This will once and for all get rid of all double-quoting issues. On windows the quoting
is doubly bad because it escaped all \ to \\, which is very bad for error messages. In
order to prevent future regression std::filesystem::path formatting now must use a special
type PathFmt (like Magenta). In the future we could even change how we render filesystem paths.
This factors out the helper function from seekableGetNarBytes into copyFdRange
and adds some more sanity checks for offset/length truncation/wrapping at that
API boundary where we work with NAR-style offsets and convert to native off_t.
Instead of mutating the file pointer we can instead safely do
preads. That makes the local-nar-info cache once again thread safe
without the overhead of reopening the file that we used to have prior
to b9b6defca6 which broke the thread safety
by persisting the file descriptor.
We can get rid of `NarMember`, because it is just `NarListing` in
disguise! The use of `std::variant` makes clear that certain stat fields
we don't care about in the non-regular-file case too.
This also adds a utility for opening a file descriptor from a path in readonly mode.
Previous commit helps a bit with error handling, since now we just throw a NativeSysError.
For windows we should live fully in the HANDLE land instead
of converting back-n-forth (which sometimes is destructive).
Using native API is much better for this.
The whole NarAccessor -> listing -> lazy NarAccessor is very weird. Source
can now be seek-ed over when supported, so we can support it pretty easily.
Alternatively we could also make it single-pass very easily with a custom
FileSystemObjectSink. It will get removed in a follow-up commit anyway.
Also do a better JSON and testing for deep and shallow NAR listings.
As documented, this for file system objects themselves, since
`MemorySourceAccessor` is an implementation detail.
`listNar` did the not-so-pretty thing of going straight to JSON. Now it
uses `MemorySourceAccessor::File`, or rather variations of it, to go to
a C++ data type first, and only JSON second.
To accomplish this we add some type parameters to the `File` data type.
Actually, we need to do two rounds of this, because shallow NAR
listings. There is `FileT` and `DirectoryT` accordingly.
