1 // Copyright 2022 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
11 "log/slog/internal/buffer"
18 // A Handler handles log records produced by a Logger..
20 // A typical handler may print log records to standard error,
21 // or write them to a file or database, or perhaps augment them
22 // with additional attributes and pass them on to another handler.
24 // Any of the Handler's methods may be called concurrently with itself
25 // or with other methods. It is the responsibility of the Handler to
26 // manage this concurrency.
28 // Users of the slog package should not invoke Handler methods directly.
29 // They should use the methods of [Logger] instead.
30 type Handler interface {
31 // Enabled reports whether the handler handles records at the given level.
32 // The handler ignores records whose level is lower.
33 // It is called early, before any arguments are processed,
34 // to save effort if the log event should be discarded.
35 // If called from a Logger method, the first argument is the context
36 // passed to that method, or context.Background() if nil was passed
37 // or the method does not take a context.
38 // The context is passed so Enabled can use its values
39 // to make a decision.
40 Enabled(context.Context, Level) bool
42 // Handle handles the Record.
43 // It will only be called Enabled returns true.
44 // The Context argument is as for Enabled.
45 // It is present solely to provide Handlers access to the context's values.
46 // Canceling the context should not affect record processing.
47 // (Among other things, log messages may be necessary to debug a
48 // cancellation-related problem.)
50 // Handle methods that produce output should observe the following rules:
51 // - If r.Time is the zero time, ignore the time.
52 // - If r.PC is zero, ignore it.
53 // - If an Attr's key is the empty string and the value is not a group,
55 // - If a group's key is empty, inline the group's Attrs.
56 // - If a group has no Attrs (even if it has a non-empty key),
58 Handle(context.Context, Record) error
60 // WithAttrs returns a new Handler whose attributes consist of
61 // both the receiver's attributes and the arguments.
62 // The Handler owns the slice: it may retain, modify or discard it.
63 // [Logger.With] will resolve the Attrs.
64 WithAttrs(attrs []Attr) Handler
66 // WithGroup returns a new Handler with the given group appended to
67 // the receiver's existing groups.
68 // The keys of all subsequent attributes, whether added by With or in a
69 // Record, should be qualified by the sequence of group names.
71 // How this qualification happens is up to the Handler, so long as
72 // this Handler's attribute keys differ from those of another Handler
73 // with a different sequence of group names.
75 // A Handler should treat WithGroup as starting a Group of Attrs that ends
76 // at the end of the log event. That is,
78 // logger.WithGroup("s").LogAttrs(level, msg, slog.Int("a", 1), slog.Int("b", 2))
82 // logger.LogAttrs(level, msg, slog.Group("s", slog.Int("a", 1), slog.Int("b", 2)))
84 // If the name is empty, WithGroup returns the receiver.
85 WithGroup(name string) Handler
88 type defaultHandler struct {
90 // log.Output, except for testing
91 output func(calldepth int, message string) error
94 func newDefaultHandler(output func(int, string) error) *defaultHandler {
95 return &defaultHandler{
96 ch: &commonHandler{json: false},
101 func (*defaultHandler) Enabled(_ context.Context, l Level) bool {
102 return l >= LevelInfo
105 // Collect the level, attributes and message in a string and
106 // write it with the default log.Logger.
107 // Let the log.Logger handle time and file/line.
108 func (h *defaultHandler) Handle(ctx context.Context, r Record) error {
110 buf.WriteString(r.Level.String())
112 buf.WriteString(r.Message)
113 state := h.ch.newHandleState(buf, true, " ", nil)
115 state.appendNonBuiltIns(r)
117 // skip [h.output, defaultHandler.Handle, handlerWriter.Write, log.Output]
118 return h.output(4, buf.String())
121 func (h *defaultHandler) WithAttrs(as []Attr) Handler {
122 return &defaultHandler{h.ch.withAttrs(as), h.output}
125 func (h *defaultHandler) WithGroup(name string) Handler {
126 return &defaultHandler{h.ch.withGroup(name), h.output}
129 // HandlerOptions are options for a TextHandler or JSONHandler.
130 // A zero HandlerOptions consists entirely of default values.
131 type HandlerOptions struct {
132 // When AddSource is true, the handler adds a ("source", "file:line")
133 // attribute to the output indicating the source code position of the log
134 // statement. AddSource is false by default to skip the cost of computing
138 // Level reports the minimum record level that will be logged.
139 // The handler discards records with lower levels.
140 // If Level is nil, the handler assumes LevelInfo.
141 // The handler calls Level.Level for each record processed;
142 // to adjust the minimum level dynamically, use a LevelVar.
145 // ReplaceAttr is called to rewrite each non-group attribute before it is logged.
146 // The attribute's value has been resolved (see [Value.Resolve]).
147 // If ReplaceAttr returns an Attr with Key == "", the attribute is discarded.
