libstore: generatorize protocol serializers

this is cursed. deeply and profoundly cursed. under NO CIRCUMSTANCES
must protocol serializer helpers be applied to temporaries! doing so
will inevitably cause dangling references and cause the entire thing
to crash. we need to do this even so to get rid of boost coroutines,
and likewise to encapsulate the serializers we suffer today at least
a little bit to allow a gradual migration to an actual IPC protocol.

(this isn't a problem that's unique to generators. c++ coroutines in
general cannot safely take references to arbitrary temporaries since
c++ does not have a lifetime system that can make this safe. -sigh-)

Change-Id: I2921ba451e04d86798752d140885d3c5cc08e146

1 files changed, 5 insertions, 4 deletions

diff --git a/src/libstore/serve-protocol-impl.hh b/src/libstore/serve-protocol-impl.hh
index a3ce81026..845889451 100644
--- a/src/libstore/serve-protocol-impl.hh
+++ b/src/libstore/serve-protocol-impl.hh
@@ -20,9 +20,9 @@ namespace nix {
     { \
         return LengthPrefixedProtoHelper<ServeProto, T >::read(store, conn); \
     } \
-    TEMPLATE void ServeProto::Serialise< T >::write(const Store & store, ServeProto::WriteConn conn, const T & t) \
+    TEMPLATE [[nodiscard]] WireFormatGenerator ServeProto::Serialise< T >::write(const Store & store, ServeProto::WriteConn conn, const T & t) \
     { \
-        LengthPrefixedProtoHelper<ServeProto, T >::write(store, conn, t); \
+        return LengthPrefixedProtoHelper<ServeProto, T >::write(store, conn, t); \
     }
 
 SERVE_USE_LENGTH_PREFIX_SERIALISER(template<typename T>, std::vector<T>)
@@ -46,9 +46,10 @@ struct ServeProto::Serialise
         return CommonProto::Serialise<T>::read(store,
             CommonProto::ReadConn { .from = conn.from });
     }
-    static void write(const Store & store, ServeProto::WriteConn conn, const T & t)
+    [[nodiscard]]
+    static WireFormatGenerator write(const Store & store, ServeProto::WriteConn conn, const T & t)
     {
-        CommonProto::Serialise<T>::write(store,
+        return CommonProto::Serialise<T>::write(store,
             CommonProto::WriteConn { .to = conn.to },
             t);
     }