Group template helper functions, remove `Printf`, improve template error messages (#23982)

Follow #23328 


Major changes:

* Group the function in `templates/help.go` by their purposes. It could
make future work easier.
* Remove the `Printf` helper function, there is already a builtin
`printf`.
* Remove `DiffStatsWidth`, replace with `Eval` in template
* Rename the `NewTextFuncMap` to `mailSubjectTextFuncMap`, it's for
subject text template only, no need to make it support HTML functions.


----

And fine tune template error messages, to make it more friendly to
developers and users.


![image](https://user-images.githubusercontent.com/2114189/230714245-4fd202d1-2b25-41b2-8be5-03c5fee45091.png)


![image](https://user-images.githubusercontent.com/2114189/230714277-66783577-2a03-49d5-8e8c-ceba5e07a2d4.png)

---------

Co-authored-by: silverwind <me@silverwind.io>

15 files changed, 297 insertions, 285 deletions

diff --git a/modules/context/context.go b/modules/context/context.go
index 04f8a9bd3d..bd561be0f5 100644
--- a/modules/context/context.go
+++ b/modules/context/context.go
@@ -47,7 +47,7 @@ import (
 
 // Render represents a template render
 type Render interface {
-	TemplateLookup(tmpl string) *template.Template
+	TemplateLookup(tmpl string) (*template.Template, error)
 	HTML(w io.Writer, status int, name string, data interface{}) error
 }
 
@@ -228,7 +228,7 @@ func (ctx *Context) HTML(status int, name base.TplName) {
 	}
 	if err := ctx.Render.HTML(ctx.Resp, status, string(name), templates.BaseVars().Merge(ctx.Data)); err != nil {
 		if status == http.StatusInternalServerError && name == base.TplName("status/500") {
-			ctx.PlainText(http.StatusInternalServerError, "Unable to find status/500 template")
+			ctx.PlainText(http.StatusInternalServerError, "Unable to find HTML templates, the template system is not initialized, or Gitea can't find your template files.")
 			return
 		}
 		if execErr, ok := err.(texttemplate.ExecError); ok {
@@ -247,7 +247,7 @@ func (ctx *Context) HTML(status int, name base.TplName) {
 				if errorTemplateName != string(name) {
 					filename += " (subtemplate of " + string(name) + ")"
 				}
-				err = fmt.Errorf("%w\nin template file %s:\n%s", err, filename, templates.GetLineFromTemplate(errorTemplateName, line, target, pos))
+				err = fmt.Errorf("failed to render %s, error: %w:\n%s", filename, err, templates.GetLineFromTemplate(errorTemplateName, line, target, pos))
 			} else {
 				filename, filenameErr := templates.GetAssetFilename("templates/" + execErr.Name + ".tmpl")
 				if filenameErr != nil {
@@ -256,7 +256,7 @@ func (ctx *Context) HTML(status int, name base.TplName) {
 				if execErr.Name != string(name) {
 					filename += " (subtemplate of " + string(name) + ")"
 				}
-				err = fmt.Errorf("%w\nin template file %s", err, filename)
+				err = fmt.Errorf("failed to render %s, error: %w", filename, err)
 			}
 		}
 		ctx.ServerError("Render failed", err)
diff --git a/modules/templates/dynamic.go b/modules/templates/dynamic.go
index 7d25a61fed..2f4f542e72 100644
--- a/modules/templates/dynamic.go
+++ b/modules/templates/dynamic.go
@@ -6,20 +6,13 @@
 package templates
 
 import (
-	"html/template"
 	"io/fs"
 	"os"
 	"path/filepath"
-	texttmpl "text/template"
 
 	"code.gitea.io/gitea/modules/setting"
 )
 
-var (
-	subjectTemplates = texttmpl.New("")
-	bodyTemplates    = template.New("")
-)
-
 // GetAsset returns asset content via name
 func GetAsset(name string) ([]byte, error) {
 	bs, err := os.ReadFile(filepath.Join(setting.CustomPath, name))
diff --git a/modules/templates/helper.go b/modules/templates/helper.go
index 407b2c64f5..40a79d9578 100644
--- a/modules/templates/helper.go
+++ b/modules/templates/helper.go
@@ -18,7 +18,6 @@ import (
 	"reflect"
 	"regexp"
 	"strings"
-	texttmpl "text/template"
 	"time"
 	"unicode"
 
