Paul Hauner fe52322088 Implement SSZ union type (#2579) ... ## 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.		2021-09-25 05:58:36 +00:00
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src	Implement SSZ union type (#2579)	2021-09-25 05:58:36 +00:00
Cargo.toml	Implement SSZ union type (#2579)	2021-09-25 05:58:36 +00:00