db_stress to cover total order seek (#6039)

Summary:
Right now, in db_stress, as long as prefix extractor is defined, TestIterator always uses. There is value of cover total_order_seek = true when prefix extractor is define. Add a small chance that this flag is turned on.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6039

Test Plan: Run the test for a while.

Differential Revision: D18539689

fbshipit-source-id: 568790dd7789c9986b83764b870df0423a122d99

tools/db_stress_tool.cc

@@ -2426,6 +2426,11 @@ class StressTest {
     ReadOptions readoptionscopy = read_opts;
     readoptionscopy.snapshot = snapshot;
 
+    if (thread->rand.OneIn(16)) {
+      // When prefix extractor is used, it's useful to cover total order seek.
+      readoptionscopy.total_order_seek = true;
+    }
+
     std::string upper_bound_str;
     Slice upper_bound;
     if (thread->rand.OneIn(16)) {
@@ -2585,7 +2590,8 @@ class StressTest {
       // Iterator is not valid. It can be legimate if it has already been
       // out of upper or lower bound, or filtered out by prefix iterator.
       const Slice& total_order_key = cmp_iter->key();
-      const SliceTransform* pe = options_.prefix_extractor.get();
+      const SliceTransform* pe =
+          ro.total_order_seek ? nullptr : options_.prefix_extractor.get();
       const Comparator* cmp = options_.comparator;
 
       if (pe != nullptr) {