@@ -55,6 +54,134 @@ var mailSubjectSplit = regexp.MustCompile(`(?m)^-{3,}[\s]*$`)
 // NewFuncMap returns functions for injecting to templates
 func NewFuncMap() []template.FuncMap {
 	return []template.FuncMap{map[string]interface{}{
+		// -----------------------------------------------------------------
+		// html/template related functions
+		"dict":        dict, // it's lowercase because this name has been widely used. Our other functions should have uppercase names.
+		"Eval":        Eval,
+		"Safe":        Safe,
+		"Escape":      html.EscapeString,
+		"QueryEscape": url.QueryEscape,
+		"JSEscape":    template.JSEscapeString,
+		"Str2html":    Str2html, // TODO: rename it to SanitizeHTML
+		"URLJoin":     util.URLJoin,
+
+		"PathEscape":         url.PathEscape,
+		"PathEscapeSegments": util.PathEscapeSegments,
+
+		// -----------------------------------------------------------------
+		// string / json
+		"Join":           strings.Join,
+		"DotEscape":      DotEscape,
+		"HasPrefix":      strings.HasPrefix,
+		"EllipsisString": base.EllipsisString,
+
+		"Json": func(in interface{}) string {
+			out, err := json.Marshal(in)
+			if err != nil {
+				return ""
+			}
+			return string(out)
+		},
+		"JsonPrettyPrint": func(in string) string {
+			var out bytes.Buffer
+			err := json.Indent(&out, []byte(in), "", "  ")
+			if err != nil {
+				return ""
+			}
+			return out.String()
+		},
+
+		// -----------------------------------------------------------------
+		// svg / avatar / icon
+		"svg":            svg.RenderHTML,
+		"avatar":         Avatar,
+		"avatarHTML":     AvatarHTML,
+		"avatarByAction": AvatarByAction,
+		"avatarByEmail":  AvatarByEmail,
+		"repoAvatar":     RepoAvatar,
+		"EntryIcon":      base.EntryIcon,
+		"MigrationIcon":  MigrationIcon,
+		"ActionIcon":     ActionIcon,
+
+		"SortArrow": func(normSort, revSort, urlSort string, isDefault bool) template.HTML {
+			// if needed
+			if len(normSort) == 0 || len(urlSort) == 0 {
+				return ""
+			}
+
+			if len(urlSort) == 0 && isDefault {
+				// if sort is sorted as default add arrow tho this table header
+				if isDefault {
+					return svg.RenderHTML("octicon-triangle-down", 16)
+				}
+			} else {
+				// if sort arg is in url test if it correlates with column header sort arguments
+				// the direction of the arrow should indicate the "current sort order", up means ASC(normal), down means DESC(rev)
+				if urlSort == normSort {
+					// the table is sorted with this header normal
+					return svg.RenderHTML("octicon-triangle-up", 16)
+				} else if urlSort == revSort {
+					// the table is sorted with this header reverse
+					return svg.RenderHTML("octicon-triangle-down", 16)
+				}
+			}
+			// the table is NOT sorted with this header
+			return ""
+		},
+
+		// -----------------------------------------------------------------
+		// time / number / format
+		"FileSize":      base.FileSize,
+		"LocaleNumber":  LocaleNumber,
+		"CountFmt":      base.FormatNumberSI,
+		"TimeSince":     timeutil.TimeSince,
+		"TimeSinceUnix": timeutil.TimeSinceUnix,
+		"Sec2Time":      util.SecToTime,
+		"DateFmtLong": func(t time.Time) string {
+			return t.Format(time.RFC1123Z)
+		},
+		"LoadTimes": func(startTime time.Time) string {
+			return fmt.Sprint(time.Since(startTime).Nanoseconds()/1e6) + "ms"
+		},
+
+		// -----------------------------------------------------------------
+		// slice
+		"containGeneric": func(arr, v interface{}) bool {
+			arrV := reflect.ValueOf(arr)
+			if arrV.Kind() == reflect.String && reflect.ValueOf(v).Kind() == reflect.String {
+				return strings.Contains(arr.(string), v.(string))
+			}
+			if arrV.Kind() == reflect.Slice {
+				for i := 0; i < arrV.Len(); i++ {
+					iV := arrV.Index(i)
+					if !iV.CanInterface() {
+						continue
+					}
+					if iV.Interface() == v {
+						return true
+					}
+				}
+			}
+			return false
+		},
+		"contain": func(s []int64, id int64) bool {
+			for i := 0; i < len(s); i++ {
+				if s[i] == id {
+					return true
+				}
+			}
+			return false
+		},
+		"Iterate": func(arg interface{}) (items []int64) {
+			count, _ := util.ToInt64(arg)
+			for i := int64(0); i < count; i++ {
+				items = append(items, i)
+			}
+			return items
+		},
+
+		// -----------------------------------------------------------------
+		// setting
 		"AppName": func() string {
 			return setting.AppName
 		},
@@ -89,56 +216,12 @@ func NewFuncMap() []template.FuncMap {
 		"ShowFooterTemplateLoadTime": func() bool {
 			return setting.ShowFooterTemplateLoadTime
 		},
-		"LoadTimes": func(startTime time.Time) string {
-			return fmt.Sprint(time.Since(startTime).Nanoseconds()/1e6) + "ms"
-		},
 		"AllowedReactions": func() []string {
 			return setting.UI.Reactions
 		},
 		"CustomEmojis": func() map[string]string {
 			return setting.UI.CustomEmojisMap
