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// 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 (
"context"
"fmt"
"io"
"reflect"
"strconv"
"sync"
"time"
"golang.org/x/exp/slices"
"golang.org/x/exp/slog/internal/buffer"
)
// A Handler handles log records produced by a Logger..
//
// A typical handler may print log records to standard error,
// or write them to a file or database, or perhaps augment them
// with additional attributes and pass them on to another handler.
//
// Any of the Handler's methods may be called concurrently with itself
// or with other methods. It is the responsibility of the Handler to
// manage this concurrency.
//
// Users of the slog package should not invoke Handler methods directly.
// They should use the methods of [Logger] instead.
type Handler interface {
// Enabled reports whether the handler handles records at the given level.
// The handler ignores records whose level is lower.
// It is called early, before any arguments are processed,
// to save effort if the log event should be discarded.
// If called from a Logger method, the first argument is the context
// passed to that method, or context.Background() if nil was passed
// or the method does not take a context.
// The context is passed so Enabled can use its values
// to make a decision.
Enabled(context.Context, Level) bool
// Handle handles the Record.
// It will only be called when Enabled returns true.
// The Context argument is as for Enabled.
// It is present solely to provide Handlers access to the context's values.
// Canceling the context should not affect record processing.
// (Among other things, log messages may be necessary to debug a
// cancellation-related problem.)
//
// Handle methods that produce output should observe the following rules:
// - If r.Time is the zero time, ignore the time.
// - If r.PC is zero, ignore it.
// - Attr's values should be resolved.
// - If an Attr's key and value are both the zero value, ignore the Attr.
// This can be tested with attr.Equal(Attr{}).
// - If a group's key is empty, inline the group's Attrs.
// - If a group has no Attrs (even if it has a non-empty key),
// ignore it.
Handle(context.Context, Record) error
// WithAttrs returns a new Handler whose attributes consist of
// both the receiver's attributes and the arguments.
// The Handler owns the slice: it may retain, modify or discard it.
WithAttrs(attrs []Attr) Handler
// WithGroup returns a new Handler with the given group appended to
// the receiver's existing groups.
// The keys of all subsequent attributes, whether added by With or in a
// Record, should be qualified by the sequence of group names.
//
// How this qualification happens is up to the Handler, so long as
// this Handler's attribute keys differ from those of another Handler
// with a different sequence of group names.
//
// A Handler should treat WithGroup as starting a Group of Attrs that ends
// at the end of the log event. That is,
//
// logger.WithGroup("s").LogAttrs(level, msg, slog.Int("a", 1), slog.Int("b", 2))
//
// should behave like
//
// logger.LogAttrs(level, msg, slog.Group("s", slog.Int("a", 1), slog.Int("b", 2)))
//
// If the name is empty, WithGroup returns the receiver.
WithGroup(name string) Handler
}
type defaultHandler struct {
ch *commonHandler
// log.Output, except for testing
output func(calldepth int, message string) error
}
func newDefaultHandler(output func(int, string) error) *defaultHandler {
return &defaultHandler{
ch: &commonHandler{json: false},
output: output,
}
}
func (*defaultHandler) Enabled(_ context.Context, l Level) bool {
return l >= LevelInfo
}
// Collect the level, attributes and message in a string and
// write it with the default log.Logger.
// Let the log.Logger handle time and file/line.
func (h *defaultHandler) Handle(ctx context.Context, r Record) error {
buf := buffer.New()
buf.WriteString(r.Level.String())
buf.WriteByte(' ')
buf.WriteString(r.Message)
state := h.ch.newHandleState(buf, true, " ", nil)
defer state.free()
state.appendNonBuiltIns(r)
// skip [h.output, defaultHandler.Handle, handlerWriter.Write, log.Output]
return h.output(4, buf.String())
}
func (h *defaultHandler) WithAttrs(as []Attr) Handler {
return &defaultHandler{h.ch.withAttrs(as), h.output}
}
func (h *defaultHandler) WithGroup(name string) Handler {
return &defaultHandler{h.ch.withGroup(name), h.output}
}
// HandlerOptions are options for a TextHandler or JSONHandler.
// A zero HandlerOptions consists entirely of default values.
type HandlerOptions struct {
// AddSource causes the handler to compute the source code position
// of the log statement and add a SourceKey attribute to the output.
