Add nicer error handling on template compile errors (#21350)

There are repeated issues reported whereby users are unable to interpret
the template errors. This PR adds some (somewhat complex) error handling
to the panic recovery for template renderering but hopefully makes the
interpretation of the error easier.

Reference #21344

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

3 files changed, 225 insertions, 1 deletions

diff --git a/modules/templates/dynamic.go b/modules/templates/dynamic.go
index 4896580f62..a86e71a8c8 100644
--- a/modules/templates/dynamic.go
+++ b/modules/templates/dynamic.go
@@ -33,6 +33,21 @@ func GetAsset(name string) ([]byte, error) {
 	return os.ReadFile(filepath.Join(setting.StaticRootPath, name))
 }
 
+// GetAssetFilename returns the filename of the provided asset
+func GetAssetFilename(name string) (string, error) {
+	filename := filepath.Join(setting.CustomPath, name)
+	_, err := os.Stat(filename)
+	if err != nil && !os.IsNotExist(err) {
+		return filename, err
+	} else if err == nil {
+		return filename, nil
+	}
+
+	filename = filepath.Join(setting.StaticRootPath, name)
+	_, err = os.Stat(filename)
+	return filename, err
+}
+
 // walkTemplateFiles calls a callback for each template asset
 func walkTemplateFiles(callback func(path, name string, d fs.DirEntry, err error) error) error {
 	if err := walkAssetDir(filepath.Join(setting.CustomPath, "templates"), true, callback); err != nil && !os.IsNotExist(err) {
diff --git a/modules/templates/htmlrenderer.go b/modules/templates/htmlrenderer.go
index 210bb5e73c..81ea660161 100644
--- a/modules/templates/htmlrenderer.go
+++ b/modules/templates/htmlrenderer.go
@@ -5,7 +5,12 @@
 package templates
 
 import (
+	"bytes"
 	"context"
+	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
 
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
@@ -14,7 +19,14 @@ import (
 	"github.com/unrolled/render"
 )
 
-var rendererKey interface{} = "templatesHtmlRendereer"
+var (
+	rendererKey interface{} = "templatesHtmlRenderer"
+
+	templateError    = regexp.MustCompile(`^template: (.*):([0-9]+): (.*)`)
+	notDefinedError  = regexp.MustCompile(`^template: (.*):([0-9]+): function "(.*)" not defined`)
+	unexpectedError  = regexp.MustCompile(`^template: (.*):([0-9]+): unexpected "(.*)" in operand`)
+	expectedEndError = regexp.MustCompile(`^template: (.*):([0-9]+): expected end; found (.*)`)
+)
 
 // HTMLRenderer returns the current html renderer for the context or creates and stores one within the context for future use
 func HTMLRenderer(ctx context.Context) (context.Context, *render.Render) {
@@ -32,6 +44,25 @@ func HTMLRenderer(ctx context.Context) (context.Context, *render.Render) {
 	}
 	log.Log(1, log.DEBUG, "Creating "+rendererType+" HTML Renderer")
 