 		},
-		"Safe":          Safe,
-		"JSEscape":      JSEscape,
-		"Str2html":      Str2html,
-		"TimeSince":     timeutil.TimeSince,
-		"TimeSinceUnix": timeutil.TimeSinceUnix,
-		"FileSize":      base.FileSize,
-		"LocaleNumber":  LocaleNumber,
-		"EntryIcon":     base.EntryIcon,
-		"MigrationIcon": MigrationIcon,
-		"ActionIcon":    ActionIcon,
-		"DateFmtLong": func(t time.Time) string {
-			return t.Format(time.RFC1123Z)
-		},
-		"CountFmt":                       base.FormatNumberSI,
-		"EllipsisString":                 base.EllipsisString,
-		"DiffLineTypeToStr":              DiffLineTypeToStr,
-		"ShortSha":                       base.ShortSha,
-		"ActionContent2Commits":          ActionContent2Commits,
-		"PathEscape":                     url.PathEscape,
-		"PathEscapeSegments":             util.PathEscapeSegments,
-		"URLJoin":                        util.URLJoin,
-		"RenderCommitMessage":            RenderCommitMessage,
-		"RenderCommitMessageLinkSubject": RenderCommitMessageLinkSubject,
-		"RenderCommitBody":               RenderCommitBody,
-		"RenderCodeBlock":                RenderCodeBlock,
-		"RenderIssueTitle":               RenderIssueTitle,
-		"RenderEmoji":                    RenderEmoji,
-		"RenderEmojiPlain":               emoji.ReplaceAliases,
-		"ReactionToEmoji":                ReactionToEmoji,
-		"RenderNote":                     RenderNote,
-		"RenderMarkdownToHtml": func(ctx context.Context, input string) template.HTML {
-			output, err := markdown.RenderString(&markup.RenderContext{
-				Ctx:       ctx,
-				URLPrefix: setting.AppSubURL,
-			}, input)
-			if err != nil {
-				log.Error("RenderString: %v", err)
-			}
-			return template.HTML(output)
-		},
-		"IsMultilineCommitMessage": IsMultilineCommitMessage,
 		"ThemeColorMetaTag": func() string {
 			return setting.UI.ThemeColorMetaTag
 		},
@@ -157,58 +240,6 @@ func NewFuncMap() []template.FuncMap {
 		"EnableTimetracking": func() bool {
 			return setting.Service.EnableTimetracking
 		},
-		"FilenameIsImage": func(filename string) bool {
-			mimeType := mime.TypeByExtension(filepath.Ext(filename))
-			return strings.HasPrefix(mimeType, "image/")
-		},
-		"TabSizeClass": func(ec interface{}, filename string) string {
-			var (
-				value *editorconfig.Editorconfig
-				ok    bool
-			)
-			if ec != nil {
-				if value, ok = ec.(*editorconfig.Editorconfig); !ok || value == nil {
-					return "tab-size-8"
-				}
-				def, err := value.GetDefinitionForFilename(filename)
-				if err != nil {
-					log.Error("tab size class: getting definition for filename: %v", err)
-					return "tab-size-8"
-				}
-				if def.TabWidth > 0 {
-					return fmt.Sprintf("tab-size-%d", def.TabWidth)
-				}
-			}
-			return "tab-size-8"
-		},
-		"SubJumpablePath": func(str string) []string {
-			var path []string
-			index := strings.LastIndex(str, "/")
-			if index != -1 && index != len(str) {
-				path = append(path, str[0:index+1], str[index+1:])
-			} else {
-				path = append(path, str)
-			}
-			return path
-		},
-		"DiffStatsWidth": func(adds, dels int) string {
-			return fmt.Sprintf("%f", float64(adds)/(float64(adds)+float64(dels))*100)
-		},
-		"Json": func(in interface{}) string {
-			out, err := json.Marshal(in)
-			if err != nil {
-				return ""
-			}
-			return string(out)
-		},
-		"JsonPrettyPrint": func(in string) string {
-			var out bytes.Buffer
-			err := json.Indent(&out, []byte(in), "", "  ")
-			if err != nil {
-				return ""
-			}
-			return out.String()
-		},
 		"DisableGitHooks": func() bool {
 			return setting.DisableGitHooks
 		},
@@ -218,18 +249,9 @@ func NewFuncMap() []template.FuncMap {
 		"DisableImportLocal": func() bool {
 			return !setting.ImportLocalPaths
 		},
-		"Printf":   fmt.Sprintf,
-		"Escape":   Escape,
-		"Sec2Time": util.SecToTime,
-		"ParseDeadline": func(deadline string) []string {
-			return strings.Split(deadline, "|")
-		},
 		"DefaultTheme": func() string {
 			return setting.UI.DefaultTheme
 		},
-		"dict":                dict,
-		"CommentMustAsDiff":   gitdiff.CommentMustAsDiff,
-		"MirrorRemoteAddress": mirrorRemoteAddress,
 		"NotificationSettings": func() map[string]interface{} {
 			return map[string]interface{}{
 				"MinTimeout":            int(setting.UI.Notification.MinTimeout / time.Millisecond),
@@ -238,64 +260,32 @@ func NewFuncMap() []template.FuncMap {
 				"EventSourceUpdateTime": int(setting.UI.Notification.EventSourceUpdateTime / time.Millisecond),
 			}
 		},
-		"containGeneric": func(arr, v interface{}) bool {
-			arrV := reflect.ValueOf(arr)
-			if arrV.Kind() == reflect.String && reflect.ValueOf(v).Kind() == reflect.String {
-				return strings.Contains(arr.(string), v.(string))
-			}
+		"MermaidMaxSourceCharacters": func() int {
+			return setting.MermaidMaxSourceCharacters
+		},
 
