log/slog: add LogLoggerLevel to enable setting level on the default logger

Fixes #62418

Change-Id: I889a53d00c8a463b4d7ddb41893c000d7cd0e7b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/525096
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

4 files changed, 162 insertions, 6 deletions

diff --git a/src/log/slog/example_log_level_test.go b/src/log/slog/example_log_level_test.go
new file mode 100644
index 0000000000..ca8db416e5
--- /dev/null
+++ b/src/log/slog/example_log_level_test.go
@@ -0,0 +1,58 @@
+// Copyright 2023 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_test
+
+import (
+	"log"
+	"log/slog"
+	"log/slog/internal/slogtest"
+	"os"
+)
+
+// This example shows how to use slog.SetLogLoggerLevel to change the minimal level
+// of the internal default handler for slog package before calling slog.SetDefault.
+func ExampleSetLogLoggerLevel_log() {
+	defer log.SetFlags(log.Flags()) // revert changes after the example
+	log.SetFlags(0)
+	defer log.SetOutput(log.Writer()) // revert changes after the example
+	log.SetOutput(os.Stdout)
+
+	// Default logging level is slog.LevelInfo.
+	log.Print("log debug") // log debug
+	slog.Debug("debug")    // no output
+	slog.Info("info")      // INFO info
+
+	// Set the default logging level to slog.LevelDebug.
+	currentLogLevel := slog.SetLogLoggerLevel(slog.LevelDebug)
+	defer slog.SetLogLoggerLevel(currentLogLevel) // revert changes after the example
+
+	log.Print("log debug") // log debug
+	slog.Debug("debug")    // DEBUG debug
+	slog.Info("info")      // INFO info
+
+	// Output:
+	// log debug
+	// INFO info
+	// log debug
+	// DEBUG debug
+	// INFO info
+}
+
+// This example shows how to use slog.SetLogLoggerLevel to change the minimal level
+// of the internal writer that uses the custom handler for log package after
+// calling slog.SetDefault.
+func ExampleSetLogLoggerLevel_slog() {
+	// Set the default logging level to slog.LevelError.
+	currentLogLevel := slog.SetLogLoggerLevel(slog.LevelError)
+	defer slog.SetLogLoggerLevel(currentLogLevel) // revert changes after the example
+
+	defer slog.SetDefault(slog.Default()) // revert changes after the example
+	slog.SetDefault(slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{ReplaceAttr: slogtest.RemoveTime})))
+
+	log.Print("error") // level=ERROR msg=error
+
+	// Output:
+	// level=ERROR msg=error
+}
diff --git a/src/log/slog/handler.go b/src/log/slog/handler.go
index a6c643cdb9..9f6d88b169 100644
--- a/src/log/slog/handler.go
+++ b/src/log/slog/handler.go
@@ -100,7 +100,7 @@ func newDefaultHandler(output func(uintptr, []byte) error) *defaultHandler {
 }
 
 func (*defaultHandler) Enabled(_ context.Context, l Level) bool {
-	return l >= LevelInfo
+	return l >= logLoggerLevel.Level()
 }
 
 // Collect the level, attributes and message in a string and
diff --git a/src/log/slog/logger.go b/src/log/slog/logger.go
index fceafe0cba..f03aeec295 100644
--- a/src/log/slog/logger.go
+++ b/src/log/slog/logger.go
@@ -16,6 +16,36 @@ import (
 
 var defaultLogger atomic.Pointer[Logger]
 
+var logLoggerLevel LevelVar
+
+// SetLogLoggerLevel controls the level for the bridge to the [log] package.
+//
+// Before [SetDefault] is called, slog top-level logging functions call the default [log.Logger].
+// In that mode, SetLogLoggerLevel sets the minimum level for those calls.
+// By default, the minimum level is Info, so calls to [Debug]
+// (as well as top-level logging calls at lower levels)
+// will not be passed to the log.Logger. After calling
+//
+//	slog.SetLogLoggerLevel(slog.LevelDebug)
+//
+// calls to [Debug] will be passed to the log.Logger.
+//
+// After [SetDefault] is called, calls to the default [log.Logger] are passed to the
+// slog default handler. In that mode,
+// SetLogLoggerLevel sets the level at which those calls are logged.
+// That is, after calling
+//
+//	slog.SetLogLoggerLevel(slog.LevelDebug)
+//
+// A call to [log.Printf] will result in output at level [LevelDebug].
+//
+// SetLogLoggerLevel returns the previous value.
+func SetLogLoggerLevel(level Level) (oldLevel Level) {
+	oldLevel = logLoggerLevel.Level()
+	logLoggerLevel.Set(level)
+	return
+}
+
 func init() {
 	defaultLogger.Store(New(newDefaultHandler(loginternal.DefaultOutput)))
 }
@@ -25,7 +55,8 @@ func Default() *Logger { return defaultLogger.Load() }
 