149 // The built-in attributes with keys "time", "level", "source", and "msg"
150 // are passed to this function, except that time is omitted
151 // if zero, and source is omitted if AddSource is false.
153 // The first argument is a list of currently open groups that contain the
154 // Attr. It must not be retained or modified. ReplaceAttr is never called
155 // for Group attributes, only their contents. For example, the attribute
158 // Int("a", 1), Group("g", Int("b", 2)), Int("c", 3)
160 // results in consecutive calls to ReplaceAttr with the following arguments:
163 // []string{"g"}, Int("b", 2)
166 // ReplaceAttr can be used to change the default keys of the built-in
167 // attributes, convert types (for example, to replace a `time.Time` with the
168 // integer seconds since the Unix epoch), sanitize personal information, or
169 // remove attributes from the output.
170 ReplaceAttr func(groups []string, a Attr) Attr
173 // Keys for "built-in" attributes.
175 // TimeKey is the key used by the built-in handlers for the time
176 // when the log method is called. The associated Value is a [time.Time].
178 // LevelKey is the key used by the built-in handlers for the level
179 // of the log call. The associated value is a [Level].
181 // MessageKey is the key used by the built-in handlers for the
182 // message of the log call. The associated value is a string.
184 // SourceKey is the key used by the built-in handlers for the source file
185 // and line of the log call. The associated value is a string.
189 type commonHandler struct {
190 json bool // true => output JSON; false => output text
192 preformattedAttrs []byte
193 groupPrefix string // for text: prefix of groups opened in preformatting
194 groups []string // all groups started from WithGroup
195 nOpenGroups int // the number of groups opened in preformattedAttrs
200 func (h *commonHandler) clone() *commonHandler {
201 // We can't use assignment because we can't copy the mutex.
202 return &commonHandler{
205 preformattedAttrs: slices.Clip(h.preformattedAttrs),
206 groupPrefix: h.groupPrefix,
207 groups: slices.Clip(h.groups),
208 nOpenGroups: h.nOpenGroups,
213 // enabled reports whether l is greater than or equal to the
215 func (h *commonHandler) enabled(l Level) bool {
216 minLevel := LevelInfo
217 if h.opts.Level != nil {
218 minLevel = h.opts.Level.Level()
223 func (h *commonHandler) withAttrs(as []Attr) *commonHandler {
225 // Pre-format the attributes as an optimization.
226 prefix := buffer.New()
228 prefix.WriteString(h.groupPrefix)
229 state := h2.newHandleState((*buffer.Buffer)(&h2.preformattedAttrs), false, "", prefix)
231 if len(h2.preformattedAttrs) > 0 {
232 state.sep = h.attrSep()
235 for _, a := range as {
238 // Remember the new prefix for later keys.
239 h2.groupPrefix = state.prefix.String()
240 // Remember how many opened groups are in preformattedAttrs,
241 // so we don't open them again when we handle a Record.
242 h2.nOpenGroups = len(h2.groups)
246 func (h *commonHandler) withGroup(name string) *commonHandler {
251 h2.groups = append(h2.groups, name)
255 func (h *commonHandler) handle(r Record) error {
256 state := h.newHandleState(buffer.New(), true, "", nil)
259 state.buf.WriteByte('{')
261 // Built-in attributes. They are not in a group.
262 stateGroups := state.groups
263 state.groups = nil // So ReplaceAttrs sees no groups instead of the pre groups.
264 rep := h.opts.ReplaceAttr
266 if !r.Time.IsZero() {
268 val := r.Time.Round(0) // strip monotonic to match Attr behavior
271 state.appendTime(val)
273 state.appendAttr(Time(key, val))
281 state.appendString(val.String())
283 state.appendAttr(Any(key, val))
286 if h.opts.AddSource {
288 if frame.File != "" {
292 state.appendSource(frame.File, frame.Line)
295 buf.WriteString(frame.File) // TODO: escape?
297 buf.WritePosInt(frame.Line)
300 state.appendAttr(String(key, s))
308 state.appendString(msg)
310 state.appendAttr(String(key, msg))
312 state.groups = stateGroups // Restore groups passed to ReplaceAttrs.
313 state.appendNonBuiltIns(r)
314 state.buf.WriteByte('\n')
318 _, err := h.w.Write(*state.buf)
322 func (s *handleState) appendNonBuiltIns(r Record) {
323 // preformatted Attrs
324 if len(s.h.preformattedAttrs) > 0 {
325 s.buf.WriteString(s.sep)
326 s.buf.Write(s.h.preformattedAttrs)
327 s.sep = s.h.attrSep()
329 // Attrs in Record -- unlike the built-in ones, they are in groups started
331 s.prefix = buffer.New()
332 defer s.prefix.Free()
333 s.prefix.WriteString(s.h.groupPrefix)
335 r.Attrs(func(a Attr) {
339 // Close all open groups.
