Store: expose the protocol version used by a store

src/libstore/legacy-ssh-store.cc

@@ -303,6 +303,12 @@ struct LegacySSHStore : public Store
     {
         auto conn(connections->get());
     }
+
+    unsigned int getProtocol() override
+    {
+        auto conn(connections->get());
+        return conn->remoteVersion;
+    }
 };
 
 static RegisterStoreImplementation regStore([](

src/libstore/local-store.cc

@@ -1332,6 +1332,12 @@ void LocalStore::verifyPath(const Path & path, const PathSet & store,
 }
 
 
+unsigned int LocalStore::getProtocol()
+{
+    return PROTOCOL_VERSION;
+}
+
+
 #if defined(FS_IOC_SETFLAGS) && defined(FS_IOC_GETFLAGS) && defined(FS_IMMUTABLE_FL)
 
 static void makeMutable(const Path & path)

src/libstore/local-store.hh

@@ -209,6 +209,8 @@ public:
 
     void registerValidPaths(const ValidPathInfos & infos);
 
+    unsigned int getProtocol() override;
+
     void vacuumDB();
 
     /* Repair the contents of the given path by redownloading it using

src/libstore/remote-store.cc

@@ -646,6 +646,13 @@ void RemoteStore::connect()
 }
 
 
+unsigned int RemoteStore::getProtocol()
+{
+    auto conn(connections->get());
+    return conn->daemonVersion;
+}
+
+
 void RemoteStore::flushBadConnections()
 {
     connections->flushBad();

src/libstore/remote-store.hh

@@ -97,6 +97,8 @@ public:
 
     void connect() override;
 
+    unsigned int getProtocol() override;
+
     void flushBadConnections();
 
 protected:

src/libstore/store-api.hh

@@ -598,6 +598,12 @@ public:
        a notion of connection. Otherwise this is a no-op. */
     virtual void connect() { };
 
+    /* Get the protocol version of this store or it's connection. */
+    virtual unsigned int getProtocol()
+    {
+        return 0;
+    };
+
     /* Get the priority of the store, used to order substituters. In
        particular, binary caches can specify a priority field in their
        "nix-cache-info" file. Lower value means higher priority. */