read_storage panics

[pepyakin]

There is a problem with read_storage, if an offset passed that is out of bounds of the length of the value the implementation of an extrinsic will panic. This PR solves it.

There is another problem with the API itself: there is no way to indicate how many bytes were read. So even if we fix the panic, the API wouldn't be universally usable since the API cannot indicate how many bytes it wrote to the buffer.

The API is not used within Substrate, except for implementing exists (e.g. this).

[gavofyork]

alternative here: #3597 which i prefer since it just forces a cap on the value rather than introducing a full conditional.

the API cannot indicate how many bytes it wrote to the buffer

let offset = /* whatever offset into the value we want to read from */;
let mut out = [0u8; /* whatever the buffer size is */]; 
if let Some(value_len) = read_storage(key, &mut out, offset) {
    // offset will be truncated to the value's size.
    let offset = offset.min(value_len);
    // number of bytes written is the size of the value minus any offset into it that we start at
    // and it will never be more than the size of the output buffer.
    let bytes_written = out.len().min(value_len - offset);
}

[pepyakin]

Ah, but in this case we don't return the original value's length here which panics with the following error

  left: `Some(0)`,
 right: `Some(4)`', /builds/parity/substrate/core/test-runtime/src/lib.rs:919:2

[gavofyork]

should do the trick.