Introduce a ThreadGuard class and use it in ExternalSSTFileTest.PickedLevelBug (#8112)

Summary:
The patch adds a resource management/RAII class called `ThreadGuard`,
which can be used to ensure that the managed thread is joined when the
`ThreadGuard` is destroyed, regardless of whether it is due to the
object going out of scope, an early return, an exception etc. This is
important because if an `std::thread` object is destroyed without having
been joined (or detached) first, the process is aborted (via
`std::terminate`).

For now, `ThreadGuard` is only used in the test case
`ExternalSSTFileTest.PickedLevelBug`; however, it could come in handy
elsewhere in the codebase as well (both in test code and "real" code).
Case in point: in the `PickedLevelBug` test case, with the earlier code we
could end up in the above situation when the following assertion (which is
before the threads are joined) is triggered:

```
ASSERT_FALSE(bg_compact_started.load());
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8112

Test Plan:
```
make check
gtest-parallel --repeat=10000 ./external_sst_file_test --gtest_filter="*PickedLevelBug"
```

Reviewed By: riversand963

Differential Revision: D27343185

Pulled By: ltamasi

fbshipit-source-id: 2a8c3aa68bc78cc03ec0dbae909fb25c2cd15c69

db/external_sst_file_test.cc

@@ -16,6 +16,7 @@
 #include "rocksdb/sst_file_writer.h"
 #include "test_util/testutil.h"
 #include "util/random.h"
+#include "util/thread_guard.h"
 #include "utilities/fault_injection_env.h"
 
 namespace ROCKSDB_NAMESPACE {
@@ -1305,20 +1306,22 @@ TEST_F(ExternalSSTFileTest, PickedLevelBug) {
 
   ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->EnableProcessing();
 
-  // While writing the MANIFEST start a thread that will ask for compaction
   Status bg_compact_status;
-  ROCKSDB_NAMESPACE::port::Thread bg_compact([&]() {
+  Status bg_addfile_status;
+
+  {
+    // While writing the MANIFEST start a thread that will ask for compaction
+    ThreadGuard bg_compact(port::Thread([&]() {
       bg_compact_status =
           db_->CompactRange(CompactRangeOptions(), nullptr, nullptr);
-  });
+    }));
     TEST_SYNC_POINT("ExternalSSTFileTest::PickedLevelBug:2");
 
     // Start a thread that will ingest a new file
-  Status bg_addfile_status;
-  ROCKSDB_NAMESPACE::port::Thread bg_addfile([&]() {
+    ThreadGuard bg_addfile(port::Thread([&]() {
       file_keys = {1, 2, 3};
       bg_addfile_status = GenerateAndAddExternalFile(options, file_keys, 1);
-  });
+    }));
 
     // Wait for AddFile to start picking levels and writing MANIFEST
     TEST_SYNC_POINT("ExternalSSTFileTest::PickedLevelBug:0");
@@ -1334,9 +1337,7 @@ TEST_F(ExternalSSTFileTest, PickedLevelBug) {
 
     // Hold AddFile from finishing writing the MANIFEST
     TEST_SYNC_POINT("ExternalSSTFileTest::PickedLevelBug:1");
-
-  bg_addfile.join();
-  bg_compact.join();
+  }
 
   ASSERT_OK(bg_addfile_status);
   ASSERT_OK(bg_compact_status);

util/thread_guard.h

@@ -0,0 +1,41 @@
+//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+
+#pragma once
+
+#include "port/port.h"
+#include "rocksdb/rocksdb_namespace.h"
+
+namespace ROCKSDB_NAMESPACE {
+
+// Resource management object for threads that joins the thread upon
+// destruction. Has unique ownership of the thread object, so copying it is not
+// allowed, while moving it transfers ownership.
+class ThreadGuard {
+ public:
+  ThreadGuard() = default;
+
+  explicit ThreadGuard(port::Thread&& thread) : thread_(std::move(thread)) {}
+
+  ThreadGuard(const ThreadGuard&) = delete;
+  ThreadGuard& operator=(const ThreadGuard&) = delete;
+
+  ThreadGuard(ThreadGuard&&) noexcept = default;
+  ThreadGuard& operator=(ThreadGuard&&) noexcept = default;
+
+  ~ThreadGuard() {
+    if (thread_.joinable()) {
+      thread_.join();
+    }
+  }
+
+  const port::Thread& GetThread() const { return thread_; }
+  port::Thread& GetThread() { return thread_; }
+
+ private:
+  port::Thread thread_;
+};
+
+}  // namespace ROCKSDB_NAMESPACE