log/slog: log and logAttrs initialize ctx at top

In extreme cases (e.g., ctx = nil), it is recommended to initialize the
context only once at the entry point before using log and logAttrs.

Change-Id: Ib191963f52183406d7fcd5104b60fea1a9e1bc80
GitHub-Last-Rev: e1719b9539
GitHub-Pull-Request: golang/go#73066
Reviewed-on: https://go-review.googlesource.com/c/go/+/661255
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/log/slog/logger.go

@@ -238,6 +238,9 @@ func (l *Logger) ErrorContext(ctx context.Context, msg string, args ...any) {
 // It must always be called directly by an exported logging method
 // or function, because it uses a fixed call depth to obtain the pc.
 func (l *Logger) log(ctx context.Context, level Level, msg string, args ...any) {
+	if ctx == nil {
+		ctx = context.Background()
+	}
 	if !l.Enabled(ctx, level) {
 		return
 	}
@@ -250,14 +253,14 @@ func (l *Logger) log(ctx context.Context, level Level, msg string, args ...any)
 	}
 	r := NewRecord(time.Now(), level, msg, pc)
 	r.Add(args...)
-	if ctx == nil {
-		ctx = context.Background()
-	}
 	_ = l.Handler().Handle(ctx, r)
 }
 
 // logAttrs is like [Logger.log], but for methods that take ...Attr.
 func (l *Logger) logAttrs(ctx context.Context, level Level, msg string, attrs ...Attr) {
+	if ctx == nil {
+		ctx = context.Background()
+	}
 	if !l.Enabled(ctx, level) {
 		return
 	}
@@ -270,9 +273,6 @@ func (l *Logger) logAttrs(ctx context.Context, level Level, msg string, attrs ..
 	}
 	r := NewRecord(time.Now(), level, msg, pc)
 	r.AddAttrs(attrs...)
-	if ctx == nil {
-		ctx = context.Background()
-	}
 	_ = l.Handler().Handle(ctx, r)
 }