* remove protoc and token from network tests github action
* delete unused beacon chain methods
* downgrade writing blobs to store log
* reduce diff in block import logic
* remove some todo's and deneb built in network
* remove unnecessary error, actually use some added metrics
* remove some metrics, fix missing components on publish funcitonality
* fix status tests
* rename sidecar by root to blobs by root
* clean up some metrics
* remove unnecessary feature gate from attestation subnet tests, clean up blobs by range response code
* pawan's suggestion in `protocol_info`, peer score in matching up batch sync block and blobs
* fix range tests for deneb
* pub block and blob db cache behind the same mutex
* remove unused errs and an empty file
* move sidecar trait to new file
* move types from payload to eth2 crate
* update comment and add flag value name
* make function private again, remove allow unused
* use reth rlp for tx decoding
* fix compile after merge
* rename kzg commitments
* cargo fmt
* remove unused dep
* Update beacon_node/execution_layer/src/lib.rs
Co-authored-by: Pawan Dhananjay <pawandhananjay@gmail.com>
* Update beacon_node/beacon_processor/src/lib.rs
Co-authored-by: Pawan Dhananjay <pawandhananjay@gmail.com>
* pawan's suggestiong for vec capacity
* cargo fmt
* Revert "use reth rlp for tx decoding"
This reverts commit 5181837d81c66dcca4c960a85989ac30c7f806e2.
* remove reth rlp
---------
Co-authored-by: Pawan Dhananjay <pawandhananjay@gmail.com>
## Issue Addressed
Closes#4245
## Proposed Changes
- If an SSE channel fills up, send a comment instead of terminating the stream.
- Add a CLI flag for scaling up the SSE buffer: `--http-sse-capacity-multiplier N`.
## Additional Info
~~Blocked on #4462. I haven't rebased on that PR yet for initial testing, because it still needs some more work to handle long-running HTTP threads.~~
- [x] Add CLI flag tests.
## Issue Addressed
NA
## Proposed Changes
Rather than spawning new tasks on the tokio executor to process each HTTP API request, send the tasks to the `BeaconProcessor`. This achieves:
1. Places a bound on how many concurrent requests are being served (i.e., how many we are actually trying to compute at one time).
1. Places a bound on how many requests can be awaiting a response at one time (i.e., starts dropping requests when we have too many queued).
1. Allows the BN prioritise HTTP requests with respect to messages coming from the P2P network (i.e., proiritise importing gossip blocks rather than serving API requests).
Presently there are two levels of priorities:
- `Priority::P0`
- The beacon processor will prioritise these above everything other than importing new blocks.
- Roughly all validator-sensitive endpoints.
- `Priority::P1`
- The beacon processor will prioritise practically all other P2P messages over these, except for historical backfill things.
- Everything that's not `Priority::P0`
The `--http-enable-beacon-processor false` flag can be supplied to revert back to the old behaviour of spawning new `tokio` tasks for each request:
```
--http-enable-beacon-processor <BOOLEAN>
The beacon processor is a scheduler which provides quality-of-service and DoS protection. When set to
"true", HTTP API requests will queued and scheduled alongside other tasks. When set to "false", HTTP API
responses will be executed immediately. [default: true]
```
## New CLI Flags
I added some other new CLI flags:
```
--beacon-processor-aggregate-batch-size <INTEGER>
Specifies the number of gossip aggregate attestations in a signature verification batch. Higher values may
reduce CPU usage in a healthy network while lower values may increase CPU usage in an unhealthy or hostile
network. [default: 64]
--beacon-processor-attestation-batch-size <INTEGER>
Specifies the number of gossip attestations in a signature verification batch. Higher values may reduce CPU
usage in a healthy network whilst lower values may increase CPU usage in an unhealthy or hostile network.
[default: 64]
--beacon-processor-max-workers <INTEGER>
Specifies the maximum concurrent tasks for the task scheduler. Increasing this value may increase resource
consumption. Reducing the value may result in decreased resource usage and diminished performance. The
default value is the number of logical CPU cores on the host.
--beacon-processor-reprocess-queue-len <INTEGER>
Specifies the length of the queue for messages requiring delayed processing. Higher values may prevent
messages from being dropped while lower values may help protect the node from becoming overwhelmed.
