libstore: Introduce WorkerProto::FeatureSet alias

Unfortunately Feature is just an alias to `std::string`
and not a new-type, so a ton of code relies on it being
exactly a `std::string`.

Using transparent comparators just for StringSet necessitates
using it here as well.

src/libstore-tests/worker-protocol.cc

@@ -685,7 +685,7 @@ TEST_F(WorkerProtoTest, handshake_features)
     toClient.create();
     toServer.create();
 
-    std::tuple<WorkerProto::Version, std::set<WorkerProto::Feature>> clientResult;
+    std::tuple<WorkerProto::Version, WorkerProto::FeatureSet> clientResult;
 
     auto clientThread = std::thread([&]() {
         FdSink out { toServer.writeSide.get() };
@@ -703,7 +703,7 @@ TEST_F(WorkerProtoTest, handshake_features)
 
     EXPECT_EQ(clientResult, daemonResult);
     EXPECT_EQ(std::get<0>(clientResult), 123u);
-    EXPECT_EQ(std::get<1>(clientResult), std::set<WorkerProto::Feature>({"bar", "xyzzy"}));
+    EXPECT_EQ(std::get<1>(clientResult), WorkerProto::FeatureSet({"bar", "xyzzy"}));
 }
 
 /// Has to be a `BufferedSink` for handshake.

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

@@ -26,7 +26,7 @@ struct WorkerProto::BasicConnection
     /**
      * The set of features that both sides support.
      */
-    std::set<Feature> features;
+    FeatureSet features;
 
     /**
      * Coercion to `WorkerProto::ReadConn`. This makes it easy to use the
@@ -92,11 +92,8 @@ struct WorkerProto::BasicClientConnection : WorkerProto::BasicConnection
      * @param supportedFeatures The protocol features that we support.
      */
     // FIXME: this should probably be a constructor.
-    static std::tuple<Version, std::set<Feature>> handshake(
+    static std::tuple<Version, FeatureSet> handshake(
-        BufferedSink & to,
+        BufferedSink & to, Source & from, WorkerProto::Version localVersion, const FeatureSet & supportedFeatures);
-        Source & from,
-        WorkerProto::Version localVersion,
-        const std::set<Feature> & supportedFeatures);
 
     /**
      * After calling handshake, must call this to exchange some basic
@@ -155,11 +152,8 @@ struct WorkerProto::BasicServerConnection : WorkerProto::BasicConnection
      * @param supportedFeatures The protocol features that we support.
      */
     // FIXME: this should probably be a constructor.
-    static std::tuple<Version, std::set<Feature>> handshake(
+    static std::tuple<Version, FeatureSet> handshake(
-        BufferedSink & to,
+        BufferedSink & to, Source & from, WorkerProto::Version localVersion, const FeatureSet & supportedFeatures);
-        Source & from,
-        WorkerProto::Version localVersion,
-        const std::set<Feature> & supportedFeatures);
 
     /**
      * After calling handshake, must call this to exchange some basic

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

@@ -135,8 +135,9 @@ struct WorkerProto
     }
 
     using Feature = std::string;
+    using FeatureSet = std::set<Feature>;
 
-    static const std::set<Feature> allFeatures;
+    static const FeatureSet allFeatures;
 };
 
 enum struct WorkerProto::Op : uint64_t

src/libstore/worker-protocol-connection.cc

@@ -5,7 +5,7 @@
 
 namespace nix {
 
-const std::set<WorkerProto::Feature> WorkerProto::allFeatures{};
+const WorkerProto::FeatureSet WorkerProto::allFeatures{};
 
 WorkerProto::BasicClientConnection::~BasicClientConnection()
 {
@@ -146,21 +146,20 @@ void WorkerProto::BasicClientConnection::processStderr(
     }
 }
 
-static std::set<WorkerProto::Feature>
+static WorkerProto::FeatureSet intersectFeatures(const WorkerProto::FeatureSet & a, const WorkerProto::FeatureSet & b)
-intersectFeatures(const std::set<WorkerProto::Feature> & a, const std::set<WorkerProto::Feature> & b)
 {
-    std::set<WorkerProto::Feature> res;
+    WorkerProto::FeatureSet res;
     for (auto & x : a)
         if (b.contains(x))
             res.insert(x);
     return res;
 }
 
-std::tuple<WorkerProto::Version, std::set<WorkerProto::Feature>> WorkerProto::BasicClientConnection::handshake(
+std::tuple<WorkerProto::Version, WorkerProto::FeatureSet> WorkerProto::BasicClientConnection::handshake(
     BufferedSink & to,
     Source & from,
     WorkerProto::Version localVersion,
-    const std::set<WorkerProto::Feature> & supportedFeatures)
+    const WorkerProto::FeatureSet & supportedFeatures)
 {
     to << WORKER_MAGIC_1 << localVersion;
     to.flush();
@@ -178,21 +177,21 @@ std::tuple<WorkerProto::Version, std::set<WorkerProto::Feature>> WorkerProto::Ba
     auto protoVersion = std::min(daemonVersion, localVersion);
 
     /* Exchange features. */
-    std::set<WorkerProto::Feature> daemonFeatures;
+    WorkerProto::FeatureSet daemonFeatures;
     if (GET_PROTOCOL_MINOR(protoVersion) >= 38) {
         to << supportedFeatures;
         to.flush();
-        daemonFeatures = readStrings<std::set<WorkerProto::Feature>>(from);
+        daemonFeatures = readStrings<WorkerProto::FeatureSet>(from);
     }
 
     return {protoVersion, intersectFeatures(daemonFeatures, supportedFeatures)};
 }
 
-std::tuple<WorkerProto::Version, std::set<WorkerProto::Feature>> WorkerProto::BasicServerConnection::handshake(
+std::tuple<WorkerProto::Version, WorkerProto::FeatureSet> WorkerProto::BasicServerConnection::handshake(
     BufferedSink & to,
     Source & from,
     WorkerProto::Version localVersion,
-    const std::set<WorkerProto::Feature> & supportedFeatures)
+    const WorkerProto::FeatureSet & supportedFeatures)
 {
     unsigned int magic = readInt(from);
     if (magic != WORKER_MAGIC_1)
@@ -204,9 +203,9 @@ std::tuple<WorkerProto::Version, std::set<WorkerProto::Feature>> WorkerProto::Ba
     auto protoVersion = std::min(clientVersion, localVersion);
 
     /* Exchange features. */
-    std::set<WorkerProto::Feature> clientFeatures;
+    WorkerProto::FeatureSet clientFeatures;
     if (GET_PROTOCOL_MINOR(protoVersion) >= 38) {
-        clientFeatures = readStrings<std::set<WorkerProto::Feature>>(from);
+        clientFeatures = readStrings<WorkerProto::FeatureSet>(from);
         to << supportedFeatures;
         to.flush();
     }