Add custom vector-element formatter

This allows a very compact notation for serialization of vectors whose
elements are not serialized using their default encoding.

src/serialize.h

@@ -596,6 +596,53 @@ public:
 template<typename I>
 BigEndian<I> WrapBigEndian(I& n) { return BigEndian<I>(n); }
 
+/** Formatter to serialize/deserialize vector elements using another formatter
+ *
+ * Example:
+ *   struct X {
+ *     std::vector<uint64_t> v;
+ *     SERIALIZE_METHODS(X, obj) { READWRITE(Using<VectorFormatter<VarInt>>(obj.v)); }
+ *   };
+ * will define a struct that contains a vector of uint64_t, which is serialized
+ * as a vector of VarInt-encoded integers.
+ *
+ * V is not required to be an std::vector type. It works for any class that
+ * exposes a value_type, size, reserve, push_back, and const iterators.
+ */
+template<class Formatter>
+struct VectorFormatter
+{
+    template<typename Stream, typename V>
+    void Ser(Stream& s, const V& v)
+    {
+        WriteCompactSize(s, v.size());
+        for (const typename V::value_type& elem : v) {
+            s << Using<Formatter>(elem);
+        }
+    }
+
+    template<typename Stream, typename V>
+    void Unser(Stream& s, V& v)
+    {
+        v.clear();
+        size_t size = ReadCompactSize(s);
+        size_t allocated = 0;
+        while (allocated < size) {
+            // For DoS prevention, do not blindly allocate as much as the stream claims to contain.
+            // Instead, allocate in 5MiB batches, so that an attacker actually needs to provide
+            // X MiB of data to make us allocate X+5 Mib.
+            static_assert(sizeof(typename V::value_type) <= MAX_VECTOR_ALLOCATE, "Vector element size too large");
+            allocated = std::min(size, allocated + MAX_VECTOR_ALLOCATE / sizeof(typename V::value_type));
+            v.reserve(allocated);
+            while (v.size() < allocated) {
+                typename V::value_type val;
+                s >> Using<Formatter>(val);
+                v.push_back(std::move(val));
+            }
+        }
+    };
+};
+
 /**
  * Forward declarations
  */