From ba7c46a3d873a2595cfb68ed97bc3e2710b457bf Mon Sep 17 00:00:00 2001
From: Andrew Kryczka <ajkr@users.noreply.github.com>
Date: Thu, 21 Jan 2021 08:47:06 -0800
Subject: [PATCH] workaround race conditions during `PeriodicWorkScheduler`
 registration (#7888)

Summary:
This provides a workaround for two race conditions that will be fixed in
a more sophisticated way later. This PR:

(1) Makes the client serialize calls to `Timer::Start()` and `Timer::Shutdown()` (see https://github.com/facebook/rocksdb/issues/7711). The long-term fix will be to make those functions thread-safe.
(2) Makes `PeriodicWorkScheduler` atomically add/cancel work together with starting/shutting down its `Timer`. The long-term fix will be for `Timer` API to offer more specialized APIs so the client will not need to synchronize.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7888

Test Plan: ran the repro provided in https://github.com/facebook/rocksdb/issues/7881

Reviewed By: jay-zhuang

Differential Revision: D25990891

Pulled By: ajkr

fbshipit-source-id: a97fdaebbda6d7db7ddb1b146738b68c16c5be38
---
 HISTORY.md                    | 3 +++
 db/periodic_work_scheduler.cc | 9 +++++++--
 db/periodic_work_scheduler.h  | 6 ++++++
 util/timer.h                  | 3 +++
 4 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/HISTORY.md b/HISTORY.md
index acb00a34c..2adef8d7f 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,4 +1,7 @@
 # Rocksdb Change Log
+## Unreleased
+### Bug Fixes
+* Fix a race condition between DB startups and shutdowns in managing the periodic background worker threads. One effect of this race condition could be the process being terminated.
 
 ## 6.16.1 (1/20/2021)
 ### Bug Fixes
diff --git a/db/periodic_work_scheduler.cc b/db/periodic_work_scheduler.cc
index cc6f714d9..da0bc1e4b 100644
--- a/db/periodic_work_scheduler.cc
+++ b/db/periodic_work_scheduler.cc
@@ -10,13 +10,14 @@
 #ifndef ROCKSDB_LITE
 namespace ROCKSDB_NAMESPACE {
 
-PeriodicWorkScheduler::PeriodicWorkScheduler(Env* env) {
+PeriodicWorkScheduler::PeriodicWorkScheduler(Env* env) : timer_mu_(env) {
   timer = std::unique_ptr<Timer>(new Timer(env));
 }
 
 void PeriodicWorkScheduler::Register(DBImpl* dbi,
                                      unsigned int stats_dump_period_sec,
                                      unsigned int stats_persist_period_sec) {
+  MutexLock l(&timer_mu_);
   static std::atomic<uint64_t> initial_delay(0);
   timer->Start();
   if (stats_dump_period_sec > 0) {
@@ -41,6 +42,7 @@ void PeriodicWorkScheduler::Register(DBImpl* dbi,
 }
 
 void PeriodicWorkScheduler::Unregister(DBImpl* dbi) {
+  MutexLock l(&timer_mu_);
   timer->Cancel(GetTaskName(dbi, "dump_st"));
   timer->Cancel(GetTaskName(dbi, "pst_st"));
   timer->Cancel(GetTaskName(dbi, "flush_info_log"));
@@ -78,7 +80,10 @@ PeriodicWorkTestScheduler* PeriodicWorkTestScheduler::Default(Env* env) {
     MutexLock l(&mutex);
     if (scheduler.timer.get() != nullptr &&
         scheduler.timer->TEST_GetPendingTaskNum() == 0) {
-      scheduler.timer->Shutdown();
+      {
+        MutexLock timer_mu_guard(&scheduler.timer_mu_);
+        scheduler.timer->Shutdown();
+      }
       scheduler.timer.reset(new Timer(env));
     }
   }
diff --git a/db/periodic_work_scheduler.h b/db/periodic_work_scheduler.h
index 6c1ce314c..9382adc44 100644
--- a/db/periodic_work_scheduler.h
+++ b/db/periodic_work_scheduler.h
@@ -42,6 +42,12 @@ class PeriodicWorkScheduler {
 
  protected:
   std::unique_ptr<Timer> timer;
+  // `timer_mu_` serves two purposes currently:
+  // (1) to ensure calls to `Start()` and `Shutdown()` are serialized, as
+  //     they are currently not implemented in a thread-safe way; and
+  // (2) to ensure the `Timer::Add()`s and `Timer::Start()` run atomically, and
+  //     the `Timer::Cancel()`s and `Timer::Shutdown()` run atomically.
+  port::Mutex timer_mu_;
 
   explicit PeriodicWorkScheduler(Env* env);
 
diff --git a/util/timer.h b/util/timer.h
index 8e12d7d7b..b6ee42ed0 100644
--- a/util/timer.h
+++ b/util/timer.h
@@ -22,6 +22,9 @@ namespace ROCKSDB_NAMESPACE {
 
 // A Timer class to handle repeated work.
 //
+// `Start()` and `Shutdown()` are currently not thread-safe. The client must
+// serialize calls to these two member functions.
+//
 // A single timer instance can handle multiple functions via a single thread.
 // It is better to leave long running work to a dedicated thread pool.
 //