Re: [PATCH] copy.c: make copy_fd preserve meaningful errno

> Do any callers care about errno?  Does the function's API
> documentation say it will make errno meaningful on error, so people
> making changes to copy_fd in the future know to maintain that
> property?
>
> *searches*
>
> Looks like callers are:
>
>  convert.c::filter_buffer_or_fd, which doesn't care
>
>  copy.c::copy_file, which also doesn't care
>
>  lockfile.c::hold_lock_file_for_append, which started caring
>  in order to propagate errno in v2.2.0-rc0~53^2~2 (restore errno
>  before returning, 2014-10-01).  But no callers of that function
>  care yet.
>
> So this is about fixing a bug-waiting-to-happen in
> hold_lock_file_for_append.  That would be enough to motivate the
> change.

>> +			int save_errno = errno;
>> +			error("copy-fd: read returned %s", strerror(errno));
>> +			errno = save_errno;
>> +			return -1;
>
> Any caller is presumably going to turn around and print strerror(errno)
> again, producing repetitive output.
>
> Can we do better?  E.g., if the signature were
>
> 	int copy_fd(int ifd, int ofd, struct strbuf *err);
>
> then we could write the error message to the err strbuf for the
> caller to print.