Add repl completion test

flake.nix

@@ -393,6 +393,7 @@
           in pkgs.buildPackages.runCommand "test-rl-next-release-notes" { } ''
           LANG=C.UTF-8 ${pkgs.changelog-d-nix}/bin/changelog-d ${./doc/manual/rl-next} >$out
         '';
+        repl-completion = nixpkgsFor.${system}.native.callPackage ./tests/repl-completion.nix { };
       } // (lib.optionalAttrs (builtins.elem system linux64BitSystems)) {
         dockerImage = self.hydraJobs.dockerImage.${system};
       } // (lib.optionalAttrs (!(builtins.elem system linux32BitSystems))) {

tests/repl-completion.nix

@@ -0,0 +1,40 @@
+{ runCommand, nix, expect }:
+
+# We only use expect when necessary, e.g. for testing tab completion in nix repl.
+# See also tests/functional/repl.sh
+
+runCommand "repl-completion" {
+  nativeBuildInputs = [
+    expect
+    nix
+  ];
+  expectScript = ''
+    # Regression https://github.com/NixOS/nix/pull/10778
+    spawn nix repl --offline --extra-experimental-features nix-command
+    expect "nix-repl>"
+    send "foo = import ./does-not-exist.nix\n"
+    expect "nix-repl>"
+    send "foo.\t"
+    expect {
+      "nix-repl>" {
+        puts "Got another prompt. Good."
+      }
+      eof {
+        puts "Got EOF. Bad."
+        exit 1
+      }
+    }
+    exit 0
+  '';
+  passAsFile = [ "expectScript" ];
+}
+''
+  export NIX_STORE=$TMPDIR/store
+  export NIX_STATE_DIR=$TMPDIR/state
+  export HOME=$TMPDIR/home
+  mkdir $HOME
+
+  nix-store --init
+  expect $expectScriptPath
+  touch $out
+''