Add a test ensuring compatibility with an old daemon

This requires adding `nix` to its own closure which is a bit unfortunate,
but as it is optional (the test will be disabled if `OUTER_NIX` is unset) it
shouldn't be too much of an issue.

(Ideally this should go in another derivation so that we can build Nix and run
the test independently, but as the tests are running in the same derivation
as the build it's a bit complicated to do so).

flake.nix

@@ -150,6 +150,11 @@
       # 'nix.perl-bindings' packages.
       overlay = final: prev: {
 
+        # An older version of Nix to test against when using the daemon.
+        # Currently using `nixUnstable` as the stable one doesn't respect
+        # `NIX_DAEMON_SOCKET_PATH` which is needed for the tests.
+        mainstream-nix = prev.nixUnstable;
+
         nix = with final; with commonDeps pkgs; stdenv.mkDerivation {
           name = "nix-${version}";
           inherit version;
@@ -158,6 +163,8 @@
 
           VERSION_SUFFIX = versionSuffix;
 
+          OUTER_NIX = mainstream-nix;
+
           outputs = [ "out" "dev" "doc" ];
 
           nativeBuildInputs = nativeBuildDeps;
@@ -486,6 +493,8 @@
         stdenv.mkDerivation {
           name = "nix";
 
+          OUTER_NIX = mainstream-nix;
+
           outputs = [ "out" "dev" "doc" ];
 
           nativeBuildInputs = nativeBuildDeps;

tests/common.sh.in

@@ -57,7 +57,6 @@ clearStore() {
     mkdir "$NIX_STORE_DIR"
     rm -rf "$NIX_STATE_DIR"
     mkdir "$NIX_STATE_DIR"
-    nix-store --init
     clearProfiles
 }
 
@@ -73,7 +72,7 @@ startDaemon() {
     # Start the daemon, wait for the socket to appear.  !!!
     # ‘nix-daemon’ should have an option to fork into the background.
     rm -f $NIX_STATE_DIR/daemon-socket/socket
-    nix daemon &
+    ${NIX_DAEMON_COMMAND:-nix daemon} &
     for ((i = 0; i < 30; i++)); do
         if [ -e $NIX_DAEMON_SOCKET_PATH ]; then break; fi
         sleep 1

tests/local.mk

@@ -6,7 +6,7 @@ nix_tests = \
   gc-auto.sh \
   referrers.sh user-envs.sh logging.sh nix-build.sh misc.sh fixed.sh \
   gc-runtime.sh check-refs.sh filter-source.sh \
-  local-store.sh remote-store.sh export.sh export-graph.sh \
+  local-store.sh remote-store.sh remote-store-old-daemon.sh export.sh export-graph.sh \
   timeout.sh secure-drv-outputs.sh nix-channel.sh \
   multiple-outputs.sh import-derivation.sh fetchurl.sh optimise-store.sh \
   binary-cache.sh \

tests/remote-store-old-daemon.sh

@@ -0,0 +1,7 @@
+# Test that the new Nix can properly talk to an old daemon.
+# If `$OUTER_NIX` isn't set (e.g. when bootsraping), just skip this test
+
+if [[ -n "$OUTER_NIX" ]]; then
+    export NIX_DAEMON_COMMAND=$OUTER_NIX/bin/nix-daemon
+    source remote-store.sh
+fi

tests/remote-store.sh

@@ -23,12 +23,12 @@ startDaemon
 
 storeCleared=1 NIX_REMOTE_=$NIX_REMOTE $SHELL ./user-envs.sh
 
+nix-store --gc --max-freed 1K
+
 nix-store --dump-db > $TEST_ROOT/d1
 NIX_REMOTE= nix-store --dump-db > $TEST_ROOT/d2
 cmp $TEST_ROOT/d1 $TEST_ROOT/d2
 
-nix-store --gc --max-freed 1K
-
 killDaemon
 
 user=$(whoami)