To minimize the potential for irrecoverable errors, only atomic writes
to the ESP are performed. This is implemented by first copying the file
to the destination with a `.tmp` suffix and then renaming it to the
final desintation. This is atomic because the rename operation is atomic
on POSIX platforms.
Specifically, this means that even if the system crashes during the
operation, the final desintation path will most likely be intact if it
exists at all. There are some nuances to this however. It **cannot** be
actually guaranteed that the operation was performed on the filesystem
level. However, this is the best we can do for now.
For reference:
- POSIX rename(2): https://pubs.opengroup.org/onlinepubs/9699919799/
- Rust fs::rename corresponds to rename(2) on Unix: https://doc.rust-lang.org/std/fs/fn.rename.html
- Rust fs::rename is implemented using libc's rename: https://github.com/rust-lang/rust/blob/master/library/std/src/sys/unix/fs.rs#L1397
- Renaming in libc is atomic: https://www.gnu.org/software/libc/manual/html_node/Renaming-Files.html
To make handling systemd versions more robust, they are parsed into a
u32 tuple instead of an f32. Additionally, some unit tests for correct
parsing and comparing of versions are added.
The process of installing systemd-boot is "smarter" because it now
considers a a few conditions instead of doing nothing if there is a file
at the deistination path. systemd-boot is now forcibly installed (i.e.
overwriting any file at the destination) if (1) there is no file at the
destination, OR (2) a newer version of systemd-boot is available, OR (3)
the signature of the file at the destination could not be verified.
To access paths on the ESP before or after installing generations, split
EspPaths into general EspPaths that only depend on the path to the ESP
and EspGenerationPaths which additionally depend on generation specific
information (e.g. version number and initrd filename).
Add an extension to TempDir that allows to create secure tempfiles. This
way, everything related to creating secure tempfiles is bundled in a
single place and can easily be reused.
nikstur