AddSource bool
// Level reports the minimum record level that will be logged.
// The handler discards records with lower levels.
// If Level is nil, the handler assumes LevelInfo.
// The handler calls Level.Level for each record processed;
// to adjust the minimum level dynamically, use a LevelVar.
Level Leveler
// ReplaceAttr is called to rewrite each non-group attribute before it is logged.
// The attribute's value has been resolved (see [Value.Resolve]).
// If ReplaceAttr returns an Attr with Key == "", the attribute is discarded.
//
// The built-in attributes with keys "time", "level", "source", and "msg"
// are passed to this function, except that time is omitted
// if zero, and source is omitted if AddSource is false.
//
// The first argument is a list of currently open groups that contain the
// Attr. It must not be retained or modified. ReplaceAttr is never called
// for Group attributes, only their contents. For example, the attribute
// list
//
// Int("a", 1), Group("g", Int("b", 2)), Int("c", 3)
//
// results in consecutive calls to ReplaceAttr with the following arguments:
//
// nil, Int("a", 1)
// []string{"g"}, Int("b", 2)
// nil, Int("c", 3)
//
// ReplaceAttr can be used to change the default keys of the built-in
// attributes, convert types (for example, to replace a `time.Time` with the
// integer seconds since the Unix epoch), sanitize personal information, or
// remove attributes from the output.
ReplaceAttr func(groups []string, a Attr) Attr
}
// Keys for "built-in" attributes.
const (
// TimeKey is the key used by the built-in handlers for the time
// when the log method is called. The associated Value is a [time.Time].
TimeKey = "time"
// LevelKey is the key used by the built-in handlers for the level
// of the log call. The associated value is a [Level].
LevelKey = "level"
// MessageKey is the key used by the built-in handlers for the
// message of the log call. The associated value is a string.
MessageKey = "msg"
// SourceKey is the key used by the built-in handlers for the source file
// and line of the log call. The associated value is a string.
SourceKey = "source"
)
type commonHandler struct {
json bool // true => output JSON; false => output text
opts HandlerOptions
preformattedAttrs []byte
groupPrefix string // for text: prefix of groups opened in preformatting
groups []string // all groups started from WithGroup
nOpenGroups int // the number of groups opened in preformattedAttrs
mu sync.Mutex
w io.Writer
}
func (h *commonHandler) clone() *commonHandler {
// We can't use assignment because we can't copy the mutex.
return &commonHandler{
json: h.json,
opts: h.opts,
preformattedAttrs: slices.Clip(h.preformattedAttrs),
groupPrefix: h.groupPrefix,
groups: slices.Clip(h.groups),
nOpenGroups: h.nOpenGroups,
w: h.w,
}
}
// enabled reports whether l is greater than or equal to the
// minimum level.
func (h *commonHandler) enabled(l Level) bool {
minLevel := LevelInfo
if h.opts.Level != nil {
minLevel = h.opts.Level.Level()
}
return l >= minLevel
}
func (h *commonHandler) withAttrs(as []Attr) *commonHandler {
h2 := h.clone()
// Pre-format the attributes as an optimization.
prefix := buffer.New()
defer prefix.Free()
prefix.WriteString(h.groupPrefix)
state := h2.newHandleState((*buffer.Buffer)(&h2.preformattedAttrs), false, "", prefix)
defer state.free()
if len(h2.preformattedAttrs) > 0 {
state.sep = h.attrSep()
}
state.openGroups()
for _, a := range as {
state.appendAttr(a)
}
// Remember the new prefix for later keys.
h2.groupPrefix = state.prefix.String()
// Remember how many opened groups are in preformattedAttrs,
// so we don't open them again when we handle a Record.
h2.nOpenGroups = len(h2.groups)
return h2
}
func (h *commonHandler) withGroup(name string) *commonHandler {
if name == "" {
return h
}
h2 := h.clone()
h2.groups = append(h2.groups, name)
return h2
}
func (h *commonHandler) handle(r Record) error {
state := h.newHandleState(buffer.New(), true, "", nil)
defer state.free()
if h.json {
state.buf.WriteByte('{')
}
// Built-in attributes. They are not in a group.
stateGroups := state.groups
state.groups = nil // So ReplaceAttrs sees no groups instead of the pre groups.
