Introduce separate Serve protocol serialisers

To start, it is just a clone of the common protocol. But now that we
have the separate protocol implementations, we can add versioning
information without the versions of one protocol leaking into another.

Using the infrastructure from the previous commit, we don't have to
duplicate code for shared behavior.

Motivation: No more perverse incentives. [0] did some awkward things
because the serialisers did not store the version. I don't want anyone
making changes to be pushed towards keeping the serialization logic with
the core data types just because it's easier or the alternative is
tedious.

The actual versioning of the Worker and Serve protocol serialisers
(Common remains unversioned as the underlying mini-protocols are not
versioned) will happen in subsequent commits / PRs.

[0]: fe1f34fa60

src/libstore/serve-protocol-impl.hh

@@ -0,0 +1,59 @@
+#pragma once
+/**
+ * @file
+ *
+ * Template implementations (as opposed to mere declarations).
+ *
+ * This file is an exmample of the "impl.hh" pattern. See the
+ * contributing guide.
+ */
+
+#include "serve-protocol.hh"
+#include "length-prefixed-protocol-helper.hh"
+
+namespace nix {
+
+/* protocol-agnostic templates */
+
+#define SERVE_USE_LENGTH_PREFIX_SERIALISER(TEMPLATE, T) \
+    TEMPLATE T ServeProto::Serialise< T >::read(const Store & store, ServeProto::ReadConn conn) \
+    { \
+        return LengthPrefixedProtoHelper<ServeProto, T >::read(store, conn); \
+    } \
+    TEMPLATE void ServeProto::Serialise< T >::write(const Store & store, ServeProto::WriteConn conn, const T & t) \
+    { \
+        LengthPrefixedProtoHelper<ServeProto, T >::write(store, conn, t); \
+    }
+
+SERVE_USE_LENGTH_PREFIX_SERIALISER(template<typename T>, std::vector<T>)
+SERVE_USE_LENGTH_PREFIX_SERIALISER(template<typename T>, std::set<T>)
+SERVE_USE_LENGTH_PREFIX_SERIALISER(template<typename... Ts>, std::tuple<Ts...>)
+
+#define COMMA_ ,
+SERVE_USE_LENGTH_PREFIX_SERIALISER(
+    template<typename K COMMA_ typename V>,
+    std::map<K COMMA_ V>)
+#undef COMMA_
+
+/**
+ * Use `CommonProto` where possible.
+ */
+template<typename T>
+struct ServeProto::Serialise
+{
+    static T read(const Store & store, ServeProto::ReadConn conn)
+    {
+        return CommonProto::Serialise<T>::read(store,
+            CommonProto::ReadConn { .from = conn.from });
+    }
+    static void write(const Store & store, ServeProto::WriteConn conn, const T & t)
+    {
+        CommonProto::Serialise<T>::write(store,
+            CommonProto::WriteConn { .to = conn.to },
+            t);
+    }
+};
+
+/* protocol-specific templates */
+
+}