From 1e7f9def3b0d74dccb9f3876ba3ba7666641aa52 Mon Sep 17 00:00:00 2001
From: magic_rb <richard@brezak.sk>
Date: Sat, 21 Oct 2023 17:34:58 +0200
Subject: [PATCH] Allow for symlinking files

Signed-off-by: magic_rb <richard@brezak.sk>
---
 mount-file.bash | 36 +++++++++++++++++++++++-------------
 nixos.nix       | 26 +++++++++++++++++++++-----
 2 files changed, 44 insertions(+), 18 deletions(-)

diff --git a/mount-file.bash b/mount-file.bash
index 6202d07..09d3297 100755
--- a/mount-file.bash
+++ b/mount-file.bash
@@ -10,7 +10,7 @@ shopt -s inherit_errexit  # Inherit the errexit option status in subshells.
 trap 'echo Error when executing ${BASH_COMMAND} at line ${LINENO}! >&2' ERR
 
 # Get inputs from command line arguments
-if [[ "$#" != 3 ]]; then
+if [[ "$#" != 4 ]]; then
     echo "Error: 'mount-file.bash' requires *three* args." >&2
     exit 1
 fi
@@ -18,6 +18,7 @@ fi
 mountPoint="$1"
 targetFile="$2"
 debug="$3"
+method="$4"
 
 trace() {
     if (( "$debug" )); then
@@ -28,16 +29,25 @@ if (( "$debug" )); then
     set -o xtrace
 fi
 
-if [[ -L "$mountPoint" && $(readlink -f "$mountPoint") == "$targetFile" ]]; then
-    trace "$mountPoint already links to $targetFile, ignoring"
-elif mount | grep -F "$mountPoint"' ' >/dev/null && ! mount | grep -F "$mountPoint"/ >/dev/null; then
-    trace "mount already exists at $mountPoint, ignoring"
-elif [[ -e "$mountPoint" ]]; then
-    echo "A file already exists at $mountPoint!" >&2
-    exit 1
-elif [[ -e "$targetFile" ]]; then
-    touch "$mountPoint"
-    mount -o bind "$targetFile" "$mountPoint"
-else
-    ln -s "$targetFile" "$mountPoint"
+if [[ "$method" == "bind" ]]; then
+   if [[ -L "$mountPoint" && $(readlink -f "$mountPoint") == "$targetFile" ]]; then
+       trace "$mountPoint already links to $targetFile, ignoring"
+   elif mount | grep -F "$mountPoint"' ' >/dev/null && ! mount | grep -F "$mountPoint"/ >/dev/null; then
+       trace "mount already exists at $mountPoint, ignoring"
+   elif [[ -e "$mountPoint" ]]; then
+       echo "A file already exists at $mountPoint!" >&2
+       exit 1
+   elif [[ -e "$targetFile" ]]; then
+       touch "$mountPoint"
+       mount -o bind "$targetFile" "$mountPoint"
+   else
+       ln -s "$targetFile" "$mountPoint"
+   fi
+elif [[ "$method" == "symlink" ]]; then
+    if [[ -e "$mountPoint" ]] && ! [[ -L "$mountPoint" ]]; then
+	echo "symlink requested, but something else than symlink is present at $mountPoint"
+	exit 1
+    else
+	ln -sf "$targetFile" "$mountPoint"
+    fi
 fi
diff --git a/nixos.nix b/nixos.nix
index b7a104f..bed7962 100644
--- a/nixos.nix
+++ b/nixos.nix
@@ -40,6 +40,11 @@ let
     parentsOf
     ;
 
+  isSymlink = entry:
+    entry.method == "symlink";
+  isBind = entry:
+    entry.method == "bind";
+
   cfg = config.environment.persistence;
   users = config.users.users;
   allPersistentStoragePaths = { directories = [ ]; files = [ ]; users = [ ]; }
@@ -64,7 +69,7 @@ let
 
   # Create all fileSystems bind mount entries for a specific
   # persistent storage path.
-  bindMounts = listToAttrs (map mkBindMountNameValuePair directories);
+  bindMounts = listToAttrs (map mkBindMountNameValuePair (filter isBind directories));
 in
 {
   options = {
@@ -83,6 +88,7 @@ in
             either
             str
             coercedTo
+            enum
             ;
         in
         attrsOf (
@@ -96,6 +102,13 @@ in
               };
               commonOpts = {
                 options = {
+                  method = mkOption {
+                    type = enum [ "bind" "symlink" ];
+                    default = "bind";
+                    description = ''
+                      Whether to use a bind mount or a plain symlink to persist this.
+                    '';
+                  };
                   persistentStoragePath = mkOption {
                     type = path;
                     default = cfg.${name}.persistentStoragePath;
@@ -477,7 +490,7 @@ in
   config = {
     systemd.services =
       let
-        mkPersistFileService = { filePath, persistentStoragePath, enableDebugging, ... }:
+        mkPersistFileService = { filePath, persistentStoragePath, enableDebugging, method, ... }:
           let
             targetFile = escapeShellArg (concatPaths [ persistentStoragePath filePath ]);
             mountPoint = escapeShellArg filePath;
@@ -492,7 +505,7 @@ in
               serviceConfig = {
                 Type = "oneshot";
                 RemainAfterExit = true;
-                ExecStart = "${mountFile} ${mountPoint} ${targetFile} ${escapeShellArg enableDebugging}";
+                ExecStart = "${mountFile} ${mountPoint} ${targetFile} ${escapeShellArg enableDebugging} ${escapeShellArg method}";
                 ExecStop = pkgs.writeShellScript "unbindOrUnlink-${sanitizeName targetFile}" ''
                   set -eu
                   if [[ -L ${mountPoint} ]]; then
@@ -505,8 +518,10 @@ in
               };
             };
           };
+        mkPersistDirectoryService = { dirPath, ... }@args:
+          mkPersistFileService (args // { filePath = dirPath; });
       in
-      foldl' recursiveUpdate { } (map mkPersistFileService files);
+        foldl' recursiveUpdate { } ((map mkPersistFileService files) ++ (map mkPersistDirectoryService (filter isSymlink directories)));
 
     fileSystems = bindMounts;
     # So the mounts still make it into a VM built from `system.build.vm`
@@ -639,7 +654,7 @@ in
             exit $_status
           '';
 
-        mkPersistFile = { filePath, persistentStoragePath, enableDebugging, ... }:
+        mkPersistFile = { filePath, persistentStoragePath, enableDebugging, method, ... }:
           let
             mountPoint = filePath;
             targetFile = concatPaths [ persistentStoragePath filePath ];
@@ -647,6 +662,7 @@ in
               mountPoint
               targetFile
               enableDebugging
+              method
             ];
           in
           ''