Gate the external gc behind an xp feature flag

src/libstore/gc.cc

@@ -283,6 +283,8 @@ void LocalStore::findRootsNoTemp(Roots & roots, bool censor)
     if (::connect(fd.get(), (struct sockaddr *) &addr, sizeof(addr)) == -1)
         return findRootsNoTempNoExternalDaemon(roots, censor);
 
+    settings.requireExperimentalFeature(Xp::ExternalGCDaemon);
+
     try {
         while (true) {
             auto line = readLine(fd.get());

src/libutil/experimental-features.cc

@@ -11,6 +11,7 @@ std::map<ExperimentalFeature, std::string> stringifiedXpFeatures = {
     { Xp::NixCommand, "nix-command" },
     { Xp::RecursiveNix, "recursive-nix" },
     { Xp::NoUrlLiterals, "no-url-literals" },
+    { Xp::ExternalGCDaemon, "external-gc-daemon" },
 };
 
 const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)

src/libutil/experimental-features.hh

@@ -19,7 +19,8 @@ enum struct ExperimentalFeature
     Flakes,
     NixCommand,
     RecursiveNix,
-    NoUrlLiterals
+    NoUrlLiterals,
+    ExternalGCDaemon,
 };
 
 /**

tests/gc-external-daemon.sh

@@ -1,5 +1,7 @@
 source common.sh
 
+sed -i 's/experimental-features .*/& external-gc-daemon/' "$NIX_CONF_DIR"/nix.conf
+
 export NIX_GC_SOCKET_PATH=$TEST_ROOT/gc.socket
 startGcDaemon() {
     # Start the daemon, wait for the socket to appear.  !!!