-			if arrV.Kind() == reflect.Slice {
-				for i := 0; i < arrV.Len(); i++ {
-					iV := arrV.Index(i)
-					if !iV.CanInterface() {
-						continue
-					}
-					if iV.Interface() == v {
-						return true
-					}
-				}
-			}
+		// -----------------------------------------------------------------
+		// render
+		"RenderCommitMessage":            RenderCommitMessage,
+		"RenderCommitMessageLinkSubject": RenderCommitMessageLinkSubject,
 
-			return false
-		},
-		"contain": func(s []int64, id int64) bool {
-			for i := 0; i < len(s); i++ {
-				if s[i] == id {
-					return true
-				}
-			}
-			return false
-		},
-		"svg":            svg.RenderHTML,
-		"avatar":         Avatar,
-		"avatarHTML":     AvatarHTML,
-		"avatarByAction": AvatarByAction,
-		"avatarByEmail":  AvatarByEmail,
-		"repoAvatar":     RepoAvatar,
-		"SortArrow": func(normSort, revSort, urlSort string, isDefault bool) template.HTML {
-			// if needed
-			if len(normSort) == 0 || len(urlSort) == 0 {
-				return ""
-			}
+		"RenderCommitBody": RenderCommitBody,
+		"RenderCodeBlock":  RenderCodeBlock,
+		"RenderIssueTitle": RenderIssueTitle,
+		"RenderEmoji":      RenderEmoji,
+		"RenderEmojiPlain": emoji.ReplaceAliases,
+		"ReactionToEmoji":  ReactionToEmoji,
+		"RenderNote":       RenderNote,
 
-			if len(urlSort) == 0 && isDefault {
-				// if sort is sorted as default add arrow tho this table header
-				if isDefault {
-					return svg.RenderHTML("octicon-triangle-down", 16)
-				}
-			} else {
-				// if sort arg is in url test if it correlates with column header sort arguments
-				// the direction of the arrow should indicate the "current sort order", up means ASC(normal), down means DESC(rev)
-				if urlSort == normSort {
-					// the table is sorted with this header normal
-					return svg.RenderHTML("octicon-triangle-up", 16)
-				} else if urlSort == revSort {
-					// the table is sorted with this header reverse
-					return svg.RenderHTML("octicon-triangle-down", 16)
-				}
+		"RenderMarkdownToHtml": func(ctx context.Context, input string) template.HTML {
+			output, err := markdown.RenderString(&markup.RenderContext{
+				Ctx:       ctx,
+				URLPrefix: setting.AppSubURL,
+			}, input)
+			if err != nil {
+				log.Error("RenderString: %v", err)
 			}
-			// the table is NOT sorted with this header
-			return ""
+			return template.HTML(output)
 		},
 		"RenderLabel": func(ctx context.Context, label *issues_model.Label) template.HTML {
 			return template.HTML(RenderLabel(ctx, label))
@@ -313,20 +303,53 @@ func NewFuncMap() []template.FuncMap {
 			htmlCode += "</span>"
 			return template.HTML(htmlCode)
 		},
-		"MermaidMaxSourceCharacters": func() int {
-			return setting.MermaidMaxSourceCharacters
+
+		// -----------------------------------------------------------------
+		// misc
+		"DiffLineTypeToStr":        DiffLineTypeToStr,
+		"ShortSha":                 base.ShortSha,
+		"ActionContent2Commits":    ActionContent2Commits,
+		"IsMultilineCommitMessage": IsMultilineCommitMessage,
+		"CommentMustAsDiff":        gitdiff.CommentMustAsDiff,
+		"MirrorRemoteAddress":      mirrorRemoteAddress,
+
+		"ParseDeadline": func(deadline string) []string {
+			return strings.Split(deadline, "|")
 		},
-		"Join":        strings.Join,
-		"QueryEscape": url.QueryEscape,
-		"DotEscape":   DotEscape,
-		"Iterate": func(arg interface{}) (items []int64) {
-			count, _ := util.ToInt64(arg)
-			for i := int64(0); i < count; i++ {
-				items = append(items, i)
+		"FilenameIsImage": func(filename string) bool {
+			mimeType := mime.TypeByExtension(filepath.Ext(filename))
+			return strings.HasPrefix(mimeType, "image/")
+		},
+		"TabSizeClass": func(ec interface{}, filename string) string {
+			var (
+				value *editorconfig.Editorconfig
+				ok    bool
+			)
+			if ec != nil {
+				if value, ok = ec.(*editorconfig.Editorconfig); !ok || value == nil {
+					return "tab-size-8"
+				}
+				def, err := value.GetDefinitionForFilename(filename)
+				if err != nil {
+					log.Error("tab size class: getting definition for filename: %v", err)
+					return "tab-size-8"
+				}
+				if def.TabWidth > 0 {
+					return fmt.Sprintf("tab-size-%d", def.TabWidth)
+				}
 			}
-			return items
+			return "tab-size-8"
+		},
+		"SubJumpablePath": func(str string) []string {
+			var path []string
+			index := strings.LastIndex(str, "/")
+			if index != -1 && index != len(str) {
+				path = append(path, str[0:index+1], str[index+1:])
+			} else {
+				path = append(path, str)
+			}
+			return path
 		},
-		"HasPrefix": strings.HasPrefix,
 		"CompareLink": func(baseRepo, repo *repo_model.Repository, branchName string) string {
 			var curBranch string
 			if repo.ID != baseRepo.ID {
@@ -340,45 +363,6 @@ func NewFuncMap() []template.FuncMap {
 				curBranch,
 			)
 		},
-		"Eval": Eval,
-	}}
-}
-
-// NewTextFuncMap returns functions for injecting to text templates
-// It's a subset of those used for HTML and other templates
-func NewTextFuncMap() []texttmpl.FuncMap {
-	return []texttmpl.FuncMap{map[string]interface{}{
-		"AppName": func() string {
-			return setting.AppName
-		},
-		"AppSubUrl": func() string {
-			return setting.AppSubURL
-		},
-		"AppUrl": func() string {
-			return setting.AppURL
-		},
-		"AppVer": func() string {
-			return setting.AppVer
-		},
-		"AppDomain": func() string { // documented in mail-templates.md
-			return setting.Domain
-		},
-		"TimeSince":     timeutil.TimeSince,
-		"TimeSinceUnix": timeutil.TimeSinceUnix,
-		"DateFmtLong": func(t time.Time) string {
-			return t.Format(time.RFC1123Z)
-		},
-		"EllipsisString": base.EllipsisString,
-		"URLJoin":        util.URLJoin,
-		"Printf":         fmt.Sprintf,
-		"Escape":         Escape,
-		"Sec2Time":       util.SecToTime,
-		"ParseDeadline": func(deadline string) []string {
-			return strings.Split(deadline, "|")
-		},
-		"dict":        dict,
-		"QueryEscape": url.QueryEscape,
-		"Eval":        Eval,
 	}}
 }
 
