Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libstore/common-protocol.hh

@@ -0,0 +1,106 @@
+#pragma once
+///@file
+
+#include "serialise.hh"
+
+namespace nix {
+
+struct StoreDirConfig;
+struct Source;
+
+// items being serialized
+class StorePath;
+struct ContentAddress;
+struct DrvOutput;
+struct Realisation;
+
+
+/**
+ * Shared serializers between the worker protocol, serve protocol, and a
+ * few others.
+ *
+ * This `struct` is basically just a `namespace`; We use a type rather
+ * than a namespace just so we can use it as a template argument.
+ */
+struct CommonProto
+{
+    /**
+     * A unidirectional read connection, to be used by the read half of the
+     * canonical serializers below.
+     */
+    struct ReadConn {
+        Source & from;
+    };
+
+    /**
+     * A unidirectional write connection, to be used by the write half of the
+     * canonical serializers below.
+     */
+    struct WriteConn {
+        Sink & to;
+    };
+
+    template<typename T>
+    struct Serialise;
+
+    /**
+     * Wrapper function around `CommonProto::Serialise<T>::write` that allows us to
+     * infer the type instead of having to write it down explicitly.
+     */
+    template<typename T>
+    static void write(const StoreDirConfig & store, WriteConn conn, const T & t)
+    {
+        CommonProto::Serialise<T>::write(store, conn, t);
+    }
+};
+
+#define DECLARE_COMMON_SERIALISER(T) \
+    struct CommonProto::Serialise< T > \
+    { \
+        static T read(const StoreDirConfig & store, CommonProto::ReadConn conn); \
+        static void write(const StoreDirConfig & store, CommonProto::WriteConn conn, const T & str); \
+    }
+
+template<>
+DECLARE_COMMON_SERIALISER(std::string);
+template<>
+DECLARE_COMMON_SERIALISER(StorePath);
+template<>
+DECLARE_COMMON_SERIALISER(ContentAddress);
+template<>
+DECLARE_COMMON_SERIALISER(DrvOutput);
+template<>
+DECLARE_COMMON_SERIALISER(Realisation);
+
+template<typename T>
+DECLARE_COMMON_SERIALISER(std::vector<T>);
+template<typename T>
+DECLARE_COMMON_SERIALISER(std::set<T>);
+template<typename... Ts>
+DECLARE_COMMON_SERIALISER(std::tuple<Ts...>);
+
+#define COMMA_ ,
+template<typename K, typename V>
+DECLARE_COMMON_SERIALISER(std::map<K COMMA_ V>);
+#undef COMMA_
+
+/**
+ * These use the empty string for the null case, relying on the fact
+ * that the underlying types never serialize to the empty string.
+ *
+ * We do this instead of a generic std::optional<T> instance because
+ * ordinal tags (0 or 1, here) are a bit of a compatability hazard. For
+ * the same reason, we don't have a std::variant<T..> instances (ordinal
+ * tags 0...n).
+ *
+ * We could the generic instances and then these as specializations for
+ * compatability, but that's proven a bit finnicky, and also makes the
+ * worker protocol harder to implement in other languages where such
+ * specializations may not be allowed.
+ */
+template<>
+DECLARE_COMMON_SERIALISER(std::optional<StorePath>);
+template<>
+DECLARE_COMMON_SERIALISER(std::optional<ContentAddress>);
+
+}