Feature/backport pool (#576)

* Feature/use nonpriority pool (#570)

* new thread pool

* fix

* wip

* add static

* attempt to fix compile warning (#569)

the compile option `-fsized-deallocation` should only be used for C++
code, but not for C code. otherwise clang warns with the following
message:
```
warning: command-line option ‘-fsized-deallocation’ is valid for C++/ObjC++ but not for C
```

* Improve thread pool (#571)

* WIP

* WIP

* WIP

* WIP

* Update core/utils/async_utils.cpp

* Separate WaitGroup (#572)

* Separate WaitGroup

* Update core/utils/wait_group.hpp

Co-authored-by: Andrei Lobov <andrei.lobov@arangodb.com>

* Exact count

* fix lints

---------

Co-authored-by: Andrei Lobov <andrei.lobov@arangodb.com>

* Backport and start

* fix

* fix datarace

* fix build

---------

Co-authored-by: Jan <jsteemann@users.noreply.github.com>
Co-authored-by: Valery Mironov <32071355+MBkkt@users.noreply.github.com>

CMakeLists.txt

@@ -136,6 +136,9 @@ if (MSVC)
     set(CMAKE_C_FLAGS "/Zc:static_assert- ${CMAKE_C_FLAGS}")
     set(CMAKE_CXX_FLAGS "/Zc:static_assert- ${CMAKE_CXX_FLAGS}")
   endif ()
+else ()
+  # We want to force clang/gcc to use it, but only for C++ files
+  add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-fsized-deallocation>)
 endif ()
 
 # fix strange issue with utf_8_to_32_iterator failing

core/index/index_writer.cpp

@@ -1197,9 +1197,7 @@ void IndexWriter::Clear(uint64_t tick) {
 
   to_commit.ctx = SwitchFlushContext();
   // Ensure there are no active struct update operations
-  std::unique_lock ctx_lock{to_commit.ctx->pending_.Mutex()};
-  to_commit.ctx->pending_.Wait(ctx_lock);
-  ctx_lock.unlock();
+  to_commit.ctx->pending_.Wait();
 
   Abort();  // Abort any already opened transaction
   ApplyFlush(std::move(to_commit));
@@ -1805,11 +1803,8 @@ IndexWriter::PendingContext IndexWriter::PrepareFlush(const CommitInfo& info) {
 
   // noexcept block: I'm not sure is it really necessary or not
   auto ctx = SwitchFlushContext();
-  // TODO(MBkkt) It looks like lock mutex_ completely unnecessary
   // ensure there are no active struct update operations
-  std::unique_lock lock{ctx->pending_.Mutex()};
-  ctx->pending_.Wait(lock);
-  lock.unlock();
+  ctx->pending_.Wait();
   // Stage 0
   // wait for any outstanding segments to settle to ensure that any rollbacks
   // are properly tracked in 'modification_queries_'

core/index/index_writer.hpp

@@ -45,6 +45,7 @@
 #include "utils/object_pool.hpp"
 #include "utils/string.hpp"
 #include "utils/thread_utils.hpp"
+#include "utils/wait_group.hpp"
 
 #include <absl/container/flat_hash_map.h>
 
@@ -835,36 +836,7 @@ class IndexWriter : private util::noncopyable {
     std::deque<PendingSegmentContext> pending_segments_;
     // entries from 'pending_segments_' that are available for reuse
     Freelist pending_freelist_;
-    // TODO(MBkkt) Considered to replace with YACLib in ArangoDB 3.11+ or ...
-    struct WaitGroup {
-      std::mutex& Mutex() noexcept { return m_; }
-
-      void Add() noexcept { counter_.fetch_add(1, std::memory_order_relaxed); }
-
-      void Done() noexcept {
-        if (counter_.fetch_sub(1, std::memory_order_acq_rel) == 1) {
-          std::lock_guard lock{m_};
-          cv_.notify_one();
-        }
-      }
-
-      void Wait(std::unique_lock<std::mutex>& lock) noexcept {
-        IRS_ASSERT(lock.mutex() == &m_);
-        IRS_ASSERT(lock.owns_lock());
-        if (counter_.fetch_sub(1, std::memory_order_acq_rel) != 1) {
-          do {
-            cv_.wait(lock);
-            // relaxed probably enough
-          } while (counter_.load(std::memory_order_acquire) != 0);
-        }
-        counter_.store(1, std::memory_order_relaxed);
-      }
-
-     private:
-      std::atomic_size_t counter_{1};
-      std::mutex m_;
-      std::condition_variable cv_;
-    } pending_;
+    WaitGroup pending_;
 
     // set of segments to be removed from the index upon commit
     ConsolidatingSegments segment_mask_;