install: use an existing ~/nix-config instead of stopping

Without ~/.config/home-manager and --flake, a flake in ~/nix-config
(e.g. left by uninstall) is used as it is, like --flake ~/nix-config,
and linked to ~/.config/home-manager. A directory there without
flake.nix still stops the install.

README.md

@@ -79,13 +79,14 @@ on `main`. `sudo` needs a password: run `passwd` first if you never set one.
 
 The installer installs Nix (skipped if Nix already works), uses
 `~/.config/home-manager` or `--flake <dir-or-flakeref>` (if there is none, it
-creates `~/nix-config` from the [template](template) with your user name) and
-runs `home-manager switch`; what it sets up is listed under
-[Set up by install.sh](#set-up-by-installsh). Everything works in the running
-session right away, except two settings read only at a process start: the
-[keyboard layout](docs/session.md#keyboard-layout) (next Steam session start)
-and the [SteamVR dashboard patches](docs/steamvr-debugger.md) (VR pet, VR
-keyboard suggestions, window curvature, ...; next SteamVR start). If they
+uses `~/nix-config` as it is, or creates it from the [template](template)
+with your user name) and runs `home-manager switch`; what it sets up is
+listed under [Set up by install.sh](#set-up-by-installsh). Everything works
+in the running session right away, except two settings read only at a
+process start: the [keyboard layout](docs/session.md#keyboard-layout) (next
+Steam session start) and the
+[SteamVR dashboard patches](docs/steamvr-debugger.md) (VR pet, VR keyboard
+suggestions, window curvature, ...; next SteamVR start). If they
 are waiting, the installer ends with "Restart once": reboot once. Re-running
 it just switches again (`--yes` answers every question). Afterwards edit
 `~/nix-config/home.nix` and switch (see [Usage](#usage)).

install.sh

@@ -69,7 +69,8 @@ Commands:
                              if the clone fails. Only a missing directory or
                              the unchanged template is replaced by the clone;
                              anything else there is used as it is
-        otherwise            ~/.config/home-manager
+        otherwise            ~/.config/home-manager, else the flake in
+                             ~/nix-config (linked there)
         neither exists       a new config in ~/nix-config from the
                              steam-frame-nix template, linked to
                              ~/.config/home-manager
@@ -353,9 +354,6 @@ template_untouched() { # dir
 create_config() { # dir [question]
   local dir=$1 system
   if [[ -e $dir ]] && [[ -n $(ls -A "$dir" 2>/dev/null) ]]; then
-    if [[ -e $dir/flake.nix ]]; then
-      die "$dir already contains a flake. Use it with: install.sh install --flake $dir"
-    fi
     die "$dir exists and is not empty; move it away or pass --flake <your config>"
   fi
   [[ $USER_NAME =~ ^[a-z_][a-z0-9_-]*$ ]] || die "unsupported user name '$USER_NAME' for the template"
@@ -529,6 +527,10 @@ cmd_install() {
   fi
 
   step "Finding the Home Manager configuration"
+  if [[ -z $flake && ! -e $HM_CONFIG_LINK && ! -L $HM_CONFIG_LINK && -e $DEFAULT_CONFIG_DIR/flake.nix ]]; then
+    info "$DEFAULT_CONFIG_DIR already exists: using it as it is"
+    flake=$DEFAULT_CONFIG_DIR
+  fi
   if [[ -n $flake ]]; then
     if [[ -d ${flake%%#*} ]]; then
       dir="$(cd "${flake%%#*}" && pwd -P)"

modules/cleanup/check.nix

@@ -509,6 +509,20 @@ pkgs.runCommand "cleanup-check" { nativeBuildInputs = [ cleanup pkgs.jq pkgs.git
   ! grep -q "^clone" $LOG || fail "cloned again"
   ! switched_to_template || fail "template switched again"
   [ "$(grep -c '^hm ' $LOG)" = 1 ] || fail "switches: $(cat $LOG)"
+
+  # a plain install with ~/nix-config but no link: used as it is, linked
+  rm $HOME/.config/home-manager
+  res=$(inst)
+  has "$res" "$HOME/nix-config already exists: using it as it is"
+  hasnt "$res" "Creating a Home Manager configuration"
+  has "$res" "hm switch --flake $HOME/nix-config -b"
+  has "$res" "'home-manager switch' (from"
+  [ "$(readlink $HOME/.config/home-manager)" = $HOME/nix-config ] || fail "plain install: link"
+  # not a flake: the error, nothing created
+  rm $HOME/.config/home-manager $HOME/nix-config/flake.nix
+  res=$(inst 2>&1) && fail "a directory without flake.nix accepted"
+  has "$res" "$HOME/nix-config exists and is not empty"
+  gone $HOME/.config/home-manager
   echo "G ok"
 
   # --- H: uninstall: Nix goes while programs from the store still run ---