systemd-boot: introduce options to set a sort-key for systemd-boot entries

Without sort-keys specified on entries, the entries are sorted only by
file name (in decreasing order, so starting at the end of the alphabet!),
without taking any other fields into account (see
[the boot loader specification reference][1]).
Moreover, entries without a sort-key are always ordered after all
entries with a sort-key, so by not adding a sort-key to the NixOS ones,
we cannot add a sort-key to any other entry while keeping it below the
NixOS entries.

So currently we have options to set the file names for additional entries like
memtest and netbootxyz.
However, as mentioned above, the sorting by file name is not very intuitive and
actually sorts in the opposite order of what is currently mentioned in the option
descriptions.

With this commit, we set a configurable sort-key on all NixOS entries,
and add options for setting the sort-keys for the memtest and netbootxyz
entries.
The sorting by sort-key is more intuitive (it starts at the start of the
alphabet) and also takes into account the machine-id and version for entries
with identical sort-keys.

We use a bootspec extension to store the sort keys, which allows us to
redefine the sort key for individual specialisations without needing any
special casing.

[1]: https://uapi-group.org/specifications/specs/boot_loader_specification/#sorting

1 files changed, 10 insertions, 1 deletions

diff --git a/nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py b/nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py
index 258cf622a894..03bff1dee5b9 100644
--- a/nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py
+++ b/nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py
@@ -43,6 +43,7 @@ class BootSpec:
     system: str
     toplevel: str
     specialisations: Dict[str, "BootSpec"]
+    sortKey: str
     initrdSecrets: str | None = None
 
 
@@ -73,6 +74,7 @@ def system_dir(profile: str | None, generation: int, specialisation: str | None)
         return d
 
 BOOT_ENTRY = """title {title}
+sort-key {sort_key}
 version Generation {generation} {description}
 linux {kernel}
 initrd {initrd}
@@ -123,7 +125,13 @@ def get_bootspec(profile: str | None, generation: int) -> BootSpec:
 def bootspec_from_json(bootspec_json: Dict) -> BootSpec:
     specialisations = bootspec_json['org.nixos.specialisation.v1']
     specialisations = {k: bootspec_from_json(v) for k, v in specialisations.items()}
-    return BootSpec(**bootspec_json['org.nixos.bootspec.v1'], specialisations=specialisations)
+    systemdBootExtension = bootspec_json.get('org.nixos.systemd-boot', {})
+    sortKey = systemdBootExtension.get('sortKey', 'nixos')
+    return BootSpec(
+        **bootspec_json['org.nixos.bootspec.v1'],
+        specialisations=specialisations,
+        sortKey=sortKey
+    )
 
 
 def copy_from_file(file: str, dry_run: bool = False) -> str:
@@ -170,6 +178,7 @@ def write_entry(profile: str | None, generation: int, specialisation: str | None
 
     with open(tmp_path, 'w') as f:
         f.write(BOOT_ENTRY.format(title=title,
+                    sort_key=bootspec.sortKey,
                     generation=generation,
                     kernel=kernel,
                     initrd=initrd,