Allow usage with nix deamon

This commit removes the restriction of running the project only via single-user nix installation.
To ensure disruptor is run with appropriate settings, a comparison is made betweeen global nix configuration and those specified in scripts/nix.conf file - any differences is clearly communicated to user

scripts/nix-conf-diff.nix

@@ -0,0 +1,41 @@
+let
+  lib = (import <nixpkgs> { }).lib;
+  # attrset representing current host nix settings
+  global-nix-config = builtins.mapAttrs (_: value: value.value)
+    (builtins.fromJSON (builtins.readFile ../.cache/.global-nix-conf));
+  # list of config file lines
+  config-lines = lib.lists.remove ""
+    (lib.strings.splitString "\n" (builtins.readFile ./nix.conf));
+  # list of tuples representing config lines
+  config-kv-pairs = (builtins.map (x:
+    lib.strings.splitString "=" (builtins.replaceStrings [ " = " ] [ "=" ] x))
+    config-lines);
+  # attrset with strigified nix config values (i.e. "a b" instead of [ "a", "b" ], "true" instead of true
+  disruptor-config-strs = builtins.listToAttrs (builtins.map (x: {
+    name = (builtins.elemAt x 0);
+    value = (builtins.elemAt x 1);
+  }) config-kv-pairs);
+
+  # Convert disruptor config string values into correct types (except for numbers)
+  from_str = x:
+    (if x == "true" then
+      true
+    else if x == "false" then
+      false
+    else if (builtins.length (lib.strings.splitString " " x) == 1) then
+      x
+    else
+      (lib.strings.splitString " " x));
+
+  # attrset representing disruptor required config
+  disruptor-config =
+    builtins.mapAttrs (_: v: (from_str v)) disruptor-config-strs;
+
+  # attrset representing diff in configuration
+  different_settings = lib.attrsets.filterAttrsRecursive (k: v:
+    (let global-value = builtins.getAttr k global-nix-config;
+    in if (builtins.isInt global-value) then
+      (builtins.toString global-value) != v
+    else
+      global-value != v)) disruptor-config;
+in different_settings

scripts/shell.sh

@@ -97,6 +97,42 @@ preflightCheck() {
   esac
 }
 
+verify_nix_daemon_config_compatibility() {
+  # Check if there is no discrepancies between host nix daemon configuration
+  # and project disruptor requirements.
+  # 
+  # As one cannot assume the commands will be executed by a trusted nix user,
+  # only verification remains to notify user of any potential issues in host
+  # nix configuration.
+  #
+  # This function assumes nix is installed globally and present in $PATH.
+
+  local readonly config_difference;
+  local readonly global_nix_conf_settings="${CACHE_ROOT}/.global-nix-conf"
+
+  # `nix show-config` seems to be the only available way of obtaning 
+  # "final" nix configuration, without doing the re-implementation of 
+  # said logic. 
+  # Therefore "experimental-features = nix-command" are needed.
+
+  # Why not $(nix show-config)? It fails with cryptic 'access-tokens: command not found'
+  nix show-config --json > "${global_nix_conf_settings}" 2>/dev/null
+  # shellcheck disable=SC2181
+  if [ $? -ne 0 ]; then
+   fail "nix show-config command has failed. Please enable it by following this steps: https://nixos.wiki/wiki/Nix_command"
+  fi
+
+  config_difference="$(nix-instantiate --strict --eval "${__DIR__}"/nix-conf-diff.nix)"
+  if [ "${config_difference}" == "{ }" ]; then
+    return
+  fi
+  echo "The configuration of host nix daemon differs from the expected one."
+  echo "These are the offending settings: "
+  echo "${config_difference}"
+  echo "---" 
+  echo "Please update your configuration otherwise the shell may not work as intended."
+}
+
 printHelp() {
   cat << EOF
    Usage: nix-shell.sh [--rcfile] [--vanilla] [--help] -- <PARAMS TO PASS TO NIX-SHELL>
@@ -162,6 +198,7 @@ setup_nix() {
 }
 
 ensure_direnv_is_configured() {
+  # shellcheck disable=SC2034
   local readonly project_root
   project_root=$(dirname "$(realpath "${__DIR__}"/..)")
   mkdir -p "${CACHE_ROOT}"
@@ -172,52 +209,25 @@ EOF
 }
 
 ensure_nix_is_present() {
-  if ${IS_NIXOS}; then
-    # On nixos, nix is installed by default
-    return
-  fi
-
   if command -v "nix" >/dev/null 2>&1; then
     IS_NIX_INSTALLED=true;
   else
     IS_NIX_INSTALLED=false;
   fi
-  # We need to distinguish between single-user and multi-user installs.
-  # This is difficult because there's no official way to do this.
-  # Details: https://github.com/lilyball/nix-env.fish/blob/00c6cc762427efe08ac0bd0d1b1d12048d3ca727/conf.d/nix-env.fish
 
-  # stat is not portable. Splitting the output of ls -nd is reliable on most platforms.
+  # Global nix installation
   if ${IS_NIX_INSTALLED}; then
-    local readonly nix_store_owner
-    # shellcheck disable=SC2012
-    nix_store_owner=$(ls -nd /nix/store | cut -d' ' -f3)
-    if [ "${nix_store_owner}" -eq 0 ]; then
-      local readonly is_nix_multiuser_install=true;
-    else
-      local readonly is_nix_multiuser_install=false;
-    fi
-  else
-    # shellcheck disable=SC2034
-    local readonly is_nix_multiuser_install=false;
-  fi
-
-  # Global, single-user installation
-  if ${IS_NIX_INSTALLED} && ! ${is_nix_multiuser_install}; then
+    verify_nix_daemon_config_compatibility
     return
   fi
 
-  if ${IS_NIX_INSTALLED} && ${is_nix_multiuser_install}; then
-    # TODO: Find a solution
-    fail "Daemon-based nix installation is not supported"
-  fi
-
   if [ -d "${NIX_STORE}" ]; then
     # Nix has already been set up
     echo "Detected previous setup in ${CACHE_ROOT}. Will attempt to use it."
     return
   fi
 
-  # No nix installed or nix is multi-user installation
+  # No nix installed
   setup_nix_user_chroot
   setup_nix
 }
@@ -228,9 +238,15 @@ ensure_nix_shell_rc_exists() {
   : > "${NIX_SHELL_RC}"
   if [ -n "${EXTRA_RC}" ]; then cat "${EXTRA_RC}" >> "${NIX_SHELL_RC}"; fi
 
-  if ! ${IS_NIXOS} || ${IS_NIX_INSTALLED}; then
+  # NixOS does not have/use boostraping nix.sh script
+  if ! ${IS_NIXOS}; then
      cat >> "${NIX_SHELL_RC}" <<-EOL
 	. ${USER_HOME}/.nix-profile/etc/profile.d/nix.sh
+	EOL
+  fi
+
+  if ${IS_NIX_INSTALLED}; then
+     cat >> "${NIX_SHELL_RC}" <<-EOL
 	NIX_CONF_DIR=${NIX_CONF_DIR}
 	NIX_USER_CONF_FILES=${NIX_USER_CONF_FILES}
 	EOL
@@ -279,7 +295,7 @@ ensure_nix_shell_rc_exists
 # shellcheck disable=SC1091
 . "${__DIR__}/push.sh"
 
-if ! ${IS_NIXOS} && ! ${IS_NIX_INSTALLED}; then
+if ! ${IS_NIX_INSTALLED}; then
   # shellcheck disable=SC2250
   SHELL="$NIX_USER_CHROOT_BIN ${NIX_STORE} bash"
 fi