ftruncate has unreachable error values that can be reached

[Daimanta]

Zig Version

0.13.0

Steps to Reproduce and Observed Behavior

On Linux:
ftruncate(<my_valid_handle>, <max u64>)

This triggers the .INVAL error has which triggers an unreachable

Expected Behavior

From the manual page on man7.org

`The ftruncate() function shall fail if:

   EINTR  A signal was caught during execution.

   EINVAL The length argument was less than 0.

   EFBIG or EINVAL
          The length argument was greater than the maximum file size.

   EFBIG  The file is a regular file and length is greater than the
          offset maximum established in the open file description
          associated with fildes.

   EIO    An I/O error occurred while reading from or writing to a
          file system.

   EBADF or EINVAL
          The fildes argument is not a file descriptor open for
          writing.

`

As shown, there are three possible cases where an EINVAL is returned. The zig type system prevents negative length(passed type is u64), so we can ignore that. As the ftruncate function indicates // Handle not open for writing at .INVAL I can assume that is not possible. If that is the case, the only way to trigger EINVAL is by a length exceeding maximum file size. Therefore .INVAL should do the following: return error.FileTooBig, mirroring .FBIG behavior.

[alexrp]

In principle you're supposed to be able to know the maximum file size by way of pathconf("/path/to/file", FILESIZEBITS). However, a mount change could obviously result in that value changing between the calls to pathconf and ftruncate, making such a check unreliable, so I think it's indeed fair to consider this an expected error.

[lsculv]

#20524 is a related issue regarding "unexpected" error returns from POSIX functions.