depot/third_party/nixpkgs/pkgs/os-specific/linux/systemd/0016-systemctl-edit-suggest-systemdctl-edit-runtime-on-sy.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Maximilian Bosch <maximilian@mbosch.me>
Date: Fri, 1 Sep 2023 09:57:02 +0200
Subject: [PATCH] systemctl-edit: suggest `systemdctl edit --runtime` on system
 scope

This is a NixOS-specific change. When trying to modify a unit with
`systemctl edit` on NixOS, it'll fail with "Read-only file system":

    $ systemctl edit libvirtd
    Failed to open "/etc/systemd/system/libvirtd.service.d/.#override.conffa9825a0c9a249eb": Read-only file system

This is because `/etc/systemd/system` is a symlink into the store. In
fact, I'd consider this a feature rather than a bug since this ensures I
don't introduce state imperatively.

However, people wrongly assume that it's not possible to edit units
ad-hoc and re-deploy their system for quick&dirty debugging where this
would be absolutely fine (and doable with `--runtime` which adds a
transient and non-persistent unit override in `/run`).

To make sure that people learn about it quicker, this patch
throws an error which suggests using `--runtime` when running
`systemctl edit` on the system scope.

For the user scope this isn't needed because user-level unit overrides
are written into `$XDG_CONFIG_HOME/systemd/user`.
---
 src/systemctl/systemctl-edit.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/systemctl/systemctl-edit.c b/src/systemctl/systemctl-edit.c
index 367afa20f7..5777154d01 100644
--- a/src/systemctl/systemctl-edit.c
+++ b/src/systemctl/systemctl-edit.c
@@ -322,6 +322,9 @@ int verb_edit(int argc, char *argv[], void *userdata) {
         sd_bus *bus;
         int r;
 
+        if (!arg_runtime && arg_runtime_scope == RUNTIME_SCOPE_SYSTEM)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "The unit-directory '/etc/systemd/system' is read-only on NixOS, so it's not possible to edit system-units directly. Use 'systemctl edit --runtime' instead.");
+
         if (!on_tty())
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Cannot edit units if not on a tty.");