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Implement SSZ union type (#2579)

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## Issue Addressed

NA

## Proposed Changes

Implements the "union" type from the SSZ spec for `ssz`, `ssz_derive`, `tree_hash` and `tree_hash_derive` so it may be derived for `enums`:

https://github.com/ethereum/consensus-specs/blob/v1.1.0-beta.3/ssz/simple-serialize.md#union

The union type is required for the merge, since the `Transaction` type is defined as a single-variant union `Union[OpaqueTransaction]`.

### Crate Updates

This PR will (hopefully) cause CI to publish new versions for the following crates:

- `eth2_ssz_derive`: `0.2.1` -> `0.3.0`
- `eth2_ssz`: `0.3.0` -> `0.4.0`
- `eth2_ssz_types`: `0.2.0` -> `0.2.1`
- `tree_hash`: `0.3.0` -> `0.4.0`
- `tree_hash_derive`: `0.3.0` -> `0.4.0`

These these crates depend on each other, I've had to add a workspace-level `[patch]` for these crates. A follow-up PR will need to remove this patch, ones the new versions are published.

### Union Behaviors

We already had SSZ `Encode` and `TreeHash` derive for enums, however it just did a "transparent" pass-through of the inner value. Since the "union" decoding from the spec is in conflict with the transparent method, I've required that all `enum` have exactly one of the following enum-level attributes:

#### SSZ

-  `#[ssz(enum_behaviour = "union")]`
    - matches the spec used for the merge
-  `#[ssz(enum_behaviour = "transparent")]`
    - maintains existing functionality
    - not supported for `Decode` (never was)
    
#### TreeHash

-  `#[tree_hash(enum_behaviour = "union")]`
    - matches the spec used for the merge
-  `#[tree_hash(enum_behaviour = "transparent")]`
    - maintains existing functionality

This means that we can maintain the existing transparent behaviour, but all existing users will get a compile-time error until they explicitly opt-in to being transparent.

### Legacy Option Encoding

Before this PR, we already had a union-esque encoding for `Option<T>`. However, this was with the *old* SSZ spec where the union selector was 4 bytes. During merge specification, the spec was changed to use 1 byte for the selector.

Whilst the 4-byte `Option` encoding was never used in the spec, we used it in our database. Writing a migrate script for all occurrences of `Option` in the database would be painful, especially since it's used in the `CommitteeCache`. To avoid the migrate script, I added a serde-esque `#[ssz(with = "module")]` field-level attribute to `ssz_derive` so that we can opt into the 4-byte encoding on a field-by-field basis.

The `ssz::legacy::four_byte_impl!` macro allows a one-liner to define the module required for the `#[ssz(with = "module")]` for some `Option<T> where T: Encode + Decode`.

Notably, **I have removed `Encode` and `Decode` impls for `Option`**. I've done this to force a break on downstream users. Like I mentioned, `Option` isn't used in the spec so I don't think it'll be *that* annoying. I think it's nicer than quietly having two different union implementations or quietly breaking the existing `Option` impl.

### Crate Publish Ordering

I've modified the order in which CI publishes crates to ensure that we don't publish a crate without ensuring we already published a crate that it depends upon.

## TODO

- [ ] Queue a follow-up `[patch]`-removing PR.
This commit is contained in:
Paul Hauner
2021-09-25 05:58:36 +00:00
parent a844ce5ba9
commit fe52322088
63 changed files with 1515 additions and 571 deletions
Download Patch File Download Diff File Expand all files Collapse all files

49
consensus/ssz/src/encode/impls.rs
View File

@@ -202,36 +202,6 @@ impl_encode_for_tuples! {
}
}
/// The SSZ "union" type.
impl<T: Encode> Encode for Option<T> {
fn is_ssz_fixed_len() -> bool {
false
}
fn ssz_bytes_len(&self) -> usize {
if let Some(some) = self {
let len = if <T as Encode>::is_ssz_fixed_len() {
<T as Encode>::ssz_fixed_len()
} else {
some.ssz_bytes_len()
};
len + BYTES_PER_LENGTH_OFFSET
} else {
BYTES_PER_LENGTH_OFFSET
}
}
fn ssz_append(&self, buf: &mut Vec<u8>) {
match self {
None => buf.extend_from_slice(&encode_union_index(0)),
Some(t) => {
buf.extend_from_slice(&encode_union_index(1));
t.ssz_append(buf);
}
}
}
}
impl<T: Encode> Encode for Arc<T> {
fn is_ssz_fixed_len() -> bool {
T::is_ssz_fixed_len()
@@ -456,25 +426,6 @@ mod tests {
);
}
#[test]
fn ssz_encode_option_u16() {
assert_eq!(Some(65535_u16).as_ssz_bytes(), vec![1, 0, 0, 0, 255, 255]);
let none: Option<u16> = None;
assert_eq!(none.as_ssz_bytes(), vec![0, 0, 0, 0]);
}
#[test]
fn ssz_encode_option_vec_u16() {
assert_eq!(
Some(vec![0_u16, 1]).as_ssz_bytes(),
vec![1, 0, 0, 0, 0, 0, 1, 0]
);
let none: Option<Vec<u16>> = None;
assert_eq!(none.as_ssz_bytes(), vec![0, 0, 0, 0]);
}
#[test]
fn ssz_encode_u8() {
assert_eq!(0_u8.as_ssz_bytes(), vec![0]);