nixos/systemd-boot: atomically update copied destination files

We absolutely do not want to leave an incomplete file behind in /boot
since an incomplete initrd would render the machine unbootable. Errors
while writing are relatively common, mostly due to full /boot
partitions.

systemd-boot-builder does never attempt to re-write already existing
files which means that such situations are not fixable by re-running
nixos-rebuild etc. Instead the user needs to know about internals of the
systemd-boot and manually delete the correct file to recover from a
partially written kernel or (more commonly) initrd in /boot.

Note that this used to be a non issue since systemd-boot-builder used to
always delete all kernels and initrds before copying kernels and
initrds, so dest.exist() would always return False. This was fixed in
f2ca99055823b3c2ce762ea2d4a315086a530c3d, revealing the underlying bad
assumption (that copyfile() always succeeds or fails without writing
anything).

The solution is to write to a temporary file first and move it to the
destination path only after this has succeeded. This way, if an error
occurs during copying, only a file distinct from dest is left behind
which would be cleaned up by subsequent runs of remove_old_entries().

Resolves #444066.

0 files changed, 0 insertions, 0 deletions