rep := h.opts.ReplaceAttr
// time
if !r.Time.IsZero() {
key := TimeKey
val := r.Time.Round(0) // strip monotonic to match Attr behavior
if rep == nil {
state.appendKey(key)
state.appendTime(val)
} else {
state.appendAttr(Time(key, val))
}
}
// level
key := LevelKey
val := r.Level
if rep == nil {
state.appendKey(key)
state.appendString(val.String())
} else {
state.appendAttr(Any(key, val))
}
// source
if h.opts.AddSource {
state.appendAttr(Any(SourceKey, r.source()))
}
key = MessageKey
msg := r.Message
if rep == nil {
state.appendKey(key)
state.appendString(msg)
} else {
state.appendAttr(String(key, msg))
}
state.groups = stateGroups // Restore groups passed to ReplaceAttrs.
state.appendNonBuiltIns(r)
state.buf.WriteByte('\n')
h.mu.Lock()
defer h.mu.Unlock()
_, err := h.w.Write(*state.buf)
return err
}
func (s *handleState) appendNonBuiltIns(r Record) {
// preformatted Attrs
if len(s.h.preformattedAttrs) > 0 {
s.buf.WriteString(s.sep)
s.buf.Write(s.h.preformattedAttrs)
s.sep = s.h.attrSep()
}
// Attrs in Record -- unlike the built-in ones, they are in groups started
// from WithGroup.
s.prefix = buffer.New()
defer s.prefix.Free()
s.prefix.WriteString(s.h.groupPrefix)
s.openGroups()
r.Attrs(func(a Attr) bool {
s.appendAttr(a)
return true
})
if s.h.json {
// Close all open groups.
for range s.h.groups {
s.buf.WriteByte('}')
}
// Close the top-level object.
s.buf.WriteByte('}')
}
}
// attrSep returns the separator between attributes.
func (h *commonHandler) attrSep() string {
if h.json {
return ","
}
return " "
}
// handleState holds state for a single call to commonHandler.handle.
// The initial value of sep determines whether to emit a separator
// before the next key, after which it stays true.
type handleState struct {
h *commonHandler
buf *buffer.Buffer
freeBuf bool // should buf be freed?
sep string // separator to write before next key
prefix *buffer.Buffer // for text: key prefix
groups *[]string // pool-allocated slice of active groups, for ReplaceAttr
}
var groupPool = sync.Pool{New: func() any {
s := make([]string, 0, 10)
return &s
}}
func (h *commonHandler) newHandleState(buf *buffer.Buffer, freeBuf bool, sep string, prefix *buffer.Buffer) handleState {
s := handleState{
h: h,
buf: buf,
freeBuf: freeBuf,
sep: sep,
prefix: prefix,
}
if h.opts.ReplaceAttr != nil {
s.groups = groupPool.Get().(*[]string)
*s.groups = append(*s.groups, h.groups[:h.nOpenGroups]...)
}
return s
}
func (s *handleState) free() {
if s.freeBuf {
s.buf.Free()
}
if gs := s.groups; gs != nil {
*gs = (*gs)[:0]
groupPool.Put(gs)
}
}
func (s *handleState) openGroups() {
for _, n := range s.h.groups[s.h.nOpenGroups:] {
s.openGroup(n)
}
}
// Separator for group names and keys.
const keyComponentSep = '.'
// openGroup starts a new group of attributes
// with the given name.
func (s *handleState) openGroup(name string) {
if s.h.json {
s.appendKey(name)
s.buf.WriteByte('{')
s.sep = ""
} else {
s.prefix.WriteString(name)
s.prefix.WriteByte(keyComponentSep)
}
// Collect group names for ReplaceAttr.
if s.groups != nil {
*s.groups = append(*s.groups, name)
}
}
// closeGroup ends the group with the given name.
func (s *handleState) closeGroup(name string) {
if s.h.json {
s.buf.WriteByte('}')
} else {
(*s.prefix) = (*s.prefix)[:len(*s.prefix)-len(name)-1 /* for keyComponentSep */]
}
s.sep = s.h.attrSep()
if s.groups != nil {
*s.groups = (*s.groups)[:len(*s.groups)-1]
}
}
// appendAttr appends the Attr's key and value using app.
// It handles replacement and checking for an empty key.