@@ -457,16 +441,6 @@ func Str2html(raw string) template.HTML {
 	return template.HTML(markup.Sanitize(raw))
 }
 
-// Escape escapes a HTML string
-func Escape(raw string) string {
-	return html.EscapeString(raw)
-}
-
-// JSEscape escapes a JS string
-func JSEscape(raw string) string {
-	return template.JSEscapeString(raw)
-}
-
 // DotEscape wraps a dots in names with ZWJ [U+200D] in order to prevent autolinkers from detecting these as urls
 func DotEscape(raw string) string {
 	return strings.ReplaceAll(raw, ".", "\u200d.\u200d")
@@ -771,25 +745,6 @@ func MigrationIcon(hostname string) string {
 	}
 }
 
-func buildSubjectBodyTemplate(stpl *texttmpl.Template, btpl *template.Template, name string, content []byte) {
-	// Split template into subject and body
-	var subjectContent []byte
-	bodyContent := content
-	loc := mailSubjectSplit.FindIndex(content)
-	if loc != nil {
-		subjectContent = content[0:loc[0]]
-		bodyContent = content[loc[1]:]
-	}
-	if _, err := stpl.New(name).
-		Parse(string(subjectContent)); err != nil {
-		log.Warn("Failed to parse template [%s/subject]: %v", name, err)
-	}
-	if _, err := btpl.New(name).
-		Parse(string(bodyContent)); err != nil {
-		log.Warn("Failed to parse template [%s/body]: %v", name, err)
-	}
-}
-
 type remoteAddress struct {
 	Address  string
 	Username string
diff --git a/modules/templates/htmlrenderer.go b/modules/templates/htmlrenderer.go
index fd985edc64..f2c818798c 100644
--- a/modules/templates/htmlrenderer.go
+++ b/modules/templates/htmlrenderer.go
@@ -6,6 +6,7 @@ package templates
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"html/template"
 	"io"
@@ -15,9 +16,11 @@ import (
 	"strconv"
 	"strings"
 	"sync/atomic"
+	texttemplate "text/template"
 
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/watcher"
 )
 
@@ -34,6 +37,8 @@ type HTMLRender struct {
 	templates atomic.Pointer[template.Template]
 }
 
+var ErrTemplateNotInitialized = errors.New("template system is not initialized, check your log for errors")
+
 func (h *HTMLRender) HTML(w io.Writer, status int, name string, data interface{}) error {
 	if respWriter, ok := w.(http.ResponseWriter); ok {
 		if respWriter.Header().Get("Content-Type") == "" {
@@ -41,11 +46,23 @@ func (h *HTMLRender) HTML(w io.Writer, status int, name string, data interface{}
 		}
 		respWriter.WriteHeader(status)
 	}
-	return h.templates.Load().ExecuteTemplate(w, name, data)
+	t, err := h.TemplateLookup(name)
+	if err != nil {
+		return texttemplate.ExecError{Name: name, Err: err}
+	}
+	return t.Execute(w, data)
 }
 
-func (h *HTMLRender) TemplateLookup(t string) *template.Template {
-	return h.templates.Load().Lookup(t)
+func (h *HTMLRender) TemplateLookup(name string) (*template.Template, error) {
+	tmpls := h.templates.Load()
+	if tmpls == nil {
+		return nil, ErrTemplateNotInitialized
+	}
+	tmpl := tmpls.Lookup(name)
+	if tmpl == nil {
+		return nil, util.ErrNotExist
+	}
+	return tmpl, nil
 }
 
 func (h *HTMLRender) CompileTemplates() error {
@@ -237,6 +254,12 @@ func GetLineFromTemplate(templateName string, targetLineNum int, target string,
 		}
 	}
 
+	// FIXME: this algorithm could provide incorrect results and mislead the developers.
+	// For example: Undefined function "file" in template .....
+	//     {{Func .file.Addition file.Deletion .file.Addition}}
+	//             ^^^^          ^(the real error is here)
+	// The pointer is added to the first one, but the second one is the real incorrect one.
+	//
 	// If there is a provided target to look for in the line add a pointer to it
 	// e.g.                                                        ^^^^^^^
 	if target != "" {
diff --git a/modules/templates/mailer.go b/modules/templates/mailer.go
index a257e7c1da..d0c49e1025 100644
--- a/modules/templates/mailer.go
+++ b/modules/templates/mailer.go
@@ -11,16 +11,53 @@ import (
 	"strings"
 	texttmpl "text/template"
 
+	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/watcher"
 )
 
