Introduce MINIMUM_PROTOCOL_VERSION constant

Author: edolstra

src/libstore/daemon.cc

@@ -1031,7 +1031,7 @@ void processConnection(ref<Store> store, FdSource && from, FdSink && to, Trusted
     auto [protoVersion, features] =
         WorkerProto::BasicServerConnection::handshake(to, from, PROTOCOL_VERSION, WorkerProto::allFeatures);
 
-    if (protoVersion < 256 + 18)
+    if (protoVersion < MINIMUM_PROTOCOL_VERSION)
         throw Error("the Nix client version is too old");
 
     WorkerProto::BasicServerConnection conn;

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

@@ -13,6 +13,7 @@ namespace nix {
 /* Note: you generally shouldn't change the protocol version. Define a
    new `WorkerProto::Feature` instead. */
 #define PROTOCOL_VERSION (1 << 8 | 38)
+#define MINIMUM_PROTOCOL_VERSION (1 << 8 | 18)
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 

src/libstore/remote-store.cc

@@ -73,7 +73,7 @@ void RemoteStore::initConnection(Connection & conn)
         try {
             auto [protoVersion, features] =
                 WorkerProto::BasicClientConnection::handshake(conn.to, tee, PROTOCOL_VERSION, WorkerProto::allFeatures);
-            if (protoVersion < 256 + 18)
+            if (protoVersion < MINIMUM_PROTOCOL_VERSION)
                 throw Error("the Nix daemon version is too old");
             conn.protoVersion = protoVersion;
             conn.features = features;