Preserve context in logger when calling FromContext. (#11)

Modification of #5 

Allows for passing through the logger while still using default Info
methods. InfoContext remains to override the context with another value.

This changes the stored context value to store the underlying
slog.Logger instead of the wrapped logger so we're not storing a context
within a context.

example_test.go

@@ -42,3 +42,17 @@ func ExampleLogger() {
 	// level=INFO msg="hello world" a=b foo=bar
 	// level=ERROR msg=asdf a=b baz=true
 }
+
+func ExampleFromContext_preserveContext() {
+	log := clog.NewLogger(slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
+		// Remove time for repeatable results
+		ReplaceAttr: slogtest.RemoveTime,
+	}))).With("foo", "bar")
+	ctx := clog.WithLogger(context.Background(), log)
+
+	// Previous context values are preserved when using FromContext
+	clog.FromContext(ctx).Info("hello world")
+
+	// Output:
+	// level=INFO msg="hello world" foo=bar
+}

logger.go

@@ -11,6 +11,7 @@ import (
 
 // Logger implements a wrapper around [slog.Logger] that adds formatter functions (e.g. Infof, Errorf)
 type Logger struct {
+	ctx context.Context
 	slog.Logger
 }
 
@@ -19,12 +20,20 @@ func DefaultLogger() *Logger {
 	return NewLogger(slog.Default())
 }
 
-// NewLogger returns a new logger that wraps the given [slog.Logger].
+// NewLogger returns a new logger that wraps the given [slog.Logger] with the default context.
 func NewLogger(l *slog.Logger) *Logger {
+	return NewLoggerWithContext(context.Background(), l)
+}
+
+// NewLoggerWithContext returns a new logger that wraps the given [slog.Logger].
+func NewLoggerWithContext(ctx context.Context, l *slog.Logger) *Logger {
 	if l == nil {
 		l = slog.Default()
 	}
-	return &Logger{Logger: *l}
+	return &Logger{
+		ctx:    ctx,
+		Logger: *l,
+	}
 }
 
 // New returns a new logger that wraps the given [slog.Handler].
@@ -32,6 +41,11 @@ func New(h slog.Handler) *Logger {
 	return NewLogger(slog.New(h))
 }
 
+// New returns a new logger that wraps the given [slog.Handler].
+func NewWithContext(ctx context.Context, h slog.Handler) *Logger {
+	return NewLoggerWithContext(ctx, slog.New(h))
+}
+
 // With calls [Logger.With] on the default logger.
 func With(args ...any) *Logger {
 	return DefaultLogger().With(args...)
@@ -47,9 +61,16 @@ func (l *Logger) WithGroup(name string) *Logger {
 	return NewLogger(l.Logger.WithGroup(name))
 }
 
+func (l *Logger) context() context.Context {
+	if l.ctx == nil {
+		return context.Background()
+	}
+	return l.ctx
+}
+
 // Infof logs at LevelInfo with the given format and arguments.
 func (l *Logger) Infof(format string, args ...any) {
-	wrapf(context.Background(), l, slog.LevelInfo, format, args...)
+	wrapf(l.context(), l, slog.LevelInfo, format, args...)
 }
 
 // InfoContextf logs at LevelInfo with the given context, format and arguments.
@@ -59,7 +80,7 @@ func (l *Logger) InfoContextf(ctx context.Context, format string, args ...any) {
 
 // Warnf logs at LevelWarn with the given format and arguments.
 func (l *Logger) Warnf(format string, args ...any) {
-	wrapf(context.Background(), l, slog.LevelWarn, format, args...)
+	wrapf(l.context(), l, slog.LevelWarn, format, args...)
 }
 
 // WarnContextf logs at LevelWarn with the given context, format and arguments.
@@ -69,7 +90,7 @@ func (l *Logger) WarnContextf(ctx context.Context, format string, args ...any) {
 
 // Errorf logs at LevelError with the given format and arguments.
 func (l *Logger) Errorf(format string, args ...any) {
-	wrapf(context.Background(), l, slog.LevelError, format, args...)
+	wrapf(l.context(), l, slog.LevelError, format, args...)
 }
 
 // ErrorContextf logs at LevelError with the given context, format and arguments.
@@ -79,7 +100,7 @@ func (l *Logger) ErrorContextf(ctx context.Context, format string, args ...any)
 
 // Debugf logs at LevelDebug with the given format and arguments.
 func (l *Logger) Debugf(format string, args ...any) {
-	wrapf(context.Background(), l, slog.LevelDebug, format, args...)
+	wrapf(l.context(), l, slog.LevelDebug, format, args...)
 }
 
 // DebugContextf logs at LevelDebug with the given context, format and arguments.
@@ -89,13 +110,13 @@ func (l *Logger) DebugContextf(ctx context.Context, format string, args ...any)
 
 // Fatalf logs at LevelError with the given format and arguments, then exits.
 func (l *Logger) Fatalf(format string, args ...any) {
-	wrapf(context.Background(), l, slog.LevelError, format, args...)
+	wrapf(l.context(), l, slog.LevelError, format, args...)
 	os.Exit(1)
 }
 
 // Fatal logs at LevelError with the given message, then exits.
 func (l *Logger) Fatal(msg string, args ...any) {
-	wrap(context.Background(), l, slog.LevelError, msg, args...)
+	wrap(l.context(), l, slog.LevelError, msg, args...)
 	os.Exit(1)
 }
 
@@ -149,12 +170,15 @@ func wrapf(ctx context.Context, logger *Logger, level slog.Level, format string,
 type loggerKey struct{}
 
 func WithLogger(ctx context.Context, logger *Logger) context.Context {
-	return context.WithValue(ctx, loggerKey{}, logger)
+	return context.WithValue(ctx, loggerKey{}, logger.Logger)
 }
 
 func FromContext(ctx context.Context) *Logger {
-	if logger, ok := ctx.Value(loggerKey{}).(*Logger); ok {
-		return logger
+	if logger, ok := ctx.Value(loggerKey{}).(slog.Logger); ok {
+		return &Logger{
+			ctx:    ctx,
+			Logger: logger,
+		}
 	}
-	return DefaultLogger()
+	return NewLoggerWithContext(ctx, slog.Default())
 }