+// mailSubjectTextFuncMap returns functions for injecting to text templates, it's only used for mail subject
+func mailSubjectTextFuncMap() texttmpl.FuncMap {
+	return texttmpl.FuncMap{
+		"dict": dict,
+		"Eval": Eval,
+
+		"EllipsisString": base.EllipsisString,
+		"AppName": func() string {
+			return setting.AppName
+		},
+		"AppDomain": func() string { // documented in mail-templates.md
+			return setting.Domain
+		},
+	}
+}
+
+func buildSubjectBodyTemplate(stpl *texttmpl.Template, btpl *template.Template, name string, content []byte) {
+	// Split template into subject and body
+	var subjectContent []byte
+	bodyContent := content
+	loc := mailSubjectSplit.FindIndex(content)
+	if loc != nil {
+		subjectContent = content[0:loc[0]]
+		bodyContent = content[loc[1]:]
+	}
+	if _, err := stpl.New(name).
+		Parse(string(subjectContent)); err != nil {
+		log.Warn("Failed to parse template [%s/subject]: %v", name, err)
+	}
+	if _, err := btpl.New(name).
+		Parse(string(bodyContent)); err != nil {
+		log.Warn("Failed to parse template [%s/body]: %v", name, err)
+	}
+}
+
 // Mailer provides the templates required for sending notification mails.
 func Mailer(ctx context.Context) (*texttmpl.Template, *template.Template) {
-	for _, funcs := range NewTextFuncMap() {
-		subjectTemplates.Funcs(funcs)
-	}
+	subjectTemplates := texttmpl.New("")
+	bodyTemplates := template.New("")
+
+	subjectTemplates.Funcs(mailSubjectTextFuncMap())
 	for _, funcs := range NewFuncMap() {
 		bodyTemplates.Funcs(funcs)
 	}
diff --git a/modules/test/context_tests.go b/modules/test/context_tests.go
index 5e66049535..94dbd2f290 100644
--- a/modules/test/context_tests.go
+++ b/modules/test/context_tests.go
@@ -133,8 +133,8 @@ func (rw *mockResponseWriter) Push(target string, opts *http.PushOptions) error
 
 type mockRender struct{}
 
-func (tr *mockRender) TemplateLookup(tmpl string) *template.Template {
-	return nil
+func (tr *mockRender) TemplateLookup(tmpl string) (*template.Template, error) {
+	return nil, nil
 }
 
 func (tr *mockRender) HTML(w io.Writer, status int, _ string, _ interface{}) error {
diff --git a/routers/web/auth/oauth.go b/routers/web/auth/oauth.go
index 7de63dbe94..b3c4a234c1 100644
--- a/routers/web/auth/oauth.go
+++ b/routers/web/auth/oauth.go
@@ -578,12 +578,15 @@ func GrantApplicationOAuth(ctx *context.Context) {
 
 // OIDCWellKnown generates JSON so OIDC clients know Gitea's capabilities
 func OIDCWellKnown(ctx *context.Context) {
-	t := ctx.Render.TemplateLookup("user/auth/oidc_wellknown")
+	t, err := ctx.Render.TemplateLookup("user/auth/oidc_wellknown")
+	if err != nil {
+		ctx.ServerError("unable to find template", err)
+		return
+	}
 	ctx.Resp.Header().Set("Content-Type", "application/json")
 	ctx.Data["SigningKey"] = oauth2.DefaultSigningKey
-	if err := t.Execute(ctx.Resp, ctx.Data); err != nil {
-		log.Error("%v", err)
-		ctx.Error(http.StatusInternalServerError)
+	if err = t.Execute(ctx.Resp, ctx.Data); err != nil {
+		ctx.ServerError("unable to execute template", err)
 	}
 }
 
diff --git a/routers/web/swagger_json.go b/routers/web/swagger_json.go
index 2d626c558e..1844b90d95 100644
--- a/routers/web/swagger_json.go
+++ b/routers/web/swagger_json.go
@@ -4,11 +4,8 @@
 package web
 
 import (
-	"net/http"
-
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
-	"code.gitea.io/gitea/modules/log"
 )
 
 // tplSwaggerV1Json swagger v1 json template
@@ -16,10 +13,13 @@ const tplSwaggerV1Json base.TplName = "swagger/v1_json"
 
