Improve template error reporting (go-gitea#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 go-gitea#23274

---------

Signed-off-by: Andrew Thornton <[email protected]>

Author: zeripath

modules/context/context.go

@@ -17,8 +17,10 @@ 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)
 	}
 }

modules/templates/htmlrenderer.go

@@ -118,7 +118,7 @@ func handleGenericTemplateError(err error) (string, []interface{}) {
 
 	lineNumber, _ := strconv.Atoi(lineNumberStr)
 
-	line := getLineFromAsset(templateName, lineNumber, "")
+	line := GetLineFromTemplate(templateName, lineNumber, "", -1)
 
 	return "PANIC: Unable to compile templates!\n%s in template file %s at line %d:\n\n%s\nStacktrace:\n\n%s", []interface{}{message, filename, lineNumber, log.NewColoredValue(line, log.Reset), log.Stack(2)}
 }
@@ -140,7 +140,7 @@ func handleNotDefinedPanicError(err error) (string, []interface{}) {
 
 	lineNumber, _ := strconv.Atoi(lineNumberStr)
 
-	line := getLineFromAsset(templateName, lineNumber, functionName)
+	line := GetLineFromTemplate(templateName, lineNumber, functionName, -1)
 
 	return "PANIC: Unable to compile templates!\nUndefined function %q in template file %s at line %d:\n\n%s", []interface{}{functionName, filename, lineNumber, log.NewColoredValue(line, log.Reset)}
 }
@@ -161,7 +161,7 @@ func handleUnexpected(err error) (string, []interface{}) {
 
 	lineNumber, _ := strconv.Atoi(lineNumberStr)
 
-	line := getLineFromAsset(templateName, lineNumber, unexpected)
+	line := GetLineFromTemplate(templateName, lineNumber, unexpected, -1)
 
 	return "PANIC: Unable to compile templates!\nUnexpected %q in template file %s at line %d:\n\n%s", []interface{}{unexpected, filename, lineNumber, log.NewColoredValue(line, log.Reset)}
 }
@@ -181,14 +181,15 @@ func handleExpectedEnd(err error) (string, []interface{}) {
 
 	lineNumber, _ := strconv.Atoi(lineNumberStr)
 
-	line := getLineFromAsset(templateName, lineNumber, unexpected)
+	line := GetLineFromTemplate(templateName, lineNumber, unexpected, -1)
 
 	return "PANIC: Unable to compile templates!\nMissing end with unexpected %q in template file %s at line %d:\n\n%s", []interface{}{unexpected, filename, lineNumber, log.NewColoredValue(line, log.Reset)}
 }
 
 const dashSeparator = "----------------------------------------------------------------------\n"
 
-func getLineFromAsset(templateName string, targetLineNum int, target string) string {
+// GetLineFromTemplate returns a line from a template with some context
+func GetLineFromTemplate(templateName string, targetLineNum int, target string, position int) string {
 	bs, err := GetAsset("templates/" + templateName + ".tmpl")
 	if err != nil {
 		return fmt.Sprintf("(unable to read template file: %v)", err)
@@ -229,23 +230,29 @@ func getLineFromAsset(templateName string, targetLineNum int, target string) str
 	// If there is a provided target to look for in the line add a pointer to it
 	// e.g.                                                        ^^^^^^^
 	if target != "" {
-		idx := bytes.Index(lineBs, []byte(target))
-
-		if idx >= 0 {
-			// take the current line and replace preceding text with whitespace (except for tab)
-			for i := range lineBs[:idx] {
-				if lineBs[i] != '\t' {
-					lineBs[i] = ' '
-				}
+		targetPos := bytes.Index(lineBs, []byte(target))
+		if targetPos >= 0 {
+			position = targetPos
+		}
+	}
+	if position >= 0 {
+		// take the current line and replace preceding text with whitespace (except for tab)
+		for i := range lineBs[:position] {
+			if lineBs[i] != '\t' {
+				lineBs[i] = ' '
 			}
+		}
 
-			// write the preceding "space"
-			_, _ = sb.Write(lineBs[:idx])
+		// write the preceding "space"
+		_, _ = sb.Write(lineBs[:position])
 
-			// Now write the ^^ pointer
-			_, _ = sb.WriteString(strings.Repeat("^", len(target)))
-			_ = sb.WriteByte('\n')
+		// Now write the ^^ pointer
+		targetLen := len(target)
+		if targetLen == 0 {
+			targetLen = 1
 		}
+		_, _ = sb.WriteString(strings.Repeat("^", targetLen))
+		_ = sb.WriteByte('\n')
 	}
 
 	// Finally write the footer