Make the db_stress reopen loop in OperateDb() more robust (#5893)

Summary:
The loop in OperateDb() is getting quite complicated with the introduction of multiple key operations such as MultiGet and Reseeks. This is resulting in a number of corner cases that hangs db_stress due to synchronization problems during reopen (i.e when -reopen=<> option is specified). This PR makes it more robust by ensuring all db_stress threads vote to reopen the DB the exact same number of times.
Most of the changes in this diff are due to indentation.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5893

Test Plan: Run crash test

Differential Revision: D17823827

Pulled By: anand1976

fbshipit-source-id: ec893829f611ac7cac4057c0d3d99f9ffb6a6dd9

tools/db_stress.cc

@@ -2087,18 +2087,11 @@ class StressTest {
     const uint64_t ops_per_open = FLAGS_ops_per_thread / (FLAGS_reopen + 1);
 
     thread->stats.Start();
-    for (uint64_t i = 0, prev_i = 0; i < FLAGS_ops_per_thread; i++) {
+    for (int open_cnt = 0; open_cnt <= FLAGS_reopen; ++open_cnt) {
       if (thread->shared->HasVerificationFailedYet()) {
         break;
       }
-      // In case i is incremented more than once due to multiple operations,
-      // such as MultiGet or iterator seeks, check whether we have crossed
-      // the ops_per_open boundary in the previous iteration. If it did,
-      // then vote to reopen
-      if (i != 0 &&
-          (i % ops_per_open == 0 ||
-           i % ops_per_open < prev_i % ops_per_open)) {
-        {
+      if (open_cnt != 0) {
           thread->stats.FinishedSingleOp();
           MutexLock l(thread->shared->GetMutex());
           while (!thread->snapshot_queue.empty()) {
@@ -2117,8 +2110,11 @@ class StressTest {
           // Commenting this out as we don't want to reset stats on each open.
           // thread->stats.Start();
       }
+
+      for (uint64_t i = 0; i < ops_per_open; i++) {
+        if (thread->shared->HasVerificationFailedYet()) {
+          break;
         }
-      prev_i = i;
 
         // Change Options
         if (FLAGS_set_options_one_in > 0 &&
@@ -2372,6 +2368,7 @@ class StressTest {
         }
 #endif
       }
+    }
     while (!thread->snapshot_queue.empty()) {
       db_->ReleaseSnapshot(thread->snapshot_queue.front().second.snapshot);
       delete thread->snapshot_queue.front().second.key_vec;