bug#18316: [PATCH] warn on too large file copies

[Paul Eggert]

Adam wrote:
> I believe when copying files as large as these, if the maximum transfer
> were to be reached then the copy should not return false (as this will
> delete the failed destination file), but instead produce a warning and
> return true.

Sorry, I don't see the bug.  max_n_nread is normally UINTMAX_MAX, which 
is effectively infinity, so it shouldn't be an issue then.  When 
max_n_read is less than UINTMAX_MAX, we're copying one extent of a 
sparse file, and it's not an error to read less than the extent we 
expect to find, as that can happen when the file changed as we read it.

Perhaps cp should optionally warn if the input file changes while we 
read it, but that's a bigger change and one that should be done elsewhere.