Add utility class Defer (#6382)

Summary:
Add a utility class `Defer` to defer the execution of a function until the Defer object goes out of scope.
Used in VersionSet:: ProcessManifestWrites as an example.
The inline comments for class `Defer` have more details.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6382

Test Plan: `make defer_test version_set_test && ./defer_test && ./version_set_test`

Differential Revision: D19797538

Pulled By: cheng-chang

fbshipit-source-id: b1a9b7306e4fd4f48ec2ab55783caa561a315f0f

CMakeLists.txt

@@ -1026,6 +1026,7 @@ if(WITH_TESTS)
         util/bloom_test.cc
         util/coding_test.cc
         util/crc32c_test.cc
+        util/defer_test.cc
         util/dynamic_bloom_test.cc
         util/file_reader_writer_test.cc
         util/filelock_test.cc

Makefile

@@ -596,6 +596,7 @@ TESTS = \
 	db_secondary_test \
 	block_cache_tracer_test \
 	block_cache_trace_analyzer_test \
+	defer_test \
 
 ifeq ($(USE_FOLLY_DISTRIBUTED_MUTEX),1)
 	TESTS += folly_synchronization_distributed_mutex_test
@@ -1714,6 +1715,9 @@ block_cache_tracer_test: trace_replay/block_cache_tracer_test.o trace_replay/blo
 block_cache_trace_analyzer_test: tools/block_cache_analyzer/block_cache_trace_analyzer_test.o tools/block_cache_analyzer/block_cache_trace_analyzer.o $(LIBOBJECTS) $(TESTHARNESS)
 	$(AM_LINK)
 
+defer_test: util/defer_test.o $(LIBOBJECTS) $(TESTHARNESS)
+	$(AM_LINK)
+
 #-------------------------------------------------
 # make install related stuff
 INSTALL_PATH ?= /usr/local

TARGETS

@@ -904,6 +904,13 @@ ROCKS_TESTS = [
         [],
         [],
     ],
+    [
+        "defer_test",
+        "util/defer_test.cc",
+        "serial",
+        [],
+        [],
+    ],
     [
         "delete_scheduler_test",
         "file/delete_scheduler_test.cc",

db/version_set.cc

@@ -50,6 +50,7 @@
 #include "table/two_level_iterator.h"
 #include "test_util/sync_point.h"
 #include "util/coding.h"
+#include "util/defer.h"
 #include "util/stop_watch.h"
 #include "util/string_util.h"
 #include "util/user_comparator_wrapper.h"
@@ -3604,6 +3605,14 @@ Status VersionSet::ProcessManifestWrites(
   autovector<Version*> versions;
   autovector<const MutableCFOptions*> mutable_cf_options_ptrs;
   std::vector<std::unique_ptr<BaseReferencedVersionBuilder>> builder_guards;
+  Status s;
+  Defer defer([&s, &versions]() {
+    if (!s.ok()) {
+      for (auto v : versions) {
+        delete v;
+      }
+    }
+  });
 
   if (first_writer.edit_list.front()->IsColumnFamilyManipulation()) {
     // No group commits for column family add or drop
@@ -3689,12 +3698,8 @@ Status VersionSet::ProcessManifestWrites(
         } else if (group_start != std::numeric_limits<size_t>::max()) {
           group_start = std::numeric_limits<size_t>::max();
         }
-        Status s = LogAndApplyHelper(last_writer->cfd, builder, e, mu);
+        s = LogAndApplyHelper(last_writer->cfd, builder, e, mu);
         if (!s.ok()) {
-          // free up the allocated memory
-          for (auto v : versions) {
-            delete v;
-          }
           return s;
         }
         batch_edits.push_back(e);
@@ -3704,12 +3709,8 @@ Status VersionSet::ProcessManifestWrites(
       assert(!builder_guards.empty() &&
              builder_guards.size() == versions.size());
       auto* builder = builder_guards[i]->version_builder();
-      Status s = builder->SaveTo(versions[i]->storage_info());
+      s = builder->SaveTo(versions[i]->storage_info());
       if (!s.ok()) {
-        // free up the allocated memory
-        for (auto v : versions) {
-          delete v;
-        }
         return s;
       }
     }
@@ -3752,7 +3753,6 @@ Status VersionSet::ProcessManifestWrites(
 #endif  // NDEBUG
 
   uint64_t new_manifest_file_size = 0;
-  Status s;
 
   assert(pending_manifest_file_number_ == 0);
   if (!descriptor_log_ ||
@@ -3959,9 +3959,6 @@ Status VersionSet::ProcessManifestWrites(
     ROCKS_LOG_ERROR(db_options_->info_log,
                     "Error in committing version edit to MANIFEST: %s",
                     version_edits.c_str());
-    for (auto v : versions) {
-      delete v;
-    }
     // If manifest append failed for whatever reason, the file could be
     // corrupted. So we need to force the next version update to start a
     // new manifest file.

src.mk

@@ -419,6 +419,7 @@ MAIN_SOURCES =                                                          \
   util/bloom_test.cc                                                    \
   util/coding_test.cc                                                   \
   util/crc32c_test.cc                                                   \
+  util/defer_test.cc                                                    \
   util/dynamic_bloom_test.cc                                            \
   util/filelock_test.cc                                                 \
   util/log_write_bench.cc                                               \

util/defer.h

@@ -0,0 +1,52 @@
+// 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 <functional>
+
+namespace rocksdb {
+
+// Defers the execution of the provided function until the Defer
+// object goes out of scope.
+//
+// Usage example:
+//
+// Status DeferTest() {
+//   Status s;
+//   Defer defer([&s]() {
+//     if (!s.ok()) {
+//       // do cleanups ...
+//     }
+//   });
+//   // do something ...
+//   if (!s.ok()) return;
+//   // do some other things ...
+//   return s;
+// }
+//
+// The above code ensures that cleanups will always happen on returning.
+//
+// Without the help of Defer, you can
+// 1. every time when !s.ok(), do the cleanup;
+// 2. instead of returning when !s.ok(), continue the work only when s.ok(),
+//    but sometimes, this might lead to nested blocks of "if (s.ok()) {...}".
+//
+// With the help of Defer, you can centralize the cleanup logic inside the
+// lambda passed to Defer, and you can return immediately on failure when necessary.
+class Defer final {
+ public:
+  Defer(std::function<void()>&& fn) : fn_(std::move(fn)) {}
+  ~Defer() { fn_(); }
+
+  // Disallow copy.
+  Defer(const Defer&) = delete;
+  Defer& operator=(const Defer&) = delete;
+
+ private:
+  std::function<void()> fn_;
+};
+
+} // namespace rocksdb

util/defer_test.cc

@@ -0,0 +1,39 @@
+//  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).
+
+#include "port/port.h"
+#include "port/stack_trace.h"
+#include "test_util/testharness.h"
+#include "util/defer.h"
+
+namespace rocksdb {
+
+class DeferTest {};
+
+TEST(DeferTest, BlockScope) {
+  int v = 1;
+  {
+    Defer defer([&v]() { v *= 2; });
+  }
+  ASSERT_EQ(2, v);
+}
+
+TEST(DeferTest, FunctionScope) {
+  int v = 1;
+  auto f = [&v]() {
+    Defer defer([&v]() { v *= 2; });
+    v = 2;
+  };
+  f();
+  ASSERT_EQ(4, v);
+}
+
+} // namespace rocksdb
+
+int main(int argc, char** argv) {
+  rocksdb::port::InstallStackTraceHandler();
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}