nixos/make-disk-image: fix contents dir paths

`make-disk-image` is a tool for creating VM images. It takes an argument
`contents` that allows one to specify files and directories that should
be copied into the VM image. However, directories end up not at the
specified target, but instead at a subdirectory of the target, with a
nix-store-like path, e.g.
`/target/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-source`. See issue
https://github.com/NixOS/nixpkgs/issues/226203 .

This change adds a test for make-disk-image's contents directory
handling and adds a fix (appending `/` to rsync input directory names).

This closes issue https://github.com/NixOS/nixpkgs/issues/226203 .

nixos/doc/manual/release-notes/rl-2305.section.md

@@ -415,6 +415,8 @@ In addition to numerous new and upgraded packages, this release has the followin
 
 - The `bind` module now allows the per-zone `allow-query` setting to be configured (previously it was hard-coded to `any`; it still defaults to `any` to retain compatibility).
 
+- `make-disk-image` handles `contents` arguments that are directories better, fixing a bug where it used to put them in a subdirectory of the intended `target`.
+
 ## Detailed migration information {#sec-release-23.05-migration}
 
 ### Pipewire configuration overrides {#sec-release-23.05-migration-pipewire}

nixos/lib/make-disk-image.nix

@@ -402,11 +402,16 @@ let format' = format; in let
         done
       else
         mkdir -p $root/$(dirname $target)
-        if ! [ -e $root/$target ]; then
+        if [ -e $root/$target ]; then
-          rsync $rsync_flags $source $root/$target
-        else
           echo "duplicate entry $target -> $source"
           exit 1
+        elif [ -d $source ]; then
+          # Append a slash to the end of source to get rsync to copy the
+          # directory _to_ the target instead of _inside_ the target.
+          # (See `man rsync`'s note on a trailing slash.)
+          rsync $rsync_flags $source/ $root/$target
+        else
+          rsync $rsync_flags $source $root/$target
         fi
       fi
     done

nixos/tests/image-contents.nix

@@ -27,13 +27,19 @@ let
     inherit pkgs config;
     lib = pkgs.lib;
     format = "qcow2";
-    contents = [{
+    contents = [
+      {
         source = pkgs.writeText "testFile" "contents";
         target = "/testFile";
         user = "1234";
         group = "5678";
         mode = "755";
-    }];
+      }
+      {
+        source = ./.;
+        target = "/testDir";
+      }
+    ];
   }) + "/nixos.qcow2";
 
 in makeEc2Test {
@@ -42,10 +48,15 @@ in makeEc2Test {
   userData = null;
   script = ''
     machine.start()
+    # Test that if contents includes a file, it is copied to the target.
     assert "content" in machine.succeed("cat /testFile")
     fileDetails = machine.succeed("ls -l /testFile")
     assert "1234" in fileDetails
     assert "5678" in fileDetails
     assert "rwxr-xr-x" in fileDetails
+
+    # Test that if contents includes a directory, it is copied to the target.
+    dirList = machine.succeed("ls /testDir")
+    assert "image-contents.nix" in dirList
   '';
 }