slog: Logger

The Logger type and its methods.

Change-Id: I2447ec7a3dab5c10c8851adcc2b5ae8e7b8e210d
Reviewed-on: https://go-review.googlesource.com/c/exp/+/429436
Reviewed-by: Alan Donovan <[email protected]>
Run-TryBot: Jonathan Amsterdam <[email protected]>

slog/logger.go

@@ -0,0 +1,237 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slog
+
+import (
+	"log"
+	"sync/atomic"
+	"time"
+)
+
+var defaultLogger atomic.Value
+
+func init() {
+	defaultLogger.Store(&Logger{
+		handler: &defaultHandler{},
+	})
+}
+
+// Default returns the default Logger.
+func Default() *Logger { return defaultLogger.Load().(*Logger) }
+
+// SetDefault makes l the default Logger.
+// After this call, output from the log package's default Logger
+// (as with [log.Print], etc.) will be logged at InfoLevel using l's Handler.
+func SetDefault(l *Logger) {
+	defaultLogger.Store(l)
+	log.SetOutput(&handlerWriter{l.Handler(), log.Flags()})
+	log.SetFlags(0) // we want just the log message, no time or location
+}
+
+// handlerWriter is an io.Writer that calls a Handler.
+// It is used to link the default log.Logger to the default slog.Logger.
+type handlerWriter struct {
+	h     Handler
+	flags int
+}
+
+func (w *handlerWriter) Write(buf []byte) (int, error) {
+	var depth int
+	if w.flags&(log.Lshortfile|log.Llongfile) != 0 {
+		depth = 2
+	}
+	// Remove final newline.
+	origLen := len(buf) // Report that the entire buf was written.
+	if len(buf) > 0 && buf[len(buf)-1] == '\n' {
+		buf = buf[:len(buf)-1]
+	}
+	r := MakeRecord(time.Now(), InfoLevel, string(buf), depth)
+	return origLen, w.h.Handle(r)
+}
+
+// A Logger records structured information about each call to its
+// Log, Debug, Info, Warn, and Error methods.
+// For each call, it creates a Record and passes it to a Handler.
+//
+// Loggers are immutable; to create a new one, call [New] or [Logger.With].
+type Logger struct {
+	handler Handler // for structured logging
+}
+
+// Handler returns l's Handler.
+func (l *Logger) Handler() Handler { return l.handler }
+
+// With returns a new Logger whose handler's attributes are a concatenation of
+// l's attributes and the given arguments, converted to Attrs as in
+// [Logger.Log].
+func (l *Logger) With(attrs ...any) *Logger {
+	return &Logger{handler: l.handler.With(argsToAttrs(attrs))}
+}
+
+func argsToAttrs(args []any) []Attr {
+	var r Record
+	setAttrs(&r, args)
+	return r.Attrs()
+}
+
+// New creates a new Logger with the given Handler.
+func New(h Handler) *Logger { return &Logger{handler: h} }
+
+// With calls Logger.With on the default logger.
+func With(attrs ...any) *Logger {
+	return Default().With(attrs...)
+}
+
+// Enabled reports whether l emits log records at the given level.
+func (l *Logger) Enabled(level Level) bool {
+	return l.Handler().Enabled(level)
+}
+
+// Log emits a log record with the current time and the given level and message.
+// The Record's Attrs consist of the Logger's attributes followed by
+// the Attrs specified by args.
+//
+// The attribute arguments are processed as follows:
+//   - If an argument is an Attr, it is used as is.
+//   - If an argument is a string and this is not the last argument,
+//     the following argument is treated as the value and the two are combined
+//     into an Attr.
+//   - Otherwise, the argument is treated as a value with key "!BADKEY".
+func (l *Logger) Log(level Level, msg string, args ...any) {
+	l.LogDepth(0, level, msg, args...)
+}
+
+// LogDepth is like [Logger.Log], but accepts a call depth to adjust the
+// file and line number in the log record. 0 refers to the caller
