gda/ro: validate and exit cleanly when forced GDA config is invalid (#354)

* gda: validate and exit cleanly when forced GDA config is invalid

* ro: validate and exit cleanly when forced RO config is invalid

src/rocshmem.cpp

@@ -93,9 +93,21 @@ static BackendType select_backend_type() {
   if (!envstr.empty()) {
     DPRINTF("Found environment variable ROCSHMEM_BACKEND, value is %s\n", envstr.c_str());
     if (envstr.find("gda") != std::string::npos) {
+      if (GDABackend::backend_can_run() != ROCSHMEM_SUCCESS) {
+        fprintf(stderr, "Error: ROCSHMEM_BACKEND=gda requested but GDA backend cannot run.\n"
+                        "No active RDMA interface found for the requested provider.\n"
+                        "Check that the correct NIC hardware is present and the link is active.\n");
+        exit(1);
+      }
       return BackendType::GDA_BACKEND;
     }
     if (envstr.find("ro") != std::string::npos) {
+      if (ROBackend::backend_can_run() != ROCSHMEM_SUCCESS) {
+        fprintf(stderr, "Error: ROCSHMEM_BACKEND=ro requested but RO backend cannot run.\n"
+                        "MPI library could not be loaded.\n"
+                        "Check that MPI is properly installed and accessible.\n");
+        exit(1);
+      }
       return BackendType::RO_BACKEND;
     }
     if (envstr.find("ipc") != std::string::npos) {