Marshal lastUsageTime over worker protocol

Serialize the lastUsageTime path info attribute as part of the worker protocol, making it accessible to e.g. non-root users using the nix daemon.

This is accomplished using a new fine-grained worker protocol feature, along with some additional plumbing to make the set of negotiated features accessible to the serializer code.

Author: Popax21

src/libstore/include/nix/store/worker-protocol-connection.hh

@@ -41,6 +41,7 @@ struct WorkerProto::BasicConnection
         return WorkerProto::ReadConn{
             .from = from,
             .version = protoVersion,
+            .features = &features,
         };
     }
 
@@ -57,6 +58,7 @@ struct WorkerProto::BasicConnection
         return WorkerProto::WriteConn{
             .to = to,
             .version = protoVersion,
+            .features = &features,
         };
     }
 };

src/libstore/include/nix/store/worker-protocol.hh

@@ -58,6 +58,11 @@ struct WorkerProto
      */
     using Version = unsigned int;
 
+    using Feature = std::string;
+    using FeatureSet = std::set<Feature, std::less<>>;
+
+    static const FeatureSet allFeatures;
+
     /**
      * A unidirectional read connection, to be used by the read half of the
      * canonical serializers below.
@@ -66,6 +71,7 @@ struct WorkerProto
     {
         Source & from;
         Version version;
+        const FeatureSet * features{nullptr};
     };
 
     /**
@@ -76,6 +82,7 @@ struct WorkerProto
     {
         Sink & to;
         Version version;
+        const FeatureSet * features{nullptr};
     };
 
     /**
@@ -132,11 +139,6 @@ struct WorkerProto
     {
         WorkerProto::Serialise<T>::write(store, conn, t);
     }
-
-    using Feature = std::string;
-    using FeatureSet = std::set<Feature, std::less<>>;
-
-    static const FeatureSet allFeatures;
 };
 
 enum struct WorkerProto::Op : uint64_t {

src/libstore/worker-protocol-connection.cc

@@ -5,7 +5,7 @@
 
 namespace nix {
 
-const WorkerProto::FeatureSet WorkerProto::allFeatures{};
+const WorkerProto::FeatureSet WorkerProto::allFeatures{"path-last-usage-time"};
 
 WorkerProto::BasicClientConnection::~BasicClientConnection()
 {

src/libstore/worker-protocol.cc

@@ -262,6 +262,9 @@ UnkeyedValidPathInfo WorkerProto::Serialise<UnkeyedValidPathInfo>::read(const St
         info.sigs = readStrings<StringSet>(conn.from);
         info.ca = ContentAddress::parseOpt(readString(conn.from));
     }
+    if (conn.features && conn.features->contains("last-usage-time")) {
+        conn.from >> info.lastUsageTime;
+    }
     return info;
 }
 
@@ -275,6 +278,9 @@ void WorkerProto::Serialise<UnkeyedValidPathInfo>::write(
     if (GET_PROTOCOL_MINOR(conn.version) >= 16) {
         conn.to << pathInfo.ultimate << pathInfo.sigs << renderContentAddress(pathInfo.ca);
     }
+    if (conn.features && conn.features->contains("last-usage-time")) {
+        conn.to << pathInfo.lastUsageTime;
+    }
 }
 
 WorkerProto::ClientHandshakeInfo