pkg/log: fix panic when using custom Logger (scionproto#4452)

log.FromCtx could panic if a custom Logger was used and a span was
present in the context. This commit allows the custom logger to
implement the `WithOptions(...zap.Option) Logger` method so that the
CallerSkip can still be applied. In case the logger can't be casted to
anything the caller skip is not applied, but we also don't panic
anymore.

Author: lukedirtwalker

pkg/log/context.go

@@ -65,8 +65,21 @@ func WithLabels(ctx context.Context, labels ...interface{}) (context.Context, Lo
 
 func attachSpan(ctx context.Context, l Logger) Logger {
 	if span := opentracing.SpanFromContext(ctx); span != nil {
+		if optioner, ok := l.(interface{ WithOptions(...zap.Option) Logger }); ok {
+			return Span{
+				Logger: optioner.WithOptions(zap.AddCallerSkip(1)),
+				Span:   span,
+			}
+		}
+		if il, ok := l.(*logger); ok {
+			return Span{
+				Logger: &logger{logger: il.logger.WithOptions(zap.AddCallerSkip(1))},
+				Span:   span,
+			}
+		}
+		// Pessimistic fallback, we don't have access to the underlying zap logger:
 		return Span{
-			Logger: &logger{logger: l.(*logger).logger.WithOptions(zap.AddCallerSkip(1))},
+			Logger: l,
 			Span:   span,
 		}
 	}