Fix #88: Timeout handling. (#91)

fix timeout handling

* Despite what is documented in sd_bus_get_timeout(3), the timeout
  returned is actually an absolute time point of Linux's CLOCK_MONOTONIC
  clock. Hence, we first have to subtract the current time from the
  timeout in order to get a relative time that can be passed to poll.

* For async call timeouts to reliably work, we need a way to notify the
  event loop of a connection that is currently blocked waiting in poll.
  I.e. assume the event loop thread entered poll with a timeout set to
  T1. Afterwards, the main thread starts an async call C with a timeout
  T2 < T1. In order for C to be canceled after its timeout T1 has
  elapsed, we have to be able to notify the event loop so that it can
  update its poll data.

Co-authored-by: Urs Ritzmann <ursritzmann@protonmail.ch>
Co-authored-by: Lukasz Marcul <lukasz.marcul@onemeter.com>

tests/integrationtests/Defs.h

@@ -28,6 +28,8 @@
 #define SDBUS_CPP_INTEGRATIONTESTS_DEFS_H_
 
 #include "sdbus-c++/Types.h"
+#include <chrono>
+#include <ostream>
 
 namespace sdbus { namespace test {
 
@@ -54,6 +56,20 @@ const bool DEFAULT_BLOCKING_VALUE{true};
 
 constexpr const double DOUBLE_VALUE{3.24L};
 
+/** Duration stream operator for human readable gtest value output.
+ *
+ * Note that the conversion to double is lossy if the input type has 64 or more bits.
+ * This is ok for our integration tests because they don't have very
+ * accurate timing requirements.
+ *
+ * @return human readable duration in seconds
+ */
+template< class Rep, class Period >
+static std::ostream& operator<<(std::ostream& os, const std::chrono::duration<Rep, Period>& d)
+{
+    auto seconds = std::chrono::duration_cast<std::chrono::duration<double>>(d);
+    return os << seconds.count() << " s";
+}
 }}
 
 #endif /* SDBUS_CPP_INTEGRATIONTESTS_DEFS_H_ */