Improve template error reporting (#23396)

There are multiple duplicate reports of errors during template rendering
due to broken custom templates.

Unfortunately the error returned here is somewhat difficult for users to
understand and it doesn't return the context of the error.

This PR attempts to parse the error returned by the template renderer to
add in some further context including the filename of the template AND
the preceding lines within that template file.

Ref #23274

---------

Signed-off-by: Andrew Thornton <art27@cantab.net>

1 files changed, 32 insertions, 0 deletions

diff --git a/modules/context/context.go b/modules/context/context.go
index 50c34edae2..5876e23cc4 100644
--- a/modules/context/context.go
+++ b/modules/context/context.go
@@ -16,8 +16,10 @@ import (
 	"net/http"
 	"net/url"
 	"path"
+	"regexp"
 	"strconv"
 	"strings"
+	texttemplate "text/template"
 	"time"
 
 	"code.gitea.io/gitea/models/db"
@@ -213,6 +215,8 @@ func (ctx *Context) RedirectToFirst(location ...string) {
 	ctx.Redirect(setting.AppSubURL + "/")
 }
 
+var templateExecutingErr = regexp.MustCompile(`^template: (.*):([1-9][0-9]*):([1-9][0-9]*): executing (?:"(.*)" at <(.*)>: )?`)
+
 // HTML calls Context.HTML and renders the template to HTTP response
 func (ctx *Context) HTML(status int, name base.TplName) {
 	log.Debug("Template: %s", name)
@@ -228,6 +232,34 @@ func (ctx *Context) HTML(status int, name base.TplName) {
 			ctx.PlainText(http.StatusInternalServerError, "Unable to find status/500 template")
 			return
 		}
+		if execErr, ok := err.(texttemplate.ExecError); ok {
+			if groups := templateExecutingErr.FindStringSubmatch(err.Error()); len(groups) > 0 {
+				errorTemplateName, lineStr, posStr := groups[1], groups[2], groups[3]
+				target := ""
+				if len(groups) == 6 {
+					target = groups[5]
+				}
+				line, _ := strconv.Atoi(lineStr) // Cannot error out as groups[2] is [1-9][0-9]*
+				pos, _ := strconv.Atoi(posStr)   // Cannot error out as groups[3] is [1-9][0-9]*
+				filename, filenameErr := templates.GetAssetFilename("templates/" + errorTemplateName + ".tmpl")
+				if filenameErr != nil {
+					filename = "(template) " + errorTemplateName
+				}
+				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))
+			} else {
+				filename, filenameErr := templates.GetAssetFilename("templates/" + execErr.Name + ".tmpl")
+				if filenameErr != nil {
+					filename = "(template) " + execErr.Name
+				}
+				if execErr.Name != string(name) {
+					filename += " (subtemplate of " + string(name) + ")"
+				}
+				err = fmt.Errorf("%w\nin template file %s", err, filename)
+			}
+		}
 		ctx.ServerError("Render failed", err)
 	}
 }