 // 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 [LevelInfo] using l's Handler.
+// (as with [log.Print], etc.) will be logged using l's Handler,
+// at a level controlled by [SetLogLoggerLevel].
 func SetDefault(l *Logger) {
 	defaultLogger.Store(l)
 	// If the default's handler is a defaultHandler, then don't use a handleWriter,
@@ -36,7 +67,7 @@ func SetDefault(l *Logger) {
 	// See TestSetDefault.
 	if _, ok := l.Handler().(*defaultHandler); !ok {
 		capturePC := log.Flags()&(log.Lshortfile|log.Llongfile) != 0
-		log.SetOutput(&handlerWriter{l.Handler(), LevelInfo, capturePC})
+		log.SetOutput(&handlerWriter{l.Handler(), &logLoggerLevel, capturePC})
 		log.SetFlags(0) // we want just the log message, no time or location
 	}
 }
@@ -45,12 +76,13 @@ func SetDefault(l *Logger) {
 // It is used to link the default log.Logger to the default slog.Logger.
 type handlerWriter struct {
 	h         Handler
-	level     Level
+	level     Leveler
 	capturePC bool
 }
 
 func (w *handlerWriter) Write(buf []byte) (int, error) {
-	if !w.h.Enabled(context.Background(), w.level) {
+	level := w.level.Level()
+	if !w.h.Enabled(context.Background(), level) {
 		return 0, nil
 	}
 	var pc uintptr
@@ -66,7 +98,7 @@ func (w *handlerWriter) Write(buf []byte) (int, error) {
 	if len(buf) > 0 && buf[len(buf)-1] == '\n' {
 		buf = buf[:len(buf)-1]
 	}
-	r := NewRecord(time.Now(), w.level, string(buf), pc)
+	r := NewRecord(time.Now(), level, string(buf), pc)
 	return origLen, w.h.Handle(context.Background(), r)
 }
 
diff --git a/src/log/slog/logger_test.go b/src/log/slog/logger_test.go
index 88aa38ee0c..bb1c8a16ea 100644
--- a/src/log/slog/logger_test.go
+++ b/src/log/slog/logger_test.go
@@ -191,6 +191,7 @@ func TestCallDepth(t *testing.T) {
 		}
 	}
 
+	defer SetDefault(Default()) // restore
 	logger := New(h)
 	SetDefault(logger)
 
@@ -363,6 +364,71 @@ func TestSetDefault(t *testing.T) {
 	}
 }
 
+// Test defaultHandler minimum level without calling slog.SetDefault.
+func TestLogLoggerLevelForDefaultHandler(t *testing.T) {
+	// Revert any changes to the default logger, flags, and level of log and slog.
+	currentLogLoggerLevel := logLoggerLevel.Level()
+	currentLogWriter := log.Writer()
+	currentLogFlags := log.Flags()
+	t.Cleanup(func() {
+		logLoggerLevel.Set(currentLogLoggerLevel)
+		log.SetOutput(currentLogWriter)
+		log.SetFlags(currentLogFlags)
+	})
+
+	var logBuf bytes.Buffer
+	log.SetOutput(&logBuf)
+	log.SetFlags(0)
+
+	for _, test := range []struct {
+		logLevel Level
+		logFn    func(string, ...any)
+		want     string
+	}{
+		{LevelDebug, Debug, "DEBUG a"},
+		{LevelDebug, Info, "INFO a"},
+		{LevelInfo, Debug, ""},
+		{LevelInfo, Info, "INFO a"},
+	} {
+		SetLogLoggerLevel(test.logLevel)
+		test.logFn("a")
+		checkLogOutput(t, logBuf.String(), test.want)
+		logBuf.Reset()
+	}
+}
+
+// Test handlerWriter minimum level by calling slog.SetDefault.
+func TestLogLoggerLevelForHandlerWriter(t *testing.T) {
+	removeTime := func(_ []string, a Attr) Attr {
+		if a.Key == TimeKey {
+			return Attr{}
+		}
+		return a
+	}
+
+	// Revert any changes to the default logger. This is important because other
+	// tests might change the default logger using SetDefault. Also ensure we
+	// restore the default logger at the end of the test.
+	currentLogger := Default()
+	currentLogLoggerLevel := logLoggerLevel.Level()
+	currentLogWriter := log.Writer()
+	currentFlags := log.Flags()
+	t.Cleanup(func() {
+		SetDefault(currentLogger)
+		logLoggerLevel.Set(currentLogLoggerLevel)
+		log.SetOutput(currentLogWriter)
+		log.SetFlags(currentFlags)
+	})
+
+	var logBuf bytes.Buffer
+	log.SetOutput(&logBuf)
+	log.SetFlags(0)
+	SetLogLoggerLevel(LevelError)
+	SetDefault(New(NewTextHandler(&logBuf, &HandlerOptions{ReplaceAttr: removeTime})))
+	log.Print("error")
+	checkLogOutput(t, logBuf.String(), `level=ERROR msg=error`)
+}
+
 func TestLoggerError(t *testing.T) {
 	var buf bytes.Buffer