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

common/loggers/loggers.go

@@ -14,6 +14,8 @@
 package loggers
 
 import (
+	"bytes"
+	"io"
 	"io/ioutil"
 	"log"
 	"os"
@@ -21,17 +23,78 @@ import (
 	jww "github.com/spf13/jwalterweatherman"
 )
 
+var (
+	// Counts ERROR logs to the global jww logger.
+	GlobalErrorCounter *jww.Counter
+)
+
+func init() {
+	GlobalErrorCounter = &jww.Counter{}
+	jww.SetLogListeners(jww.LogCounter(GlobalErrorCounter, jww.LevelError))
+}
+
+// Logger wraps a *loggers.Logger and some other related logging state.
+type Logger struct {
+	*jww.Notepad
+	ErrorCounter *jww.Counter
+
+	// This is only set in server mode.
+	Errors *bytes.Buffer
+}
+
+// Reset resets the logger's internal state.
+func (l *Logger) Reset() {
+	l.ErrorCounter.Reset()
+	if l.Errors != nil {
+		l.Errors.Reset()
+	}
+}
+
+//  NewLogger creates a new Logger for the given thresholds
+func NewLogger(stdoutThreshold, logThreshold jww.Threshold, outHandle, logHandle io.Writer, saveErrors bool) *Logger {
+	return newLogger(stdoutThreshold, logThreshold, outHandle, logHandle, saveErrors)
+}
+
 // NewDebugLogger is a convenience function to create a debug logger.
-func NewDebugLogger() *jww.Notepad {
-	return jww.NewNotepad(jww.LevelDebug, jww.LevelError, os.Stdout, ioutil.Discard, "", log.Ldate|log.Ltime)
+func NewDebugLogger() *Logger {
+	return newBasicLogger(jww.LevelDebug)
 }
 
 // NewWarningLogger is a convenience function to create a warning logger.
-func NewWarningLogger() *jww.Notepad {
-	return jww.NewNotepad(jww.LevelWarn, jww.LevelError, os.Stdout, ioutil.Discard, "", log.Ldate|log.Ltime)
+func NewWarningLogger() *Logger {
+	return newBasicLogger(jww.LevelWarn)
 }
 
 // NewErrorLogger is a convenience function to create an error logger.
-func NewErrorLogger() *jww.Notepad {
-	return jww.NewNotepad(jww.LevelError, jww.LevelError, os.Stdout, ioutil.Discard, "", log.Ldate|log.Ltime)
+func NewErrorLogger() *Logger {
+	return newBasicLogger(jww.LevelError)
+}
+
+func newLogger(stdoutThreshold, logThreshold jww.Threshold, outHandle, logHandle io.Writer, saveErrors bool) *Logger {
+	errorCounter := &jww.Counter{}
+	listeners := []jww.LogListener{jww.LogCounter(errorCounter, jww.LevelError)}
+	var errorBuff *bytes.Buffer
+	if saveErrors {
+		errorBuff = new(bytes.Buffer)
+		errorCapture := func(t jww.Threshold) io.Writer {
+			if t != jww.LevelError {
+				// Only interested in ERROR
+				return nil
+			}
+
+			return errorBuff
+		}
+
+		listeners = append(listeners, errorCapture)
+	}
+
+	return &Logger{
+		Notepad:      jww.NewNotepad(stdoutThreshold, logThreshold, outHandle, logHandle, "", log.Ldate|log.Ltime, listeners...),
+		ErrorCounter: errorCounter,
+		Errors:       errorBuff,
+	}
+}
+
+func newBasicLogger(t jww.Threshold) *Logger {
+	return newLogger(t, jww.LevelError, os.Stdout, ioutil.Discard, false)
 }