1 files changed, 192 insertions, 0 deletions
diff --git a/src/common/StackStringStream.h b/src/common/StackStringStream.h
new file mode 100644
index 000000000..8bd70fc1a
--- /dev/null
+++ b/src/common/StackStringStream.h
@@ -0,0 +1,192 @@
+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * Copyright (C) 2018 Red Hat, Inc.
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software
+ * Foundation.	See file COPYING.
+ *
+ */
+
+#ifndef COMMON_STACKSTRINGSTREAM_H
+#define COMMON_STACKSTRINGSTREAM_H
+
+#include <boost/container/small_vector.hpp>
+
+#include <algorithm>
+#include <iostream>
+#include <memory>
+#include <ostream>
+#include <sstream>
+#include <string_view>
+#include <vector>
+
+#include "include/inline_memory.h"
+
+template<std::size_t SIZE>
+class StackStringBuf : public std::basic_streambuf<char>
+{
+public:
+  StackStringBuf()
+    : vec{SIZE, boost::container::default_init_t{}}
+  {
+    setp(vec.data(), vec.data() + vec.size());
+  }
+  StackStringBuf(const StackStringBuf&) = delete;
+  StackStringBuf& operator=(const StackStringBuf&) = delete;
+  StackStringBuf(StackStringBuf&& o) = delete;
+  StackStringBuf& operator=(StackStringBuf&& o) = delete;
+  ~StackStringBuf() override = default;
+
+  void clear()
+  {
+    vec.resize(SIZE);
+    setp(vec.data(), vec.data() + SIZE);
+  }
+
+  std::string_view strv() const
+  {
+    return std::string_view(pbase(), pptr() - pbase());
+  }
+
+protected:
+  std::streamsize xsputn(const char *s, std::streamsize n)
+  {
+    std::streamsize capacity = epptr() - pptr();
+    std::streamsize left = n;
+    if (capacity >= left) {
+      maybe_inline_memcpy(pptr(), s, left, 32);
+      pbump(left);
+    } else {
+      maybe_inline_memcpy(pptr(), s, capacity, 64);
+      s += capacity;
+      left -= capacity;
+      vec.insert(vec.end(), s, s + left);
+      setp(vec.data(), vec.data() + vec.size());
+      pbump(vec.size());
+    }
+    return n;
+  }
+
+  int overflow(int c)
+  {
+    if (traits_type::not_eof(c)) {
+      char str = traits_type::to_char_type(c);
+      vec.push_back(str);
+      return c;
+    } else {
+      return traits_type::eof();
+    }
+  }
+
+private:
+
+  boost::container::small_vector<char, SIZE> vec;
+};
+
+template<std::size_t SIZE>
+class StackStringStream : public std::basic_ostream<char>
+{
+public:
+  StackStringStream() : basic_ostream<char>(&ssb), default_fmtflags(flags()) {}
+  StackStringStream(const StackStringStream& o) = delete;
+  StackStringStream& operator=(const StackStringStream& o) = delete;
+  StackStringStream(StackStringStream&& o) = delete;
+  StackStringStream& operator=(StackStringStream&& o) = delete;
+  ~StackStringStream() override = default;
+
+  void reset() {
+    clear(); /* reset state flags */
+    flags(default_fmtflags); /* reset fmtflags to constructor defaults */
+    ssb.clear();
+  }
+
+  std::string_view strv() const {
+    return ssb.strv();
+  }
+  std::string str() const {
+    return std::string(ssb.strv());
+  }
+
+private:
+  StackStringBuf<SIZE> ssb;
+  fmtflags const default_fmtflags;
+};
+
+/* In an ideal world, we could use StackStringStream indiscriminately, but alas
+ * it's very expensive to construct/destruct. So, we cache them in a
+ * thread_local vector. DO NOT share these with other threads. The copy/move
+ * constructors are deliberately restrictive to make this more difficult to
+ * accidentally do.
+ */
+class CachedStackStringStream {
+public:
+  using sss = StackStringStream<4096>;
+  using osptr = std::unique_ptr<sss>;
+
+  CachedStackStringStream() {
+    if (cache.destructed || cache.c.empty()) {
+      osp = std::make_unique<sss>();
+    } else {
+      osp = std::move(cache.c.back());
+      cache.c.pop_back();
+      osp->reset();
+    }
+  }
+  CachedStackStringStream(const CachedStackStringStream&) = delete;
+  CachedStackStringStream& operator=(const CachedStackStringStream&) = delete;
+  CachedStackStringStream(CachedStackStringStream&&) = delete;
+  CachedStackStringStream& operator=(CachedStackStringStream&&) = delete;
+  ~CachedStackStringStream() {
+    if (!cache.destructed && cache.c.size() < max_elems) {
+      cache.c.emplace_back(std::move(osp));
+    }
+  }
+
+  sss& operator*() {
+    return *osp;
+  }
+  sss const& operator*() const {
+    return *osp;
+  }
+  sss* operator->() {
+    return osp.get();
+  }
+  sss const* operator->() const {
+    return osp.get();
+  }
+
+  sss const* get() const {
+    return osp.get();
+  }
+  sss* get() {
+    return osp.get();
+  }
+
+private:
+  static constexpr std::size_t max_elems = 8;
+
+  /* The thread_local cache may be destructed before other static structures.
+   * If those destructors try to create a CachedStackStringStream (e.g. for
+   * logging) and access this cache, that access will be undefined. So note if
+   * the cache has been destructed and check before use.
+   */
+  struct Cache {
+    using container = std::vector<osptr>;
+
+    Cache() {}
+    ~Cache() { destructed = true; }
+
+    container c;
+    bool destructed = false;
+  };
+
+  inline static thread_local Cache cache;
+  osptr osp;
+};
+
+#endif