+// of LogDepth; 1 refers to the caller's caller; and so on.
+func (l *Logger) LogDepth(calldepth int, level Level, msg string, args ...any) {
+	if !l.Enabled(level) {
+		return
+	}
+	r := l.makeRecord(msg, level, calldepth)
+	setAttrs(&r, args)
+	_ = l.Handler().Handle(r)
+}
+
+var useSourceLine = true
+
+// Temporary, for benchmarking.
+// Eventually, getting the pc should be fast.
+func disableSourceLine() { useSourceLine = false }
+
+func (l *Logger) makeRecord(msg string, level Level, depth int) Record {
+	if useSourceLine {
+		depth += 5
+	}
+	return MakeRecord(time.Now(), level, msg, depth)
+}
+
+const badKey = "!BADKEY"
+
+func setAttrs(r *Record, args []any) {
+	var attr Attr
+	for len(args) > 0 {
+		attr, args = argsToAttr(args)
+		r.AddAttr(attr)
+	}
+}
+
+// argsToAttrs turns a prefix of the args slice into an Attr and returns
+// the unused portion of the slice.
+// If args[0] is an Attr, it returns it.
+// If args[0] is a string, it treats the first two elements as
+// a key-value pair.
+// Otherwise, it treats args[0] as a value with a missing key.
+func argsToAttr(args []any) (Attr, []any) {
+	switch x := args[0].(type) {
+	case string:
+		if len(args) == 1 {
+			return String(badKey, x), nil
+		}
+		return Any(x, args[1]), args[2:]
+
+	case Attr:
+		return x, args[1:]
+
+	default:
+		return Any(badKey, x), args[1:]
+	}
+}
+
+// LogAttrs is a more efficient version of [Logger.Log] that accepts only Attrs.
+func (l *Logger) LogAttrs(level Level, msg string, attrs ...Attr) {
+	l.LogAttrsDepth(0, level, msg, attrs...)
+}
+
+// LogAttrsDepth is like [Logger.LogAttrs], but accepts a call depth argument
+// which it interprets like [Logger.LogDepth].
+func (l *Logger) LogAttrsDepth(calldepth int, level Level, msg string, attrs ...Attr) {
+	if !l.Enabled(level) {
+		return
+	}
+	r := l.makeRecord(msg, level, calldepth)
+	r.addAttrs(attrs)
+	_ = l.Handler().Handle(r)
+}
+
+// Debug logs at DebugLevel.
+func (l *Logger) Debug(msg string, args ...any) {
+	l.LogDepth(0, DebugLevel, msg, args...)
+}
+
+// Info logs at InfoLevel.
+func (l *Logger) Info(msg string, args ...any) {
+	l.LogDepth(0, InfoLevel, msg, args...)
+}
+
+// Warn logs at WarnLevel.
+func (l *Logger) Warn(msg string, args ...any) {
+	l.LogDepth(0, WarnLevel, msg, args...)
+}
+
+// Error logs at ErrorLevel.
+// If err is non-nil, Error appends Any("err", err)
+// to the list of attributes.
+func (l *Logger) Error(msg string, err error, args ...any) {
+	if err != nil {
+		// TODO: avoid the copy.
+		args = append(args[:len(args):len(args)], Any("err", err))
+	}
+	l.LogDepth(0, ErrorLevel, msg, args...)
+}
+
+// Debug calls Logger.Debug on the default logger.
+func Debug(msg string, args ...any) {
+	Default().LogDepth(0, DebugLevel, msg, args...)
+}
+
+// Info calls Logger.Info on the default logger.
+func Info(msg string, args ...any) {
+	Default().LogDepth(0, InfoLevel, msg, args...)
+}
+
+// Warn calls Logger.Warn on the default logger.
+func Warn(msg string, args ...any) {
+	Default().LogDepth(0, WarnLevel, msg, args...)
+}
+
+// Error calls Logger.Error on the default logger.
+func Error(msg string, err error, args ...any) {
+	if err != nil {
+		// TODO: avoid the copy.
+		args = append(args[:len(args):len(args)], Any("err", err))
+	}
+	Default().LogDepth(0, ErrorLevel, msg, args...)
+}
+
+// Log calls Logger.Log on the default logger.
+func Log(level Level, msg string, args ...any) {
+	Default().LogDepth(0, level, msg, args...)
+}
+
+// LogAttrs calls Logger.LogAttrs on the default logger.
+func LogAttrs(level Level, msg string, attrs ...Attr) {
+	Default().LogAttrsDepth(0, level, msg, attrs...)
+}