[BUG] Graceful handling of random walking with no successors (#433)

src/graph/sampler.cc

@@ -504,12 +504,9 @@ IdArray SamplerOp::RandomWalk(
       DLContext{kDLCPU, 0});
   dgl_id_t *trace_data = static_cast<dgl_id_t *>(traces->data);
 
-#pragma omp parallel
-  {
-    // get per-thread seed
-    unsigned int random_seed = time(nullptr) ^ omp_get_thread_num();
+  // FIXME: does OpenMP work with exceptions?  Especially without throwing SIGABRT?
+  unsigned int random_seed = time(nullptr);
 
-#pragma omp for
   for (int i = 0; i < num_nodes; ++i) {
     const dgl_id_t seed_id = seed_ids[i];
 
@@ -523,8 +520,11 @@ IdArray SamplerOp::RandomWalk(
 
         const auto succ = gptr->SuccVec(cur);
         const size_t size = succ.size();
-          cur = succ[rand_r(&random_seed) % size];
+        if (size == 0) {
+          LOG(FATAL) << "no successors from vertex " << cur;
+          return traces;
         }
+        cur = succ[rand_r(&random_seed) % size];
       }
     }
   }