log/slog: add DiscardHandler

This adds a package-level variable, slog.DiscardHandler, which is a
slog.Handler which performs no output. This serves a similar purpose
to io.Discard.

Fixes #62005

Change-Id: Ia8babc55f860dec9b663a5c400090a7669608fd5
GitHub-Last-Rev: 0a611174ee8819a2f4e1b8e196a60d5acc6ef9f7
GitHub-Pull-Request: golang/go#70296
Reviewed-on: https://go-review.googlesource.com/c/go/+/626486
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

diff --git a/api/next/62005.txt b/api/next/62005.txt
new file mode 100644 (file)
index 0000000..73aff4a
--- /dev/null
+++ b/api/next/62005.txt
@@ -0,0 +1 @@
+pkg log/slog, var DiscardHandler Handler #62005

diff --git a/doc/next/6-stdlib/99-minor/log/slog/62005.md b/doc/next/6-stdlib/99-minor/log/slog/62005.md
new file mode 100644 (file)
index 0000000..de88024
--- /dev/null
+++ b/doc/next/6-stdlib/99-minor/log/slog/62005.md
@@ -0,0 +1 @@
+The new [DiscardHandler] is a handler that is never enabled and always discards its output.

diff --git a/src/log/slog/example_discard_test.go b/src/log/slog/example_discard_test.go
new file mode 100644 (file)
index 0000000..a1aec65
--- /dev/null
+++ b/src/log/slog/example_discard_test.go
@@ -0,0 +1,23 @@
+package slog_test
+
+import (
+       "log/slog"
+       "log/slog/internal/slogtest"
+       "os"
+)
+
+func ExampleDiscardHandler() {
+       // A slog.TextHandler can output log messages.
+       logger1 := slog.New(slog.NewTextHandler(
+               os.Stdout,
+               &slog.HandlerOptions{ReplaceAttr: slogtest.RemoveTime},
+       ))
+       logger1.Info("message 1")
+
+       // A slog.DiscardHandler will discard all messages.
+       logger2 := slog.New(slog.DiscardHandler)
+       logger2.Info("message 2")
+
+       // Output:
+       // level=INFO msg="message 1"
+}

diff --git a/src/log/slog/handler.go b/src/log/slog/handler.go
index 2ff85b582ef32e6335fcd2d7d04ed7814cd7b5e7..1ca4f9dba3de414344463e1a275765163c8b8d36 100644 (file)
--- a/src/log/slog/handler.go
+++ b/src/log/slog/handler.go
@@ -602,3 +602,14 @@ func appendRFC3339Millis(b []byte, t time.Time) []byte {
        b = append(b[:n+prefixLen], b[n+prefixLen+1:]...) // drop the 4th digit
        return b
 }
+
+// DiscardHandler discards all log output.
+// DiscardHandler.Enabled returns false for all Levels.
+var DiscardHandler Handler = discardHandler{}
+
+type discardHandler struct{}
+
+func (dh discardHandler) Enabled(context.Context, Level) bool  { return false }
+func (dh discardHandler) Handle(context.Context, Record) error { return nil }
+func (dh discardHandler) WithAttrs(attrs []Attr) Handler       { return dh }
+func (dh discardHandler) WithGroup(name string) Handler        { return dh }

diff --git a/src/log/slog/handler_test.go b/src/log/slog/handler_test.go
index 8ce34526d0ba44aaa57d6bc822aff5425b5e73f8..d34025f1bb83ca21cbc4500811b80f250335c253 100644 (file)
--- a/src/log/slog/handler_test.go
+++ b/src/log/slog/handler_test.go
@@ -11,6 +11,7 @@ import (
        "context"
        "encoding/json"
        "io"
+       "os"
        "path/filepath"
        "slices"
        "strconv"
@@ -711,3 +712,23 @@ func BenchmarkWriteTime(b *testing.B) {
                buf = appendRFC3339Millis(buf[:0], tm)
        }
 }
+
+func TestDiscardHandler(t *testing.T) {
+       ctx := context.Background()
+       stdout, stderr := os.Stdout, os.Stderr
+       os.Stdout, os.Stderr = nil, nil // panic on write
+       t.Cleanup(func() {
+               os.Stdout, os.Stderr = stdout, stderr
+       })
+
+       // Just ensure nothing panics during normal usage
+       l := New(DiscardHandler)
+       l.Info("msg", "a", 1, "b", 2)
+       l.Debug("bg", Int("a", 1), "b", 2)
+       l.Warn("w", Duration("dur", 3*time.Second))
+       l.Error("bad", "a", 1)
+       l.Log(ctx, LevelWarn+1, "w", Int("a", 1), String("b", "two"))
+       l.LogAttrs(ctx, LevelInfo+1, "a b c", Int("a", 1), String("b", "two"))
+       l.Info("info", "a", []Attr{Int("i", 1)})
+       l.Info("info", "a", GroupValue(Int("i", 1)))
+}

diff --git a/src/log/slog/logger_test.go b/src/log/slog/logger_test.go
index 9efd4ed0e91f1cc329eeebcfd0b015b49bc809a2..0f1b2113c1513b2ef00e86c4831ed6abafc9bd80 100644 (file)
--- a/src/log/slog/logger_test.go
+++ b/src/log/slog/logger_test.go
@@ -231,7 +231,7 @@ func TestCallDepth(t *testing.T) {
 
 func TestAlloc(t *testing.T) {
        ctx := context.Background()
-       dl := New(discardHandler{})
+       dl := New(discardTestHandler{})
        defer SetDefault(Default()) // restore
        SetDefault(dl)
 
@@ -258,7 +258,7 @@ func TestAlloc(t *testing.T) {
                })
        })
        t.Run("2 pairs disabled inline", func(t *testing.T) {
-               l := New(discardHandler{disabled: true})
+               l := New(DiscardHandler)
                s := "abc"
                i := 2000
                wantAllocs(t, 2, func() {
@@ -269,7 +269,7 @@ func TestAlloc(t *testing.T) {
                })
        })
        t.Run("2 pairs disabled", func(t *testing.T) {
-               l := New(discardHandler{disabled: true})
+               l := New(DiscardHandler)
                s := "abc"
                i := 2000
                wantAllocs(t, 0, func() {
@@ -305,7 +305,7 @@ func TestAlloc(t *testing.T) {
                })
        })
        t.Run("attrs3 disabled", func(t *testing.T) {
-               logger := New(discardHandler{disabled: true})
+               logger := New(DiscardHandler)
                wantAllocs(t, 0, func() {
                        logger.LogAttrs(ctx, LevelInfo, "hello", Int("a", 1), String("b", "two"), Duration("c", time.Second))
                })
@@ -568,18 +568,17 @@ func (c *captureHandler) clear() {
        c.r = Record{}
 }
 
-type discardHandler struct {
-       disabled bool
-       attrs    []Attr
+type discardTestHandler struct {
+       attrs []Attr
 }
 
-func (d discardHandler) Enabled(context.Context, Level) bool { return !d.disabled }
-func (discardHandler) Handle(context.Context, Record) error  { return nil }
-func (d discardHandler) WithAttrs(as []Attr) Handler {
+func (d discardTestHandler) Enabled(context.Context, Level) bool { return true }
+func (discardTestHandler) Handle(context.Context, Record) error  { return nil }
+func (d discardTestHandler) WithAttrs(as []Attr) Handler {
        d.attrs = concat(d.attrs, as)
        return d
 }
-func (h discardHandler) WithGroup(name string) Handler {
+func (h discardTestHandler) WithGroup(name string) Handler {
        return h
 }