// after replacement).
func (s *handleState) appendAttr(a Attr) {
if rep := s.h.opts.ReplaceAttr; rep != nil && a.Value.Kind() != KindGroup {
var gs []string
if s.groups != nil {
gs = *s.groups
}
// Resolve before calling ReplaceAttr, so the user doesn't have to.
a.Value = a.Value.Resolve()
a = rep(gs, a)
}
a.Value = a.Value.Resolve()
// Elide empty Attrs.
if a.isEmpty() {
return
}
// Special case: Source.
if v := a.Value; v.Kind() == KindAny {
if src, ok := v.Any().(*Source); ok {
if s.h.json {
a.Value = src.group()
} else {
a.Value = StringValue(fmt.Sprintf("%s:%d", src.File, src.Line))
}
}
}
if a.Value.Kind() == KindGroup {
attrs := a.Value.Group()
// Output only non-empty groups.
if len(attrs) > 0 {
// Inline a group with an empty key.
if a.Key != "" {
s.openGroup(a.Key)
}
for _, aa := range attrs {
s.appendAttr(aa)
}
if a.Key != "" {
s.closeGroup(a.Key)
}
}
} else {
s.appendKey(a.Key)
s.appendValue(a.Value)
}
}
func (s *handleState) appendError(err error) {
s.appendString(fmt.Sprintf("!ERROR:%v", err))
}
func (s *handleState) appendKey(key string) {
s.buf.WriteString(s.sep)
if s.prefix != nil {
// TODO: optimize by avoiding allocation.
s.appendString(string(*s.prefix) + key)
} else {
s.appendString(key)
}
if s.h.json {
s.buf.WriteByte(':')
} else {
s.buf.WriteByte('=')
}
s.sep = s.h.attrSep()
}
func (s *handleState) appendString(str string) {
if s.h.json {
s.buf.WriteByte('"')
*s.buf = appendEscapedJSONString(*s.buf, str)
s.buf.WriteByte('"')
} else {
// text
if needsQuoting(str) {
*s.buf = strconv.AppendQuote(*s.buf, str)
} else {
s.buf.WriteString(str)
}
}
}
func (s *handleState) appendValue(v Value) {
defer func() {
if r := recover(); r != nil {
// If it panics with a nil pointer, the most likely cases are
// an encoding.TextMarshaler or error fails to guard against nil,
// in which case "<nil>" seems to be the feasible choice.
//
// Adapted from the code in fmt/print.go.
if v := reflect.ValueOf(v.any); v.Kind() == reflect.Pointer && v.IsNil() {
s.appendString("<nil>")
return
}
// Otherwise just print the original panic message.
s.appendString(fmt.Sprintf("!PANIC: %v", r))
}
}()
var err error
if s.h.json {
err = appendJSONValue(s, v)
} else {
err = appendTextValue(s, v)
}
if err != nil {
s.appendError(err)
}
}
func (s *handleState) appendTime(t time.Time) {
if s.h.json {
appendJSONTime(s, t)
} else {
writeTimeRFC3339Millis(s.buf, t)
}
}
// This takes half the time of Time.AppendFormat.
func writeTimeRFC3339Millis(buf *buffer.Buffer, t time.Time) {
year, month, day := t.Date()
buf.WritePosIntWidth(year, 4)
buf.WriteByte('-')
buf.WritePosIntWidth(int(month), 2)
buf.WriteByte('-')
buf.WritePosIntWidth(day, 2)
buf.WriteByte('T')
hour, min, sec := t.Clock()
buf.WritePosIntWidth(hour, 2)
buf.WriteByte(':')
buf.WritePosIntWidth(min, 2)
buf.WriteByte(':')
buf.WritePosIntWidth(sec, 2)
ns := t.Nanosecond()
buf.WriteByte('.')
buf.WritePosIntWidth(ns/1e6, 3)
_, offsetSeconds := t.Zone()
if offsetSeconds == 0 {
buf.WriteByte('Z')
} else {
offsetMinutes := offsetSeconds / 60
if offsetMinutes < 0 {
buf.WriteByte('-')
offsetMinutes = -offsetMinutes
} else {
buf.WriteByte('+')
}
buf.WritePosIntWidth(offsetMinutes/60, 2)
buf.WriteByte(':')
buf.WritePosIntWidth(offsetMinutes%60, 2)
}
}