Non-blocking fetch implementation. (#83)

* Non-blocking fetch implementation.

* Make fetch tests more robust to timing issues.

* Bug fix when ignoring transferred objects.

* Fix.

* Documentation fixes.

src/common/state/redis.c

@@ -364,7 +364,10 @@ void redis_object_table_get_entry(redisAsyncContext *c,
   int64_t manager_count = reply->elements;
 
   if (reply->type == REDIS_REPLY_ARRAY) {
-    const char **manager_vector = malloc(manager_count * sizeof(char *));
+    const char **manager_vector = NULL;
+    if (manager_count > 0) {
+      manager_vector = malloc(manager_count * sizeof(char *));
+    }
     for (int j = 0; j < reply->elements; ++j) {
       CHECK(reply->element[j]->type == REDIS_REPLY_STRING);
       memcpy(managers[j].id, reply->element[j]->str, sizeof(managers[j].id));
@@ -377,10 +380,13 @@ void redis_object_table_get_entry(redisAsyncContext *c,
                   callback_data->user_context);
     /* remove timer */
     destroy_timer_callback(callback_data->db_handle->loop, callback_data);
-    free(managers);
+    if (manager_count > 0) {
+      free(manager_vector);
+    }
   } else {
     LOG_FATAL("expected integer or string, received type %d", reply->type);
   }
+  free(managers);
 }
 
 void redis_object_table_subscribe_lookup(redisAsyncContext *c,

src/common/test/db_tests.c

@@ -47,7 +47,6 @@ void lookup_done_callback(object_id object_id,
              received_port2) != 2) {
     CHECK(0);
   }
-  free(manager_vector);
 }
 
 /* Entry added to database successfully. */

src/common/test/object_table_tests.c

@@ -290,7 +290,6 @@ void lookup_retry_done_callback(object_id object_id,
                                 void *context) {
   CHECK(context == (void *) lookup_retry_context);
   lookup_retry_succeeded = 1;
-  free(manager_vector);
 }
 
 void lookup_retry_fail_callback(unique_id id,