Merge pull request hyperledger#93 from ethereum/safe-head

add safeBlockHash to forkchoiceUpdated

src/engine/specification.md

@@ -142,8 +142,9 @@ This structure contains the attributes required to initiate a payload build proc
 * method: "engine_forkchoiceUpdatedV1"
 * params: 
   1. `headBlockHash`: `DATA`, 32 Bytes - block hash of the head of the canonical chain
-  2. `finalizedBlockHash`: `DATA`, 32 Bytes - block hash of the most recent finalized block
-  3. `payloadAttributesV1`: `Object|null` - instance of [`PayloadAttributesV1`](#PayloadAttributesV1) or `null`
+  2. `safeBlockHash`: `DATA`, 32 Bytes - the "safe" block hash of the canonical chain under certain synchrony and honesty assumptions. This value **MUST** be either equal to or an ancestor of `headBlockHash`
+  3. `finalizedBlockHash`: `DATA`, 32 Bytes - block hash of the most recent finalized block
+  4. `payloadAttributes`: `Object|null` - instance of [`PayloadAttributesV1`](#PayloadAttributesV1) or `null`
 
 #### Response
 
@@ -152,18 +153,20 @@ This structure contains the attributes required to initiate a payload build proc
 
 #### Specification
 
-1. This method call maps on the `POS_FORKCHOICE_UPDATED` event of [EIP-3675](https://eips.ethereum.org/EIPS/eip-3675#specification) and **MUST** be processed according to the specification defined in the EIP.
+1. The values `(headBlockHash, finalizedBlockHash)` of this method call map on the `POS_FORKCHOICE_UPDATED` event of [EIP-3675](https://eips.ethereum.org/EIPS/eip-3675#specification) and **MUST** be processed according to the specification defined in the EIP.
 
-2. Client software **MUST** return `SYNCING` status if the payload identified by either the `headBlockHash` or the `finalizedBlockHash` is unknown or if the sync process is in progress. In the event that either the `headBlockHash` or the `finalizedBlockHash` is unknown, the client software **SHOULD** initiate the sync process.
+2. All updates to the forkchoice resulting from this call **MUST** be made atomically.
 
-3. Client software **MUST** begin a payload build process building on top of `headBlockHash` if `payloadAttributes` is not `null` and the client is not `SYNCING`. The build process is specified as:
+3. Client software **MUST** return `SYNCING` status if the payload identified by either the `headBlockHash` or the `finalizedBlockHash` is unknown or if the sync process is in progress. In the event that either the `headBlockHash` or the `finalizedBlockHash` is unknown, the client software **SHOULD** initiate the sync process.
+
+4. Client software **MUST** begin a payload build process building on top of `headBlockHash` if `payloadAttributes` is not `null` and the client is not `SYNCING`. The build process is specified as:
   * The payload build process **MUST** be identifid via `payloadId` where `payloadId` is defined as the first `8` bytes of the `sha256` hash of concatenation of `headBlockHash`, `payloadAttributes.timestamp`, `payloadAttributes.random`, and `payloadAttributes.feeRecipient` where `payloadAttributes.timestamp` is encoded as big-endian and padded fully to 8 bytes -- i.e. `sha256(headBlockHash + timestamp + random + feeRecipient)[0:8]`.
   * Client software **MUST** set the payload field values according to the set of parameters passed into this method with exception of the `feeRecipient`. The prepared `ExecutionPayload` **MAY** deviate the `coinbase` field value from what is specified by the `feeRecipient` parameter.
   * Client software **SHOULD** build the initial version of the payload which has an empty transaction set.
   * Client software **SHOULD** start the process of updating the payload. The strategy of this process is implementation dependent. The default strategy is to keep the transaction set up-to-date with the state of local mempool.
   * Client software **SHOULD** stop the updating process when either a call to `engine_getPayload` with the build process's `payloadId` is made or [`SECONDS_PER_SLOT`](https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/beacon-chain.md#time-parameters-1) (currently set to 12 in the Mainnet configuration) seconds have passed since the point in time identified by the `timestamp` parameter.
 
-4. If any of the above fails due to errors unrelated to the client software's normal `SYNCING` status, the client software **MUST** return an error.
+5. If any of the above fails due to errors unrelated to the client software's normal `SYNCING` status, the client software **MUST** return an error.
 
 ### engine_getPayloadV1