Restore ServeProto::Command::ImportPaths

This partially reverts commit 5e46df973f,
partially reversing changes made to
8c789db05b.

We do this because Hydra, while using the newer version of the protocol,
still uses this command, even though Nix (as a client) doesn't use it.
On that basis, we don't want to remove it (or consider it only part of
the older versions of the protocol) until Hydra no longer uses the
Legacy SSH Protocol.

(cherry picked from commit 0deb492b3d)

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

@@ -82,6 +82,8 @@ struct ServeProto::BasicClientConnection
     BuildResult getBuildDerivationResponse(const StoreDirConfig & store);
 
     void narFromPath(const StoreDirConfig & store, const StorePath & path, std::function<void(Source &)> fun);
+
+    void importPaths(const StoreDirConfig & store, std::function<void(Sink &)> fun);
 };
 
 struct ServeProto::BasicServerConnection

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

@@ -108,6 +108,13 @@ enum struct ServeProto::Command : uint64_t {
     QueryValidPaths = 1,
     QueryPathInfos = 2,
     DumpStorePath = 3,
+    /**
+     * @note This is no longer used by Nix (as a client), but it is used
+     * by Hydra. We should therefore not remove it until Hydra no longer
+     * uses it either.
+     */
+    ImportPaths = 4,
+    // ExportPaths = 5,
     BuildPaths = 6,
     QueryClosure = 7,
     BuildDerivation = 8,

src/libstore/serve-protocol-connection.cc

@@ -93,4 +93,14 @@ void ServeProto::BasicClientConnection::narFromPath(
     fun(from);
 }
 
+void ServeProto::BasicClientConnection::importPaths(const StoreDirConfig & store, std::function<void(Sink &)> fun)
+{
+    to << ServeProto::Command::ImportPaths;
+    fun(to);
+    to.flush();
+
+    if (readInt(from) != 1)
+        throw Error("remote machine failed to import closure");
+}
+
 } // namespace nix

src/nix/nix-store/nix-store.cc

@@ -986,6 +986,16 @@ static void opServe(Strings opFlags, Strings opArgs)
             store->narFromPath(store->parseStorePath(readString(in)), out);
             break;
 
+        case ServeProto::Command::ImportPaths: {
+            if (!writeAllowed)
+                throw Error("importing paths is not allowed");
+            // FIXME: should we skip sig checking?
+            importPaths(*store, in, NoCheckSigs);
+            // indicate success
+            out << 1;
+            break;
+        }
+
         case ServeProto::Command::BuildPaths: {
 
             if (!writeAllowed)