 // SwaggerV1Json render swagger v1 json
 func SwaggerV1Json(ctx *context.Context) {
-	t := ctx.Render.TemplateLookup(string(tplSwaggerV1Json))
+	t, err := ctx.Render.TemplateLookup(string(tplSwaggerV1Json))
+	if err != nil {
+		ctx.ServerError("unable to find template", err)
+		return
+	}
 	ctx.Resp.Header().Set("Content-Type", "application/json")
-	if err := t.Execute(ctx.Resp, ctx.Data); err != nil {
-		log.Error("%v", err)
-		ctx.Error(http.StatusInternalServerError)
+	if err = t.Execute(ctx.Resp, ctx.Data); err != nil {
+		ctx.ServerError("unable to execute template", err)
 	}
 }
diff --git a/templates/repo/diff/comments.tmpl b/templates/repo/diff/comments.tmpl
index 5c26e7815f..5bcaeeb463 100644
--- a/templates/repo/diff/comments.tmpl
+++ b/templates/repo/diff/comments.tmpl
@@ -42,7 +42,7 @@
 						</div>
 					{{end}}
 				{{end}}
-				{{template "repo/issue/view_content/add_reaction" dict "ctxData" $.root "ActionURL" (Printf "%s/comments/%d/reactions" $.root.RepoLink .ID)}}
+				{{template "repo/issue/view_content/add_reaction" dict "ctxData" $.root "ActionURL" (printf "%s/comments/%d/reactions" $.root.RepoLink .ID)}}
 				{{template "repo/issue/view_content/context_menu" dict "ctxData" $.root "item" . "delete" true "issue" false "diff" true "IsCommentPoster" (and $.root.IsSigned (eq $.root.SignedUserID .PosterID))}}
 			</div>
 		</div>
@@ -60,7 +60,7 @@
 		{{$reactions := .Reactions.GroupByType}}
 		{{if $reactions}}
 			<div class="ui attached segment reactions">
-			{{template "repo/issue/view_content/reactions" dict "ctxData" $.root "ActionURL" (Printf "%s/comments/%d/reactions" $.root.RepoLink .ID) "Reactions" $reactions}}
+			{{template "repo/issue/view_content/reactions" dict "ctxData" $.root "ActionURL" (printf "%s/comments/%d/reactions" $.root.RepoLink .ID) "Reactions" $reactions}}
 			</div>
 		{{end}}
 	</div>
diff --git a/templates/repo/diff/stats.tmpl b/templates/repo/diff/stats.tmpl
index 1e466d6b46..ff7e90d29c 100644
--- a/templates/repo/diff/stats.tmpl
+++ b/templates/repo/diff/stats.tmpl
@@ -1,4 +1,5 @@
 {{Eval .file.Addition "+" .file.Deletion}}
 <span class="diff-stats-bar gt-mx-3" data-tooltip-content="{{.root.locale.Tr "repo.diff.stats_desc_file" (Eval .file.Addition "+" .file.Deletion) .file.Addition .file.Deletion | Str2html}}">
-	<div class="diff-stats-add-bar" style="width: {{DiffStatsWidth .file.Addition .file.Deletion}}%"></div>
+	{{/* if the denominator is zero, then the float result is "width: NaNpx", as before, it just works */}}
+	<div class="diff-stats-add-bar" style="width: {{Eval 100 "*" .file.Addition "/" "(" .file.Addition "+" .file.Deletion "+" 0.0 ")"}}%"></div>
 </span>
diff --git a/templates/repo/issue/view_content.tmpl b/templates/repo/issue/view_content.tmpl
index 7c94d00fe4..a5d9b0bde0 100644
--- a/templates/repo/issue/view_content.tmpl
+++ b/templates/repo/issue/view_content.tmpl
@@ -64,7 +64,7 @@
 								{{end}}
 							{{end}}
 							{{if not $.Repository.IsArchived}}
-								{{template "repo/issue/view_content/add_reaction" dict "ctxData" $ "ActionURL" (Printf "%s/issues/%d/reactions" $.RepoLink .Issue.Index)}}
+								{{template "repo/issue/view_content/add_reaction" dict "ctxData" $ "ActionURL" (printf "%s/issues/%d/reactions" $.RepoLink .Issue.Index)}}
 								{{template "repo/issue/view_content/context_menu" dict "ctxData" $ "item" .Issue "delete" false "issue" true "diff" false "IsCommentPoster" $.IsIssuePoster}}
 							{{end}}
 						</div>
@@ -86,7 +86,7 @@
 					{{$reactions := .Issue.Reactions.GroupByType}}
 					{{if $reactions}}
 						<div class="ui attached segment reactions" role="note">
-							{{template "repo/issue/view_content/reactions" dict "ctxData" $ "ActionURL" (Printf "%s/issues/%d/reactions" $.RepoLink .Issue.Index) "Reactions" $reactions}}
+							{{template "repo/issue/view_content/reactions" dict "ctxData" $ "ActionURL" (printf "%s/issues/%d/reactions" $.RepoLink .Issue.Index) "Reactions" $reactions}}
 						</div>
 					{{end}}
 				</div>
diff --git a/templates/repo/issue/view_content/comments.tmpl b/templates/repo/issue/view_content/comments.tmpl
index 52b453d0dd..91bef4fe1a 100644
--- a/templates/repo/issue/view_content/comments.tmpl
+++ b/templates/repo/issue/view_content/comments.tmpl
@@ -64,7 +64,7 @@
 								</div>
 							{{end}}
 							{{if not $.Repository.IsArchived}}
-								{{template "repo/issue/view_content/add_reaction" dict "ctxData" $ "ActionURL" (Printf "%s/comments/%d/reactions" $.RepoLink .ID)}}
+								{{template "repo/issue/view_content/add_reaction" dict "ctxData" $ "ActionURL" (printf "%s/comments/%d/reactions" $.RepoLink .ID)}}
 								{{template "repo/issue/view_content/context_menu" dict "ctxData" $ "item" . "delete" true "issue" true "diff" false "IsCommentPoster" (and $.IsSigned (eq $.SignedUserID .PosterID))}}
 							{{end}}
 						</div>
@@ -86,7 +86,7 @@
 					{{$reactions := .Reactions.GroupByType}}
 					{{if $reactions}}
 						<div class="ui attached segment reactions" role="note">
-							{{template "repo/issue/view_content/reactions" dict "ctxData" $ "ActionURL" (Printf "%s/comments/%d/reactions" $.RepoLink .ID) "Reactions" $reactions}}
