Fix segfault during shutdown caused by UnsharedNamespaceInitializer static destruction order

The simulation crashed with a segfault during program shutdown. The crash
occurred in cEnvir::removeLifecycleListener(), called from the
~cISimulationLifecycleListener() destructor, which dereferences the envir
returned by getEnvir().

Root cause: UnsharedNamespaceInitializer::singleton was a file-scope static
object registered as a lifecycle listener via EXECUTE_ON_STARTUP. This added
it to the static 'bootstrap' envir (StaticEnv), which is also a file-scope
static (in csimulation.cc). During static destruction, C++ does not guarantee
destruction order across translation units. If StaticEnv was destroyed before
the singleton, the singleton's destructor called getEnvir() on an already-
destroyed object, causing a use-after-free.

Alternatives considered:
- Having the listener destructor check getLifecycleListeners() before calling
  remove: doesn't help because the envir itself may already be destroyed.
- Reordering setActiveSimulation(nullptr) and delete simulation in startup.cc:
  the ~cSimulation destructor panics if the simulation is still active, and
  moving deletion before deactivation caused a different crash.
- Storing a back-pointer to the envir each listener was registered with:
  invasive API change for a problem specific to static-lifetime listeners.

Chosen fix: change the singleton from a static object to a heap-allocated
pointer that is intentionally leaked (never deleted). This matches the
existing pattern used by OMNeT++'s own LocalLifecycleListener in
csoftowner.cc, which is also registered via EXECUTE_ON_STARTUP and
deliberately leaked. Since the listener is never destroyed, its destructor
never runs, and the static destruction order problem is avoided entirely.

Author: levy

src/inet/common/NetworkNamespaceContext.cc

@@ -96,7 +96,7 @@ NetworkNamespaceContext::NetworkNamespaceContext(const char *name)
 #ifdef __linux__
         auto it = localNetworkNamespaces.find(name);
         if (it != localNetworkNamespaces.end()) {
-            oldFd = UnsharedNamespaceInitializer::singleton.originalNetworkNamespaceFd;
+            oldFd = UnsharedNamespaceInitializer::singleton->originalNetworkNamespaceFd;
             newFd = it->second;
             global = false;
         }

src/inet/common/UnsharedNamespaceInitializer.cc

@@ -17,11 +17,11 @@ namespace inet {
 
 Register_GlobalConfigOption(CFGID_UNSHARE_NAMESPACES, "unshare-namespaces", CFG_BOOL, "false", "Unshares the user and network namespaces using the unshare() system call. The simulation continues running as the root user in the new user namespace. This allows creating new network namespaces and assign network resources without using sudo or setting capabilities for opp_run.");
 
-UnsharedNamespaceInitializer UnsharedNamespaceInitializer::singleton;
+UnsharedNamespaceInitializer *UnsharedNamespaceInitializer::singleton = nullptr;
 
 #ifdef __linux__
 #if OMNETPP_VERSION < 0x0700
-EXECUTE_ON_STARTUP(getEnvir()->addLifecycleListener(&UnsharedNamespaceInitializer::singleton));
+EXECUTE_ON_STARTUP(UnsharedNamespaceInitializer::singleton = new UnsharedNamespaceInitializer(); getEnvir()->addLifecycleListener(UnsharedNamespaceInitializer::singleton));
 #endif
 #endif
 

src/inet/common/UnsharedNamespaceInitializer.h

@@ -21,7 +21,7 @@ namespace inet {
 class UnsharedNamespaceInitializer : public omnetpp::cISimulationLifecycleListener
 {
   public:
-    static UnsharedNamespaceInitializer singleton;
+    static UnsharedNamespaceInitializer *singleton;
 
     int originalNetworkNamespaceFd = -1;