Merge #12886: Introduce Span type and use it instead of FLATDATA

9272d70 Support serializing Span<unsigned char> and use that instead of FLATDATA (Pieter Wuille)
833bc08 Add Slice: a (pointer, size) array view that acts like a container (Pieter Wuille)

Pull request description:

  Introduce a new data type `Span`, which is an encapsulated pointer + size (like C++20's `std::span` or LevelDB's `Slice`), and represents a view to a sequence of objects laid out continuously in memory.

  The immediate use case is replacing the remaining `FLATDATA` invocations. Instead of those, we support serializing/deserializing unsigned char `Span`s (treating them as arrays).

  A longer term goal for `Span`s is making the script execution operate on them rather than on `CScript` itself. This will allow separate storage mechanisms for scripts.

Tree-SHA512: 7b0da3c802e5df367f223275004d16b04262804c007b7c73fda927176f0a9c3b2ef3225fa842cb73500b0df73175ec1419f1f5239de2402e21dd9ae8e5d05233

5 files changed, 57 insertions, 48 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index f82248fbed..1bbb92bf42 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -316,6 +316,7 @@ libbitcoin_consensus_a_SOURCES = \
   script/script_error.cpp \
   script/script_error.h \
   serialize.h \
+  span.h \
   tinyformat.h \
   uint256.cpp \
   uint256.h \
diff --git a/src/compressor.h b/src/compressor.h
index 561c8e66d0..6bd68529d4 100644
--- a/src/compressor.h
+++ b/src/compressor.h
@@ -9,6 +9,7 @@
 #include <primitives/transaction.h>
 #include <script/script.h>
 #include <serialize.h>
+#include <span.h>
 
 class CKeyID;
 class CPubKey;
@@ -51,12 +52,12 @@ public:
     void Serialize(Stream &s) const {
         std::vector<unsigned char> compr;
         if (CompressScript(script, compr)) {
-            s << CFlatData(compr);
+            s << MakeSpan(compr);
             return;
         }
         unsigned int nSize = script.size() + nSpecialScripts;
         s << VARINT(nSize);
-        s << CFlatData(script);
+        s << MakeSpan(script);
     }
 
     template<typename Stream>
@@ -65,7 +66,7 @@ public:
         s >> VARINT(nSize);
         if (nSize < nSpecialScripts) {
             std::vector<unsigned char> vch(GetSpecialScriptSize(nSize), 0x00);
-            s >> CFlatData(vch);
+            s >> MakeSpan(vch);
             DecompressScript(script, nSize, vch);
             return;
         }
@@ -76,7 +77,7 @@ public:
             s.ignore(nSize);
         } else {
             script.resize(nSize);
-            s >> CFlatData(script);
+            s >> MakeSpan(script);
         }
     }
 };
diff --git a/src/netaddress.h b/src/netaddress.h
index ad6b55eb58..b3d1407f72 100644
--- a/src/netaddress.h
+++ b/src/netaddress.h
@@ -11,6 +11,7 @@
 
 #include <compat.h>
 #include <serialize.h>
+#include <span.h>
 
 #include <stdint.h>
 #include <string>
@@ -167,10 +168,13 @@ class CService : public CNetAddr
         template <typename Stream, typename Operation>
         inline void SerializationOp(Stream& s, Operation ser_action) {
             READWRITE(ip);
+
+            // TODO: introduce native support for BE serialization in serialize.h
             unsigned short portN = htons(port);
-            READWRITE(FLATDATA(portN));
-            if (ser_action.ForRead())
+            READWRITE(Span<unsigned char>((unsigned char*)&portN, 2));
+            if (ser_action.ForRead()) {
                  port = ntohs(portN);
+            }
         }
 };
 
diff --git a/src/serialize.h b/src/serialize.h
index 247e915298..e90b041cc2 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -22,6 +22,7 @@
 #include <vector>
 
 #include <prevector.h>
+#include <span.h>
 
 static const unsigned int MAX_SIZE = 0x02000000;
 
@@ -41,7 +42,7 @@ constexpr deserialize_type deserialize {};
 
