switch-to-configuration-ng: Fix exit status on bootloader install error

The problem
-----------

When rebuilding a system, if `switch-to-configuration-ng` fails to install
bootloader files, it will (most likely) `exit(0)`.

```
/etc/nixos $ sudo nixos-rebuild --fast boot && reboot
[sudo] password for samuel:
building the system configuration...
updating GRUB 2 menu...
cannot copy /nix/store/.../initrd to /boot/kernels/...-initrd.tmp: No space left on device
Failed to install bootloader

Broadcast message from samuel@... on pts/1 (Tue 2024-12-31 16:48:26 EST):

The system will reboot now!
```

This is a quite awkward breaking change with the expected behaviour.

* * *

The investigation
-----------------

Compare:

 - https://github.com/NixOS/nixpkgs/blob/85b5f3e959327a3fa46f843848ebb8799069bb95/pkgs/by-name/sw/switch-to-configuration-ng/src/src/main.rs#L171-L179
 - https://github.com/NixOS/nixpkgs/blob/85b5f3e959327a3fa46f843848ebb8799069bb95/nixos/modules/system/activation/switch-to-configuration.pl#L115-L117

Let's see what `die()` is all about:

 - https://github.com/NixOS/nixpkgs/blob/85b5f3e959327a3fa46f843848ebb8799069bb95/pkgs/by-name/sw/switch-to-configuration-ng/src/src/main.rs#L121-L125

***sus.***

There are multiple issues converging here.

**Incorrect port**

The original implementation did not use `die`, but `exit 1`.

So porting from `perl` following the script's idiosyncrasies was not
done appropriately.

**Incorrect `die` fac-simile**

The `die` method is incomplete with regard to the semantics of perl.

 - https://perldoc.perl.org/5.40.0/functions/die

Of importance to us:

> If [die is called],  the exit code is determined from the values of
> `$!` and `$?` with this pseudocode:
>
> ```
> exit $! if $!;              # errno
> exit $? >> 8 if $? >> 8;    # child exit status
> exit 255;                   # last resort
> ```

The `die()` method in `switch-to-configuration-ng` *only* checks
`errno`, using its value directly to `exit()`.

It does not handle some form of implicit child process exit status.

And, due to incorrect assumptions, it will not fall back to anything.

**Incorrect implementation**

(Note that from this point on, I'm not a Rust expert, so bear with me if
some nuances are lost or incorrectly represented.)

The `die()` function implementation, as a port, might not even work
correctly.

Already, the `spawn` method does not mention it would be setting
`errno`, so any `die()` following a `status.success()` is *sus* and
should be investigated. Since it's not attempting to do anything "smart"
with child processes.

 - https://doc.rust-lang.org/1.83.0/std/process/struct.Command.html#method.spawn

And I'd argue that using *errno* in this manner in Rust is probably a
mistake, and should not be done.

> This should be called immediately after a call to a platform function,
> otherwise the state of the error value is indeterminate.

 - https://doc.rust-lang.org/1.83.0/std/io/struct.Error.html#method.last_os_error

Considering *platform function* is largely left undefined, I would
(probably wrongly) intuit that it should be considered undefined
behaviour to rely on it.

Note that `raw_os_error` might have a surprising interface.

> If this `Error` was constructed via `last_os_error` [...],
> then this function will return `Some`, otherwise it will return `None`.

 - https://doc.rust-lang.org/1.83.0/std/io/struct.Error.html#method.raw_os_error

Since it's used as `std::io::Error::last_os_error().raw_os_error()`,
AFAIUI it will always return `Some`.

Since this is exposing `errno`, the libc concept, it will behave the
same, and may be set to `0` by default, just like here:

```
$ printf '#include <stdlib.h>\n#include <errno.h>\nint main() { exit(errno); }' \
    | cc -x c - && ./a.out; echo $?
0
```

Which means that, since no *platform function*[sic] changed its value,
it will be zero, the `die()` function will be equivalent to `exit(errno)`,
and the program will have failed “successfully” wrongly.

* * *

The fix
-------

I've fixed the `do_pre_switch_check` and `do_install_bootloader` methods,
both of which share the same defects (the original script uses `exit 1`
for both).

They were the only `status.success()` checks using `die()`.

* * *

Reproducing the issue
---------------------

Remember how I said:

> Considering *platform function* is largely left undefined, I would
> (probably wrongly) intuit that it should be considered undefined
> behaviour to rely on it.

Here's why it's not some vague FUD.

First, make sure a `nixos-rebuild boot` would need to write new files to
the boot partition. Removing an older (but still alive) generation's
initrd can do that.

Fill the `/boot` partition to force an error.

```
 $ sudo dd if=/dev/zero of=/boot/BOGUS.FILLINGS
```

Then, and here's the fun part, observe:

```
~ $ sudo rm -r /run/nixos

~ $ sudo nixos-rebuild --fast boot ; echo $?
building the system configuration...
updating GRUB 2 menu...
cannot copy /nix/store/x91w4p91l7iclkdp38chvdxcw6nr5113-mobile-nixos-initrd-generic/initrd to /boot/kernels/x91w4p91l7iclkdp38chvdxcw6nr5113-mobile-nixos-initrd-generic-initrd.tmp: No space left on device
Failed to install bootloader
0

~ $ sudo nixos-rebuild --fast boot ; echo $?
building the system configuration...
updating GRUB 2 menu...
cannot copy /nix/store/x91w4p91l7iclkdp38chvdxcw6nr5113-mobile-nixos-initrd-generic/initrd to /boot/kernels/x91w4p91l7iclkdp38chvdxcw6nr5113-mobile-nixos-initrd-generic-initrd.tmp: No space left on device
Failed to install bootloader
warning: error(s) occurred while switching to the new configuration
1
```

So... What's the deal with /run/nixos? It's where the lock file will
reside. (And other transient files.)

 - https://github.com/NixOS/nixpkgs/blob/85b5f3e959327a3fa46f843848ebb8799069bb95/pkgs/by-name/sw/switch-to-configuration-ng/src/src/main.rs#L1018-L1027

But why does that matter here?

```
~ $ errno 17
EEXIST 17 File exists
```

This error is produced by some *platform functions*[sic] that create
either the directory, or the lockfile. The file already exists.

So the script would end-up failing this way *only for the first
invocation*. Which is why it's possible any of you all reviewing this
~~novel~~ PR haven't faced that issue.

* * *

Future work
-----------

I believe `die()` *probably* should be switched to check the value, and
`exit 255` if it's 0.

Though I also believe `die()` shouldn't try to port perl semantics into
Rust. I don't think it's working out.

Additionally, a NixOS test should be authored to ensure that errors in
these phases actually are handled appropriately.

Signed-off-by: Samuel Dionne-Riel <samuel@dionne-riel.com>

pkgs/by-name/sw/switch-to-configuration-ng/src/src/main.rs

@@ -152,7 +152,7 @@ fn do_pre_switch_check(command: &str, toplevel: &Path) -> Result<()> {
         Ok(Ok(status)) if status.success() => {}
         _ => {
             eprintln!("Pre-switch checks failed");
-            die()
+            std::process::exit(1);
         }
     }
 
@@ -174,7 +174,7 @@ fn do_install_bootloader(command: &str, toplevel: &Path) -> Result<()> {
         Ok(Ok(status)) if status.success() => {}
         _ => {
             eprintln!("Failed to install bootloader");
-            die();
+            std::process::exit(1);
         }
     }