Avoid adding SegfaultLogger if process already has sig handler. (apac…

…he#13842)

In current implemenation, we override signal handler regardless if MXNET_USE_SIGNAL_HANDLER=1.
This breaks caller process behavior and cause process exit unexpectedly.
The example use case is libmxnet.so is loadded into java process via JNI or JNA. JVM will crash
due to SegfaultLogger.

In this PR, we will not register SegfaultLogger if there is a signal handler registered.

src/initialize.cc

@@ -44,7 +44,11 @@ class LibraryInitializer {
   LibraryInitializer() {
     dmlc::InitLogging("mxnet");
 #if MXNET_USE_SIGNAL_HANDLER && DMLC_LOG_STACK_TRACE
-    signal(SIGSEGV, SegfaultLogger);
+    struct sigaction sa;
+    sigaction(SIGSEGV, NULL, &sa);
+    if (sa.sa_handler == NULL) {
+        signal(SIGSEGV, SegfaultLogger);
+    }
 #endif
 
 // disable openmp for multithreaded workers