SystemLayerImplSelect: libev: eliminate I/O wakeup thread

libev based regular builds do not need a separate I/O wakeup thread, so
the wakeup thread can be eliminated in CHIP_SYSTEM_CONFIG_USE_LIBEV case.

In normal operation, libev based builds also never call Signal(), so if it
is still called it now emits a error log message, to indicate something might
be wrong in the setup.

However we keep Signal() and related LayerSocketsLoop methods, following the
Darwin dispatch implementation, as fallback to select-based event handling
seems to be needed for some I/O tests.

As noted in a comment to the original libev PR [1], eventually, the select() based
mainloop and external mainloop based solutions like Darwin Dispatch and libev
should be detangled and extracted into separate classes, adapting all the tests
that somehow rely on the select() fallback. As a single self-funded
developer however, I cannot possibly be expected to solve this for Apple ;-)

[1] project-chip#24232 (review)

src/system/SystemLayerImplSelect.cpp

@@ -66,8 +66,10 @@ CHIP_ERROR LayerImplSelect::Init()
     mHandleSelectThread = PTHREAD_NULL;
 #endif // CHIP_SYSTEM_CONFIG_POSIX_LOCKING
 
+#if !CHIP_SYSTEM_CONFIG_USE_LIBEV
     // Create an event to allow an arbitrary thread to wake the thread in the select loop.
     ReturnErrorOnFailure(mWakeEvent.Open(*this));
+#endif // !CHIP_SYSTEM_CONFIG_USE_LIBEV
 
     VerifyOrReturnError(mLayerState.SetInitialized(), CHIP_ERROR_INCORRECT_STATE);
     return CHIP_NO_ERROR;
@@ -113,13 +115,18 @@ void LayerImplSelect::Shutdown()
     mTimerPool.ReleaseAll();
 #endif // CHIP_SYSTEM_CONFIG_USE_DISPATCH/LIBEV
 
+#if !CHIP_SYSTEM_CONFIG_USE_LIBEV
     mWakeEvent.Close(*this);
+#endif // !CHIP_SYSTEM_CONFIG_USE_LIBEV
 
     mLayerState.ResetFromShuttingDown(); // Return to uninitialized state to permit re-initialization.
 }
 
 void LayerImplSelect::Signal()
 {
+#if CHIP_SYSTEM_CONFIG_USE_LIBEV
+    ChipLogError(DeviceLayer, "Signal() should not be called in CHIP_SYSTEM_CONFIG_USE_LIBEV builds (might be ok in tests)");
+#else
     /*
      * Wake up the I/O thread by writing a single byte to the wake pipe.
      *
@@ -143,6 +150,7 @@ void LayerImplSelect::Signal()
 
         ChipLogError(chipSystemLayer, "System wake event notify failed: %" CHIP_ERROR_FORMAT, status.Format());
     }
+#endif // !CHIP_SYSTEM_CONFIG_USE_LIBEV
 }
 
 CHIP_ERROR LayerImplSelect::StartTimer(Clock::Timeout delay, TimerCompleteCallback onComplete, void * appState)
@@ -274,7 +282,10 @@ void LayerImplSelect::CancelTimer(TimerCompleteCallback onComplete, void * appSt
 #endif // CHIP_SYSTEM_CONFIG_USE_DISPATCH/LIBEV
 
     mTimerPool.Release(timer);
+#if !CHIP_SYSTEM_CONFIG_USE_LIBEV
+    // LIBEV has no I/O wakeup thread, so must not call Signal()
     Signal();
+#endif
 }
 
 CHIP_ERROR LayerImplSelect::ScheduleWork(TimerCompleteCallback onComplete, void * appState)

src/system/SystemLayerImplSelect.h

@@ -146,7 +146,9 @@ class LayerImplSelect : public LayerSocketsLoop
     int mSelectResult;
 
     ObjectLifeCycle mLayerState;
+#if !CHIP_SYSTEM_CONFIG_USE_LIBEV
     WakeEvent mWakeEvent;
+#endif
 
 #if CHIP_SYSTEM_CONFIG_POSIX_LOCKING
     std::atomic<pthread_t> mHandleSelectThread;

src/system/WakeEvent.cpp

@@ -23,7 +23,7 @@
 
 #include <system/SystemConfig.h>
 
-#if CHIP_SYSTEM_CONFIG_USE_SOCKETS
+#if CHIP_SYSTEM_CONFIG_USE_SOCKETS && !CHIP_SYSTEM_CONFIG_USE_LIBEV
 
 #include <system/WakeEvent.h>
 
@@ -207,4 +207,4 @@ CHIP_ERROR WakeEvent::Notify() const
 } // namespace System
 } // namespace chip
 
-#endif // CHIP_SYSTEM_CONFIG_USE_SOCKETS
+#endif // CHIP_SYSTEM_CONFIG_USE_SOCKETS && !CHIP_SYSTEM_CONFIG_USE_LIBEV

src/system/WakeEvent.h

@@ -25,7 +25,7 @@
 // Include configuration headers
 #include <system/SystemConfig.h>
 
-#if CHIP_SYSTEM_CONFIG_USE_SOCKETS
+#if CHIP_SYSTEM_CONFIG_USE_SOCKETS && !CHIP_SYSTEM_CONFIG_USE_LIBEV
 
 #include <lib/core/CHIPError.h>
 #include <system/SocketEvents.h>
@@ -67,4 +67,4 @@ class WakeEvent
 } // namespace System
 } // namespace chip
 
-#endif // CHIP_SYSTEM_CONFIG_USE_SOCKETS
+#endif // CHIP_SYSTEM_CONFIG_USE_SOCKETS && !CHIP_SYSTEM_CONFIG_USE_LIBEV