Copying multiple large files results in `assertion failed: (left == right)` in kernel_copy.rs:592

[archer884]

I tried this code:

fn copy_file(path: &Path, target: &str) -> io::Result<()> {
    let mut reader = File::open(path)?;
    let mut writer = File::create(make_target_path(path, target))?;

    io::copy(&mut reader, &mut writer)?;

    Ok(())
}

I expected the copy operation to succeed (that is, to return Ok(())) or to fail (that is, to return Err(BUY A BETTER HARD DRIVE!). What happens instead is a panic thrown from line 592 of kernel_copy.rs. Before I changed from the default mount options to something else, this happened pretty much every time I tried to copy multiple large files from a Linux laptop to a Linux server on my network.

The server in question has a BTRFS volume. That volume was mounted on the laptop by the Gnome shell--er, that is, just whatever Gnome does to mount shares it finds on the network. Because it gets mounted at something obscene, like /this/path/takes/forever/to/type1000/100/volumename, I added a symlink to allow access via ~/share in my home directory. (I only mention this because I already discovered that a symlink renders something like fs::copy unworkable, so.... /shrug)

Anyway, looks like the code in kernel_copy 1) encounters an error, 2) attempts to fall back to user space copy instead of using the fancy kernel copy routine, and 3) panics upon realizing that some data has already been written. I reckon there's some kind of reason for it, but I was still surprised that this is a panic instead of just, "Whoops. We encountered an error."

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rustc --version --verbose:

rustc 1.56.1 (59eed8a2a 2021-11-01)
binary: rustc
commit-hash: 59eed8a2aac0230a8b53e89d4e99d55912ba6b35
commit-date: 2021-11-01
host: x86_64-unknown-linux-gnu
release: 1.56.1
LLVM version: 13.0.0