Support more file operations on stdin/stdout/stderr

[RalfJung]

Thanks to recent work by @samrat, the standard FDs 0-2 (stdin/stdout/stderr, but I never know in which order) are now sharing the code with the FDs from opened files. However, for some operations we fall back to working on FileHandle directly.

It should be possible to stop doing that, and support all operations also on FDs 0-2. In particular, I think it would be great to support "dup" to let a program put some file as stdout, or stdin.

In the end, all uses of as_file_handle should be gone, and everything work through the trait:

• "dup"
• "fullfsync", "fsync", "fdatasync", "sync_file_range" (probably NOPs on stdin and errors for the others, I don't think we can flush in a cross-platform way)
• "close"
• "ftruncate64"... well this probably can just error
• Move FileMetadata::from_fd to the trait as well

[RalfJung]

@samrat if you are looking for more ways to contribute, this seems like an obvious next step. :)

[samrat]

@RalfJung Yes! I will take this up :)

[RalfJung]

Awesome. :) As usual, let us know if you need any help or have any questions.

[RalfJung]

My thinking is that we should move all operations to the trait so that FileHandle becomes a proper abstraction that is not used outside the trait. But do y'all think that this is worth it?
Cc @oli-obk

[oli-obk]

Hmm... "worth it" sounds like there are downsides to this? The complexitiy in #1511 seems ok to me

[RalfJung]

Well, the alternative is to keep using as_file_handle as we do now, which has the advantage of already being implemented. ;)

We have more operations that the std* handles do not support than I anticipated. I am not sure whether it is easier to just have a single method in the trait to reflect this (as_file_handle, with the actual handling code then being at the respective shim) or one method per operation (with the handling code being separated from the code to glue this to the rest of Miri).