefinition, _ := TmplText(context.Background(), tmplFromDefinition, alerts, l, &tmplDefErr)
+
 	cases := []struct {
 		templateString string
 		expected       string
@@ -305,10 +320,19 @@ Silence: [http://localhost/grafana/alerting/silence/new?alertmanager=grafana&mat
 
 	for _, c := range cases {
 		t.Run(c.templateString, func(t *testing.T) {
-			act := expand(c.templateString)
-			require.NoError(t, tmplErr)
-			require.Equal(t, c.expected, act)
+			t.Run("FromContent", func(t *testing.T) {
+				act := expand(c.templateString)
+				require.NoError(t, tmplErr)
+				require.Equal(t, c.expected, act)
+			})
+
+			t.Run("DefaultTemplate", func(t *testing.T) {
+				act := expandFromDefinition(c.templateString)
+				require.NoError(t, tmplDefErr)
+				require.Equal(t, c.expected, act)
+			})
 		})
 	}
 	require.NoError(t, tmplErr)
+	require.NoError(t, tmplDefErr)
 }
diff --git a/templates/template_data.go b/templates/template_data.go
index 0e0a6bf4..f9b45262 100644
--- a/templates/template_data.go
+++ b/templates/template_data.go
@@ -3,10 +3,14 @@ package templates
 import (
 	"context"
 	"encoding/json"
+	"fmt"
+	tmplhtml "html/template"
 	"net/url"
 	"path"
+	"slices"
 	"sort"
 	"strings"
+	tmpltext "text/template"
 	"time"
 
 	"github.com/go-kit/log"
@@ -65,6 +69,52 @@ type ExtendedData struct {
 	ExternalURL string `json:"externalURL"`
 }
 
+var DefaultTemplateName = "__default__"
+
+// DefaultTemplate returns a new Template with all default templates parsed.
+func DefaultTemplate(options ...template.Option) (TemplateDefinition, error) {
+	// We cannot simply append the text of each default file together as there can be (and are) duplicate template
+	// names. Duplicate templates should override when parsed from separate files but will fail to parse if both are in
+	// the same file.
+	// So, instead we allow tmpltext to combine the templates and then convert it to a string afterwards.
+	// The underlying template is not accessible, so we capture it via template.Option.
+	var newTextTmpl *tmpltext.Template
+	var captureTemplate template.Option = func(text *tmpltext.Template, _ *tmplhtml.Template) {
+		newTextTmpl = text
+	}
+
+	// Call FromContent without any user-provided templates to get the combined default template.
+	_, err := FromContent(nil, append(options, captureTemplate)...)
+	if err != nil {
+		return TemplateDefinition{}, err
+	}
+
+	var combinedTemplate strings.Builder
+	tmpls := newTextTmpl.Templates()
+	// Sort for a consistent order.
+	slices.SortFunc(tmpls, func(a, b *tmpltext.Template) int {
+		return strings.Compare(a.Name(), b.Name())
+	})
+
+	// Recreate the "define" blocks for all templates. Would be nice to have a more direct way to do this.
+	for _, tmpl := range tmpls {
+		if tmpl.Name() != "" {
+			def := tmpl.Tree.Root.String()
+			if tmpl.Name() == "__text_values_list" {
+				// Temporary fix for https://github.com/golang/go/commit/6fea4094242fe4e7be8bd7ec0b55df9f6df3f025.
+				// TODO: Can remove with GO v1.24.
+				def = strings.Replace(def, "$first := false", "$first = false", 1)
+			}
+
+			combinedTemplate.WriteString(fmt.Sprintf("{{ define \"%s\" }}%s{{ end }}\n\n", tmpl.Name(), def))
+		}
+	}
+	return TemplateDefinition{
+		Name:     DefaultTemplateName,
+		Template: combinedTemplate.String(),
+	}, nil
+}
+
 // FromContent calls Parse on all provided template content and returns the resulting Template. Content equivalent to templates.FromGlobs.
 func FromContent(tmpls []string, options ...template.Option) (*Template, error) {
 	t, err := newTemplate(options...)