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diff --git a/src/ceph/src/common/PrebufferedStreambuf.h b/src/ceph/src/common/PrebufferedStreambuf.h
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+++ b/src/ceph/src/common/PrebufferedStreambuf.h
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+#ifndef CEPH_COMMON_PREBUFFEREDSTREAMBUF_H
+#define CEPH_COMMON_PREBUFFEREDSTREAMBUF_H
+
+#include <streambuf>
+
+/**
+ * streambuf using existing buffer, overflowing into a std::string
+ *
+ * A simple streambuf that uses a preallocated buffer for small
+ * strings, and overflows into a std::string when necessary.  If the
+ * preallocated buffer size is chosen well, we can optimize for the
+ * common case and overflow to a slower heap-allocated buffer when
+ * necessary.
+ */
+class PrebufferedStreambuf
+  : public std::basic_streambuf<char, std::basic_string<char>::traits_type>
+{
+  char *m_buf;
+  size_t m_buf_len;
+  std::string m_overflow;
+
+  typedef std::char_traits<char> traits_ty;
+  typedef traits_ty::int_type int_type;
+  typedef traits_ty::pos_type pos_type;
+  typedef traits_ty::off_type off_type;
+
+public:
+  PrebufferedStreambuf(char *buf, size_t len);
+
+  // called when the buffer fills up
+  int_type overflow(int_type c) override;
+
+  // called when we read and need more data
+  int_type underflow() override;
+
+  /// return a string copy (inefficiently)
+  std::string get_str() const;
+
+  // returns current size of content
+  size_t size() const;
+
+  // extracts up to avail chars of content
+  int snprintf(char* dst, size_t avail) const;
+};
+
+#endif