commands: Show server error info in browser

The main item in this commit is showing of errors with a file context when running `hugo server`.

This can be turned off: `hugo server --disableBrowserError` (can also be set in `config.toml`).

But to get there, the error handling in Hugo needed a revision. There are some items left TODO for commits soon to follow, most notable errors in content and config files.

Fixes #5284
Fixes #5290
See #5325
See #5324

tpl/template.go

@@ -1,4 +1,4 @@
-// Copyright 2017-present The Hugo Authors. All rights reserved.
+// Copyright 2018 The Hugo Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -14,16 +14,26 @@
 package tpl
 
 import (
+	"fmt"
 	"io"
+	"path/filepath"
+	"regexp"
+	"strings"
 	"time"
 
-	"text/template/parse"
+	"github.com/gohugoio/hugo/common/herrors"
+
+	"github.com/gohugoio/hugo/hugofs"
+
+	"github.com/spf13/afero"
 
 	"html/template"
 	texttemplate "text/template"
+	"text/template/parse"
 
 	bp "github.com/gohugoio/hugo/bufferpool"
 	"github.com/gohugoio/hugo/metrics"
+	"github.com/pkg/errors"
 )
 
 var (
@@ -35,8 +45,7 @@ type TemplateHandler interface {
 	TemplateFinder
 	AddTemplate(name, tpl string) error
 	AddLateTemplate(name, tpl string) error
-	LoadTemplates(prefix string)
-	PrintErrors()
+	LoadTemplates(prefix string) error
 
 	NewTextTemplate() TemplateParseFinder
 
@@ -82,16 +91,122 @@ type TemplateDebugger interface {
 type TemplateAdapter struct {
 	Template
 	Metrics metrics.Provider
+
+	// The filesystem where the templates are stored.
+	Fs afero.Fs
+
+	// Maps to base template if relevant.
+	NameBaseTemplateName map[string]string
+}
+
+var baseOfRe = regexp.MustCompile("template: (.*?):")
+
+func extractBaseOf(err string) string {
+	m := baseOfRe.FindStringSubmatch(err)
+	if len(m) == 2 {
+		return m[1]
+	}
+	return ""
 }
 
 // Execute executes the current template. The actual execution is performed
 // by the embedded text or html template, but we add an implementation here so
 // we can add a timer for some metrics.
-func (t *TemplateAdapter) Execute(w io.Writer, data interface{}) error {
+func (t *TemplateAdapter) Execute(w io.Writer, data interface{}) (execErr error) {
+	defer func() {
+		// Panics in templates are a little bit too common (nil pointers etc.)
+		if r := recover(); r != nil {
+			execErr = t.addFileContext(t.Name(), fmt.Errorf("panic in Execute: %s", r))
+		}
+	}()
+
 	if t.Metrics != nil {
 		defer t.Metrics.MeasureSince(t.Name(), time.Now())
 	}
-	return t.Template.Execute(w, data)
+
+	execErr = t.Template.Execute(w, data)
+	if execErr != nil {
+		execErr = t.addFileContext(t.Name(), execErr)
+	}
+
+	return
+}
+
+var identifiersRe = regexp.MustCompile("at \\<(.*?)\\>:")
+
+func (t *TemplateAdapter) extractIdentifiers(line string) []string {
+	m := identifiersRe.FindAllStringSubmatch(line, -1)
+	identifiers := make([]string, len(m))
+	for i := 0; i < len(m); i++ {
+		identifiers[i] = m[i][1]
+	}
+	return identifiers
+}
+
+func (t *TemplateAdapter) addFileContext(name string, inerr error) error {
+	f, realFilename, err := t.fileAndFilename(t.Name())
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	master, hasMaster := t.NameBaseTemplateName[name]
+
+	ferr := errors.Wrapf(inerr, "execute of template %q failed", realFilename)
+
+	// Since this can be a composite of multiple template files (single.html + baseof.html etc.)
+	// we potentially need to look in both -- and cannot rely on line number alone.
+	lineMatcher := func(le herrors.FileError, lineNumber int, line string) bool {
+		if le.LineNumber() != lineNumber {
+			return false
+		}
+		if !hasMaster {
+			return true
+		}
+
+		identifiers := t.extractIdentifiers(le.Error())
+
+		for _, id := range identifiers {
+			if strings.Contains(line, id) {
+				return true
+			}
+		}
+		return false
+	}
+
+	// TODO(bep) 2errors text vs HTML
+	fe, ok := herrors.WithFileContext(ferr, f, "go-html-template", lineMatcher)
+	if ok || !hasMaster {
+		return fe
+	}
+
+	// Try the base template if relevant
+	f, realFilename, err = t.fileAndFilename(master)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	ferr = errors.Wrapf(inerr, "execute of template %q failed", realFilename)
+	fe, _ = herrors.WithFileContext(ferr, f, "go-html-template", lineMatcher)
+	return fe
+
+}
+
+func (t *TemplateAdapter) fileAndFilename(name string) (afero.File, string, error) {
+	fs := t.Fs
+	filename := filepath.FromSlash(name)
+
+	fi, err := fs.Stat(filename)
+	if err != nil {
+		return nil, "", errors.Wrapf(err, "failed to Stat %q", filename)
+	}
+	f, err := fs.Open(filename)
+	if err != nil {
+		return nil, "", errors.Wrapf(err, "failed to open template file %q:", filename)
+	}
+
+	return f, fi.(hugofs.RealFilenameInfo).RealFilename(), nil
 }
 
 // ExecuteToString executes the current template and returns the result as a