340 for range s.h.groups {
343 // Close the top-level object.
348 // attrSep returns the separator between attributes.
349 func (h *commonHandler) attrSep() string {
356 // handleState holds state for a single call to commonHandler.handle.
357 // The initial value of sep determines whether to emit a separator
358 // before the next key, after which it stays true.
359 type handleState struct {
362 freeBuf bool // should buf be freed?
363 sep string // separator to write before next key
364 prefix *buffer.Buffer // for text: key prefix
365 groups *[]string // pool-allocated slice of active groups, for ReplaceAttr
368 var groupPool = sync.Pool{New: func() any {
369 s := make([]string, 0, 10)
373 func (h *commonHandler) newHandleState(buf *buffer.Buffer, freeBuf bool, sep string, prefix *buffer.Buffer) handleState {
381 if h.opts.ReplaceAttr != nil {
382 s.groups = groupPool.Get().(*[]string)
383 *s.groups = append(*s.groups, h.groups[:h.nOpenGroups]...)
388 func (s *handleState) free() {
392 if gs := s.groups; gs != nil {
398 func (s *handleState) openGroups() {
399 for _, n := range s.h.groups[s.h.nOpenGroups:] {
404 // Separator for group names and keys.
405 const keyComponentSep = '.'
407 // openGroup starts a new group of attributes
408 // with the given name.
409 func (s *handleState) openGroup(name string) {
415 s.prefix.WriteString(name)
416 s.prefix.WriteByte(keyComponentSep)
418 // Collect group names for ReplaceAttr.
420 *s.groups = append(*s.groups, name)
425 // closeGroup ends the group with the given name.
426 func (s *handleState) closeGroup(name string) {
430 (*s.prefix) = (*s.prefix)[:len(*s.prefix)-len(name)-1 /* for keyComponentSep */]
432 s.sep = s.h.attrSep()
434 *s.groups = (*s.groups)[:len(*s.groups)-1]
438 // appendAttr appends the Attr's key and value using app.
439 // It handles replacement and checking for an empty key.
440 // after replacement).
441 func (s *handleState) appendAttr(a Attr) {
443 // Elide a non-group with an empty key.
444 if a.Key == "" && v.Kind() != KindGroup {
447 if rep := s.h.opts.ReplaceAttr; rep != nil && v.Kind() != KindGroup {
452 a = rep(gs, Attr{a.Key, v})
456 // Although all attributes in the Record are already resolved,
457 // This one came from the user, so it may not have been.
458 v = a.Value.Resolve()
460 if v.Kind() == KindGroup {
462 // Output only non-empty groups.
464 // Inline a group with an empty key.
468 for _, aa := range attrs {
481 func (s *handleState) appendError(err error) {
482 s.appendString(fmt.Sprintf("!ERROR:%v", err))
485 func (s *handleState) appendKey(key string) {
486 s.buf.WriteString(s.sep)
488 // TODO: optimize by avoiding allocation.
489 s.appendString(string(*s.prefix) + key)
498 s.sep = s.h.attrSep()
501 func (s *handleState) appendSource(file string, line int) {
504 *s.buf = appendEscapedJSONString(*s.buf, file)
506 s.buf.WritePosInt(line)
510 if needsQuoting(file) {
511 s.appendString(file + ":" + strconv.Itoa(line))
513 // common case: no quoting needed.
516 s.buf.WritePosInt(line)
521 func (s *handleState) appendString(str string) {
524 *s.buf = appendEscapedJSONString(*s.buf, str)
528 if needsQuoting(str) {
529 *s.buf = strconv.AppendQuote(*s.buf, str)
531 s.buf.WriteString(str)
536 func (s *handleState) appendValue(v Value) {
539 err = appendJSONValue(s, v)
541 err = appendTextValue(s, v)
548 func (s *handleState) appendTime(t time.Time) {
552 writeTimeRFC3339Millis(s.buf, t)
556 // This takes half the time of Time.AppendFormat.
557 func writeTimeRFC3339Millis(buf *buffer.Buffer, t time.Time) {
558 year, month, day := t.Date()
559 buf.WritePosIntWidth(year, 4)
561 buf.WritePosIntWidth(int(month), 2)
563 buf.WritePosIntWidth(day, 2)
565 hour, min, sec := t.Clock()
566 buf.WritePosIntWidth(hour, 2)
568 buf.WritePosIntWidth(min, 2)
570 buf.WritePosIntWidth(sec, 2)
573 buf.WritePosIntWidth(ns/1e6, 3)
574 _, offsetSeconds := t.Zone()
575 if offsetSeconds == 0 {
578 offsetMinutes := offsetSeconds / 60
579 if offsetMinutes < 0 {
581 offsetMinutes = -offsetMinutes
585 buf.WritePosIntWidth(offsetMinutes/60, 2)
587 buf.WritePosIntWidth(offsetMinutes%60, 2)