+	compilingTemplates := true
+	defer func() {
+		if !compilingTemplates {
+			return
+		}
+
+		panicked := recover()
+		if panicked == nil {
+			return
+		}
+
+		// OK try to handle the panic...
+		err, ok := panicked.(error)
+		if ok {
+			handlePanicError(err)
+		}
+		log.Fatal("PANIC: Unable to compile templates!\n%v\n\nStacktrace:\n%s", panicked, log.Stack(2))
+	}()
+
 	renderer := render.New(render.Options{
 		Extensions:                []string{".tmpl"},
 		Directory:                 "templates",
@@ -42,6 +73,7 @@ func HTMLRenderer(ctx context.Context) (context.Context, *render.Render) {
 		IsDevelopment:             false,
 		DisableHTTPErrorRendering: true,
 	})
+	compilingTemplates = false
 	if !setting.IsProd {
 		watcher.CreateWatcher(ctx, "HTML Templates", &watcher.CreateWatcherOpts{
 			PathsCallback:   walkTemplateFiles,
@@ -50,3 +82,168 @@ func HTMLRenderer(ctx context.Context) (context.Context, *render.Render) {
 	}
 	return context.WithValue(ctx, rendererKey, renderer), renderer
 }
+
+func handlePanicError(err error) {
+	wrapFatal(handleNotDefinedPanicError(err))
+	wrapFatal(handleUnexpected(err))
+	wrapFatal(handleExpectedEnd(err))
+	wrapFatal(handleGenericTemplateError(err))
+}
+
+func wrapFatal(format string, args []interface{}) {
+	if format == "" {
+		return
+	}
+	log.FatalWithSkip(1, format, args...)
+}
+
+func handleGenericTemplateError(err error) (string, []interface{}) {
+	groups := templateError.FindStringSubmatch(err.Error())
+	if len(groups) != 4 {
+		return "", nil
+	}
+
+	templateName, lineNumberStr, message := groups[1], groups[2], groups[3]
+
+	filename, assetErr := GetAssetFilename("templates/" + templateName + ".tmpl")
+	if assetErr != nil {
+		return "", nil
+	}
+
+	lineNumber, _ := strconv.Atoi(lineNumberStr)
+
+	line := getLineFromAsset(templateName, lineNumber, "")
+
+	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)}
+}
+
+func handleNotDefinedPanicError(err error) (string, []interface{}) {
+	groups := notDefinedError.FindStringSubmatch(err.Error())
+	if len(groups) != 4 {
+		return "", nil
+	}
+
+	templateName, lineNumberStr, functionName := groups[1], groups[2], groups[3]
+
+	functionName, _ = strconv.Unquote(`"` + functionName + `"`)
+
+	filename, assetErr := GetAssetFilename("templates/" + templateName + ".tmpl")
+	if assetErr != nil {
+		return "", nil
+	}
+
+	lineNumber, _ := strconv.Atoi(lineNumberStr)
+
+	line := getLineFromAsset(templateName, lineNumber, functionName)
+
+	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)}
+}
+
+func handleUnexpected(err error) (string, []interface{}) {
+	groups := unexpectedError.FindStringSubmatch(err.Error())
+	if len(groups) != 4 {
+		return "", nil
+	}
+
+	templateName, lineNumberStr, unexpected := groups[1], groups[2], groups[3]
+	unexpected, _ = strconv.Unquote(`"` + unexpected + `"`)
+
+	filename, assetErr := GetAssetFilename("templates/" + templateName + ".tmpl")
+	if assetErr != nil {
+		return "", nil
+	}
+
+	lineNumber, _ := strconv.Atoi(lineNumberStr)
+
+	line := getLineFromAsset(templateName, lineNumber, unexpected)
+
+	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)}
+}
+
+func handleExpectedEnd(err error) (string, []interface{}) {
+	groups := expectedEndError.FindStringSubmatch(err.Error())
+	if len(groups) != 4 {
+		return "", nil
+	}
+
+	templateName, lineNumberStr, unexpected := groups[1], groups[2], groups[3]
+
+	filename, assetErr := GetAssetFilename("templates/" + templateName + ".tmpl")
+	if assetErr != nil {
+		return "", nil
+	}
+
+	lineNumber, _ := strconv.Atoi(lineNumberStr)
+
+	line := getLineFromAsset(templateName, lineNumber, unexpected)
+
+	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 {
+	bs, err := GetAsset("templates/" + templateName + ".tmpl")
+	if err != nil {
+		return fmt.Sprintf("(unable to read template file: %v)", err)
+	}
+
+	sb := &strings.Builder{}
+
+	// Write the header
+	sb.WriteString(dashSeparator)
+
+	var lineBs []byte
+
+	// Iterate through the lines from the asset file to find the target line
+	for start, currentLineNum := 0, 1; currentLineNum <= targetLineNum && start < len(bs); currentLineNum++ {
+		// Find the next new line
+		end := bytes.IndexByte(bs[start:], '\n')
+
+		// adjust the end to be a direct pointer in to []byte
+		if end < 0 {
+			end = len(bs)
+		} else {
+			end += start
+		}
+
+		// set lineBs to the current line []byte
+		lineBs = bs[start:end]
+
+		// move start to after the current new line position
+		start = end + 1
+
+		// Write 2 preceding lines + the target line
+		if targetLineNum-currentLineNum < 3 {
+			_, _ = sb.Write(lineBs)
+			_ = sb.WriteByte('\n')
+		}
+	}
+
+	// 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] = ' '
+				}
+			}
+
+			// write the preceding "space"
+			_, _ = sb.Write(lineBs[:idx])
+
+			// Now write the ^^ pointer
+			_, _ = sb.WriteString(strings.Repeat("^", len(target)))
+			_ = sb.WriteByte('\n')
+		}
+	}
+
+	// Finally write the footer
+	sb.WriteString(dashSeparator)
+
+	return sb.String()
+}
diff --git a/modules/templates/static.go b/modules/templates/static.go
index 3265bd9cfc..7f7cbe702f 100644
--- a/modules/templates/static.go
+++ b/modules/templates/static.go
@@ -31,6 +31,18 @@ func GlobalModTime(filename string) time.Time {
 	return timeutil.GetExecutableModTime()
 }
 
+// GetAssetFilename returns the filename of the provided asset
+func GetAssetFilename(name string) (string, error) {
+	filename := filepath.Join(setting.CustomPath, name)
+	_, err := os.Stat(filename)
+	if err != nil && !os.IsNotExist(err) {
+		return name, err
+	} else if err == nil {
+		return filename, nil
+	}
+	return "(builtin) " + name, nil
+}
+
 // GetAsset get a special asset, only for chi
 func GetAsset(name string) ([]byte, error) {
 	bs, err := os.ReadFile(filepath.Join(setting.CustomPath, name))