Hi,

Thanks for the bug report and nice reproducer!

Ricardo Wurmus <rekado@elephly.net> skribis:

> The guix-daemon's libutil/util.cc uses copy_file_range to copy a
> downloaded file into the store.  copy_file_range fails on files larger
> than 4GB with an error like this:
>
>     guix build: error: short write in copy_file_range `15' to `16': No such file or directory
>
> The man page for copy_file_range says that it could return EFBIG when
> the range exceeds the maximum range.  The daemon code does not check any
> limits and will attempt to copy the whole file.
>
> I believe our code ought to check the value of st.size and fall back to
> a boring copy if it exceeds some "reasonable" value.

The goal leading to this error message looks like this:

  copy_file_range(15, NULL, 16, NULL, 4294967297, 0) = 2147479552

… which is precisely 2 GiB - 4 KiB.

Reading the man page, it’s entirely fine: like ‘write’,
‘copy_file_range’ might copy less than asked for, so it’s really a
mistake of mine to assume that short writes can’t happen.  Presumably
there’s an internal limit here we’re reaching that explains why it won’t
copy more than 2 GiB at once.

With the following change, we get:

  newfstatat(15, "", {st_mode=S_IFREG|0644, st_size=4294967297, ...}, AT_EMPTY_PATH) = 0
  copy_file_range(15, NULL, 16, NULL, 4294967297, 0) = 2147479552
  copy_file_range(15, NULL, 16, NULL, 2147487745, 0) = 2147479552
  copy_file_range(15, NULL, 16, NULL, 8193, 0) = 8193
  fchown(16, 30001, 30000)          = 0

Could you confirm that it works for you?

Thanks,
Ludo’.