signal: fix race in Signal_receiver pending_signal

In the past, we had the lock of the context we found for the rest of the scope
of pending_signal. Now we use for_each_locked to find the context and the
lock is released as soon as we return from for_each_locked. Thus, we need to
lock the context again before returning it to avoid that the copy constructor
during the return reads values that are currently changing.

Fixes #2532

repos/base/src/lib/base/signal_common.cc

@@ -175,6 +175,7 @@ Signal Signal_receiver::pending_signal()
 		throw Context_ring::Break_for_each();
 	});
 	if (result.context) {
+		Lock::Guard lock_guard(result.context->_lock);
 		if (result.num == 0)
 			warning("returning signal with num == 0");