Make sure we always store record/op before attachments (#195)

* Make sure we always store record/op before attachments
We don't want to store attachments first - a GC operation could then remove attachments if triggered before storing record/op
* zen::Latch
* Use latch to wait for attachments to be stored
* use zen::latch when adding attachments from project oplog import
* changelog

1 files changed, 33 insertions, 2 deletions

diff --git a/zencore/include/zencore/thread.h b/zencore/include/zencore/thread.h
index 16d0e9dee..3c1821a62 100644
--- a/zencore/include/zencore/thread.h
+++ b/zencore/include/zencore/thread.h
@@ -4,9 +4,9 @@
 
 #include "zencore.h"
 
-#include <shared_mutex>
-
+#include <atomic>
 #include <filesystem>
+#include <shared_mutex>
 #include <string_view>
 #include <vector>
 
@@ -144,6 +144,37 @@ private:
 	void* m_MutexHandle = nullptr;
 };
 
+/**
+ * Downward counter of type std::ptrdiff_t which can be used to synchronize threads
+ */
+class Latch
+{
+public:
+	Latch(std::ptrdiff_t Count) : Counter(Count) {}
+
+	void CountDown()
+	{
+		std::ptrdiff_t Old = Counter.fetch_sub(1);
+		if (Old == 1)
+		{
+			Complete.Set();
+		}
+	}
+
+	void Wait()
+	{
+		std::ptrdiff_t Old = Counter.load();
+		if (Old != 0)
+		{
+			Complete.Wait();
+		}
+	}
+
+private:
+	std::atomic_ptrdiff_t Counter;
+	Event				  Complete;
+};
+
 /** Basic process abstraction
  */
 class ProcessHandle