Re: [PATCH v2 4/7] avoid looking at errno for short read_in_full() returns

> When a caller tries to read a particular set of bytes via
> read_in_full(), there are three possible outcomes:
>
>   1. An error, in which case -1 is returned and errno is
>      set.
>
>   2. A short read, in which fewer bytes are returned and
>      errno is unspecified (we never saw a read error, so we
>      may have some random value from whatever syscall failed
>      last).
>
>   3. The full read completed successfully.
>
> Many callers handle cases 1 and 2 together by just checking
> the result against the requested size. If their combined
> error path looks at errno (e.g., by calling die_errno), they
> may report a nonsense value.
>
> Let's fix these sites by having them distinguish between the
> two error cases. That avoids the random errno confusion, and
> lets us give more detailed error messages.