 /**
  * Used to bypass the rule against non-const reference to temporary
- * where it makes sense with wrappers such as CFlatData or CTxDB
+ * where it makes sense with wrappers.
  */
 template<typename T>
 inline T& REF(const T& val)
@@ -185,6 +186,8 @@ template<typename Stream> inline void Serialize(Stream& s, float a   ) { ser_wri
 template<typename Stream> inline void Serialize(Stream& s, double a  ) { ser_writedata64(s, ser_double_to_uint64(a)); }
 template<typename Stream, int N> inline void Serialize(Stream& s, const char (&a)[N]) { s.write(a, N); }
 template<typename Stream, int N> inline void Serialize(Stream& s, const unsigned char (&a)[N]) { s.write(CharCast(a), N); }
+template<typename Stream> inline void Serialize(Stream& s, const Span<const unsigned char>& span) { s.write(CharCast(span.data()), span.size()); }
+template<typename Stream> inline void Serialize(Stream& s, const Span<unsigned char>& span) { s.write(CharCast(span.data()), span.size()); }
 
 template<typename Stream> inline void Unserialize(Stream& s, char& a    ) { a = ser_readdata8(s); } // TODO Get rid of bare char
 template<typename Stream> inline void Unserialize(Stream& s, int8_t& a  ) { a = ser_readdata8(s); }
@@ -199,6 +202,7 @@ template<typename Stream> inline void Unserialize(Stream& s, float& a   ) { a =
 template<typename Stream> inline void Unserialize(Stream& s, double& a  ) { a = ser_uint64_to_double(ser_readdata64(s)); }
 template<typename Stream, int N> inline void Unserialize(Stream& s, char (&a)[N]) { s.read(a, N); }
 template<typename Stream, int N> inline void Unserialize(Stream& s, unsigned char (&a)[N]) { s.read(CharCast(a), N); }
+template<typename Stream> inline void Unserialize(Stream& s, Span<unsigned char>& span) { s.read(CharCast(span.data()), span.size()); }
 
 template<typename Stream> inline void Serialize(Stream& s, bool a)    { char f=a; ser_writedata8(s, f); }
 template<typename Stream> inline void Unserialize(Stream& s, bool& a) { char f=ser_readdata8(s); a=f; }
@@ -384,51 +388,10 @@ I ReadVarInt(Stream& is)
     }
 }
 
-#define FLATDATA(obj) CFlatData((char*)&(obj), (char*)&(obj) + sizeof(obj))
 #define VARINT(obj, ...) WrapVarInt<__VA_ARGS__>(REF(obj))
 #define COMPACTSIZE(obj) CCompactSize(REF(obj))
 #define LIMITED_STRING(obj,n) LimitedString< n >(REF(obj))
 
-/** 
- * Wrapper for serializing arrays and POD.
- */
-class CFlatData
-{
-protected:
-    char* pbegin;
-    char* pend;
-public:
-    CFlatData(void* pbeginIn, void* pendIn) : pbegin((char*)pbeginIn), pend((char*)pendIn) { }
-    template <class T, class TAl>
-    explicit CFlatData(std::vector<T,TAl> &v)
-    {
-        pbegin = (char*)v.data();
-        pend = (char*)(v.data() + v.size());
-    }
-    template <unsigned int N, typename T, typename S, typename D>
-    explicit CFlatData(prevector<N, T, S, D> &v)
-    {
-        pbegin = (char*)v.data();
-        pend = (char*)(v.data() + v.size());
-    }
-    char* begin() { return pbegin; }
-    const char* begin() const { return pbegin; }
-    char* end() { return pend; }
-    const char* end() const { return pend; }
-
-    template<typename Stream>
-    void Serialize(Stream& s) const
-    {
-        s.write(pbegin, pend - pbegin);
-    }
-
-    template<typename Stream>
-    void Unserialize(Stream& s)
-    {
-        s.read(pbegin, pend - pbegin);
-    }
-};
-
 template<VarIntMode Mode, typename I>
 class CVarInt
 {
diff --git a/src/span.h b/src/span.h
new file mode 100644
index 0000000000..707fc21918
--- /dev/null
+++ b/src/span.h
@@ -0,0 +1,40 @@
+// Copyright (c) 2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_SPAN_H
+#define BITCOIN_SPAN_H
+
+#include <type_traits>
+#include <cstddef>
+
+/** A Span is an object that can refer to a contiguous sequence of objects.
+ *
+ * It implements a subset of C++20's std::span.
+ */
+template<typename C>
+class Span
+{
+    C* m_data;
+    std::ptrdiff_t m_size;
+
+public:
+    constexpr Span() noexcept : m_data(nullptr), m_size(0) {}
+    constexpr Span(C* data, std::ptrdiff_t size) noexcept : m_data(data), m_size(size) {}
+
+    constexpr C* data() const noexcept { return m_data; }
+    constexpr std::ptrdiff_t size() const noexcept { return m_size; }
+};
+
+/** Create a span to a container exposing data() and size().
+ *
+ * This correctly deals with constness: the returned Span's element type will be
+ * whatever data() returns a pointer to. If either the passed container is const,
+ * or its element type is const, the resulting span will have a const element type.
+ *
+ * std::span will have a constructor that implements this functionality directly.
+ */
+template<typename V>
+constexpr Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type> MakeSpan(V& v) { return Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type>(v.data(), v.size()); }
+
+#endif