+							{{template "repo/issue/view_content/reactions" dict "ctxData" $ "ActionURL" (printf "%s/comments/%d/reactions" $.RepoLink .ID) "Reactions" $reactions}}
 						</div>
 					{{end}}
 				</div>
@@ -436,7 +436,7 @@
 										</div>
 									{{end}}
 									{{if not $.Repository.IsArchived}}
-											{{template "repo/issue/view_content/add_reaction" dict "ctxData" $ "ActionURL" (Printf "%s/comments/%d/reactions" $.RepoLink .ID)}}
+											{{template "repo/issue/view_content/add_reaction" dict "ctxData" $ "ActionURL" (printf "%s/comments/%d/reactions" $.RepoLink .ID)}}
 											{{template "repo/issue/view_content/context_menu" dict "ctxData" $ "item" . "delete" false "issue" true "diff" false "IsCommentPoster" (and $.IsSigned (eq $.SignedUserID .PosterID))}}
 									{{end}}
 							</div>
@@ -458,7 +458,7 @@
 						{{$reactions := .Reactions.GroupByType}}
 						{{if $reactions}}
 							<div class="ui attached segment reactions">
-									{{template "repo/issue/view_content/reactions" dict "ctxData" $ "ActionURL" (Printf "%s/comments/%d/reactions" $.RepoLink .ID) "Reactions" $reactions}}
+									{{template "repo/issue/view_content/reactions" dict "ctxData" $ "ActionURL" (printf "%s/comments/%d/reactions" $.RepoLink .ID) "Reactions" $reactions}}
 							</div>
 						{{end}}
 					</div>
@@ -563,7 +563,7 @@
 																	</div>
 																{{end}}
 																{{if not $.Repository.IsArchived}}
-																	{{template "repo/issue/view_content/add_reaction" dict "ctxData" $ "ActionURL" (Printf "%s/comments/%d/reactions" $.RepoLink .ID)}}
+																	{{template "repo/issue/view_content/add_reaction" dict "ctxData" $ "ActionURL" (printf "%s/comments/%d/reactions" $.RepoLink .ID)}}
 																	{{template "repo/issue/view_content/context_menu" dict "ctxData" $ "item" . "delete" true "issue" true "diff" true "IsCommentPoster" (and $.IsSigned (eq $.SignedUserID .PosterID))}}
 																{{end}}
 															</div>
@@ -582,7 +582,7 @@
 														{{$reactions := .Reactions.GroupByType}}
 														{{if $reactions}}
 															<div class="ui attached segment reactions">
-																{{template "repo/issue/view_content/reactions" dict "ctxData" $ "ActionURL" (Printf "%s/comments/%d/reactions" $.RepoLink .ID) "Reactions" $reactions}}
+																{{template "repo/issue/view_content/reactions" dict "ctxData" $ "ActionURL" (printf "%s/comments/%d/reactions" $.RepoLink .ID) "Reactions" $reactions}}
 															</div>
 														{{end}}
 													</div>
diff --git a/templates/repo/issue/view_content/context_menu.tmpl b/templates/repo/issue/view_content/context_menu.tmpl
index 0cc207afd4..aa9d52b67a 100644
--- a/templates/repo/issue/view_content/context_menu.tmpl
+++ b/templates/repo/issue/view_content/context_menu.tmpl
@@ -6,9 +6,9 @@
 	<div class="menu">
 		{{$referenceUrl := ""}}
 		{{if .issue}}
-			{{$referenceUrl = Printf "%s#%s" .ctxData.Issue.Link .item.HashTag}}
+			{{$referenceUrl = printf "%s#%s" .ctxData.Issue.Link .item.HashTag}}
 		{{else}}
-			{{$referenceUrl = Printf "%s/files#%s" .ctxData.Issue.Link .item.HashTag}}
+			{{$referenceUrl = printf "%s/files#%s" .ctxData.Issue.Link .item.HashTag}}
 		{{end}}
 		<div class="item context js-aria-clickable" data-clipboard-text-type="url" data-clipboard-text="{{AppSubUrl}}{{$referenceUrl}}">{{.ctxData.locale.Tr "repo.issues.context.copy_link"}}</div>
 		<div class="item context js-aria-clickable quote-reply {{if .diff}}quote-reply-diff{{end}}" data-target="{{.item.HashTag}}-raw">{{.ctxData.locale.Tr "repo.issues.context.quote_reply"}}</div>
diff --git a/templates/repo/issue/view_content/reference_issue_dialog.tmpl b/templates/repo/issue/view_content/reference_issue_dialog.tmpl
index e877ffb688..1bb6366ea4 100644
--- a/templates/repo/issue/view_content/reference_issue_dialog.tmpl
+++ b/templates/repo/issue/view_content/reference_issue_dialog.tmpl
@@ -3,7 +3,7 @@
 		{{.locale.Tr "repo.issues.context.reference_issue"}}
 	</div>
 	<div class="content" style="text-align:left">
-		<form class="ui form" action="{{Printf "%s/issues/new" .Repository.Link}}" method="post">
+		<form class="ui form" action="{{printf "%s/issues/new" .Repository.Link}}" method="post">
 			{{.CsrfTokenHtml}}
 			<div class="ui segment content">
 				<div class="field">
diff --git a/templates/repo/migrate/migrate.tmpl b/templates/repo/migrate/migrate.tmpl
index 9663b85eba..44769e60bd 100644
--- a/templates/repo/migrate/migrate.tmpl
+++ b/templates/repo/migrate/migrate.tmpl
@@ -6,13 +6,13 @@
 			<div class="ui three stackable cards">
 				{{range .Services}}
 					<a class="ui card gt-df gt-ac" href="{{AppSubUrl}}/repo/migrate?service_type={{.}}&org={{$.Org}}&mirror={{$.Mirror}}">
-						{{svg (Printf "gitea-%s" .Name) 184}}
+						{{svg (printf "gitea-%s" .Name) 184}}
 						<div class="content">
 							<div class="header gt-tc">
 								{{.Title}}
 							</div>
 							<div class="description gt-tc">
-								{{(Printf "repo.migrate.%s.description" .Name) | $.locale.Tr}}
+								{{(printf "repo.migrate.%s.description" .Name) | $.locale.Tr}}
 							</div>
 						</div>
 					</a>