common/item_history: container to support lockless reads, infrequent updates

Signed-off-by: Sage Weil <sage@redhat.com>

diff --git a/src/common/item_history.h b/src/common/item_history.h
new file mode 100644 (file)
index 0000000..987a7dc
--- /dev/null
+++ b/src/common/item_history.h
@@ -0,0 +1,103 @@
+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+
+#pragma once
+
+#include <list>
+#include <mutex>
+
+/*
+
+Keep a history of item values so that readers can dereference the pointer to
+the latest value and continue using it as long as they want.  This container
+is only appropriate for values that are updated a handful of times over their
+total lifetime.
+
+There is a prune() method to throw out old values, but it should only be used
+if the caller has some way of knowing all readers are done.
+
+*/
+
+template<class T>
+class mutable_item_history {
+private:
+  std::mutex lock;
+  std::list<T> history;
+  T *current = nullptr;
+
+public:
+  mutable_item_history() {
+    history.emplace_back(T());
+    current = &history.back();
+  }
+
+  // readers are lock-free
+  const T& operator*() const {
+    return *current;
+  }
+  const T *operator->() const {
+    return current;
+  }
+
+  // non-const variants (be careful!)
+  T& operator*() {
+    return *current;
+  }
+  T *operator->() {
+    return current;
+  }
+
+  // writes are serialized
+  const T& operator=(const T& other) {
+    std::lock_guard<std::mutex> l(lock);
+    history.push_back(other);
+    current = &history.back();
+    return *current;
+  }
+
+  void prune() {
+    // note: this is not necessarily thread-safe wrt readers
+    std::lock_guard<std::mutex> l(lock);
+    while (history.size() > 1) {
+      history.pop_front();
+    }
+  }
+};
+
+template<class T>
+class safe_item_history {
+private:
+  std::mutex lock;
+  std::list<T> history;
+  T *current = nullptr;
+
+public:
+  safe_item_history() {
+    history.emplace_back(T());
+    current = &history.back();
+  }
+
+  // readers are lock-free
+  const T& operator*() const {
+    return *current;
+  }
+  const T *operator->() const {
+    return current;
+  }
+
+  // writes are serialized
+  const T& operator=(const T& other) {
+    std::lock_guard<std::mutex> l(lock);
+    history.push_back(other);
+    current = &history.back();
+    return *current;
+  }
+
+  void prune() {
+    // note: this is not necessarily thread-safe wrt readers
+    std::lock_guard<std::mutex> l(lock);
+    while (history.size() > 1) {
+      history.pop_front();
+    }
+  }
+};