wsd: reap bg-save on SIGCHLD

Change-Id: I4f2941a67fc7b3871e18c6171a6714e9d021f380
Signed-off-by: Ashod Nakashian <ashod.nakashian@collabora.co.uk>

kit/Kit.cpp

@@ -3078,6 +3078,16 @@ void wakeCallback(void* pData)
 #endif
 }
 
+extern "C"
+{
+    [[maybe_unused]]
+    static void sigChildHandler(int pid)
+    {
+        // Reap the child; will log failures.
+        SigUtil::reapZombieChild(pid);
+    }
+}
+
 #ifndef BUILDING_TESTS
 
 namespace
@@ -3679,6 +3689,14 @@ void lokit_main(
 
         LOG_INF("New kit client websocket inserted.");
 
+        // Since we don't track the bg-save process,
+        // for example to prevent multiple parallel saves,
+        // we could, in principle, ignore SIGCHLD and avoid
+        // the problem of zombies and reaping. Unfortunately,
+        // ignoring SIGCHLD is not portable, according to
+        // man 2 sigaction. So we simply waitpid(2) on SIGCHLD.
+        SigUtil::setSigChildHandler(sigChildHandler);
+
 #if !MOBILEAPP
         if (bTraceStartup && LogLevel != LogLevelStartup)
         {