[default: 12288]
```
I needed to add the max-workers flag since the "simulator" flavor tests started failing with HTTP timeouts on the test assertions. I believe they were failing because the Github runners only have 2 cores and there just weren't enough workers available to process our requests in time. I added the other flags since they seem fun to fiddle with.
## Additional Info
I bumped the timeouts on the "simulator" flavor test from 4s to 8s. The prioritisation of consensus messages seems to be causing slower responses, I guess this is what we signed up for 🤷
The `validator/register` validator has some special handling because the relays have a bad habit of timing out on these calls. It seems like a waste of a `BeaconProcessor` worker to just wait for the builder API HTTP response, so we spawn a new `tokio` task to wait for a builder response.
I've added an optimisation for the `GET beacon/states/{state_id}/validators/{validator_id}` endpoint in [efbabe3](efbabe3252). That's the endpoint the VC uses to resolve pubkeys to validator indices, and it's the endpoint that was causing us grief. Perhaps I should move that into a new PR, not sure.
## Issue Addressed
Addresses [#4401](https://github.com/sigp/lighthouse/issues/4401)
## Proposed Changes
Shift some constants into ```ChainSpec``` and remove the constant values from code space.
## Additional Info
I mostly used ```MainnetEthSpec::default_spec()``` for getting ```ChainSpec```. I wonder Did I make a mistake about that.
Co-authored-by: armaganyildirak <armaganyildirak@gmail.com>
Co-authored-by: Paul Hauner <paul@paulhauner.com>
Co-authored-by: Age Manning <Age@AgeManning.com>
Co-authored-by: Diva M <divma@protonmail.com>
## Issue Addressed
Fix an issue observed by `@zlan` on Discord where Lighthouse would sometimes return this error when looking up states via the API:
> {"code":500,"message":"UNHANDLED_ERROR: ForkChoiceError(MissingProtoArrayBlock(0xc9cf1495421b6ef3215d82253b388d77321176a1dcef0db0e71a0cd0ffc8cdb7))","stacktraces":[]}
## Proposed Changes
The error stems from a faulty assumption in the HTTP API logic: that any state in the hot database must have its block in fork choice. This isn't true because the state's hot database may update much less frequently than the fork choice store, e.g. if reconstructing states (where freezer migration pauses), or if the freezer migration runs slowly. There could also be a race between loading the hot state and checking fork choice, e.g. even if the finalization migration of DB+fork choice were atomic, the update could happen between the 1st and 2nd calls.
To address this I've changed the HTTP API logic to use the finalized block's execution status as a fallback where it is safe to do so. In the case where a block is non-canonical and prior to finalization (permanently orphaned) we default `execution_optimistic` to `true`.
## Additional Info
I've also added a new CLI flag to reduce the frequency of the finalization migration as this is useful for several purposes:
- Spacing out database writes (less frequent, larger batches)
- Keeping a limited chain history with high availability, e.g. the last month in the hot database.
This new flag made it _substantially_ easier to test this change. It was extracted from `tree-states` (where it's called `--db-migration-period`), which is why this PR also carries the `tree-states` label.
## Issue Addressed
[Users on Twitter](https://twitter.com/ashekhirin/status/1676334843192397824) are getting checkpoint sync URL timeouts with the default of 60s, so this PR increases the default timeout to 3 minutes.
I've also added a short section to the book about adjusting the timeout with `--checkpoint-sync-url-timeout`.
## Issue Addressed
#4118
## Proposed Changes
This PR introduces a "progressive balances" cache on the `BeaconState`, which keeps track of the accumulated target attestation balance for the current & previous epochs. The cached values are utilised by fork choice to calculate unrealized justification and finalization (instead of converting epoch participation arrays to balances for each block we receive).
This optimization will be rolled out gradually to allow for more testing. A new `--progressive-balances disabled|checked|strict|fast` flag is introduced to support this:
- `checked`: enabled with checks against participation cache, and falls back to the existing epoch processing calculation if there is a total target attester balance mismatch. There is no performance gain from this as the participation cache still needs to be computed. **This is the default mode for now.**
- `strict`: enabled with checks against participation cache, returns error if there is a mismatch. **Used for testing only**.
- `fast`: enabled with no comparative checks and without computing the participation cache. This mode gives us the performance gains from the optimization. This is still experimental and not currently recommended for production usage, but will become the default mode in a future release.
- `disabled`: disable the usage of progressive cache, and use the existing method for FFG progression calculation. This mode may be useful if we find a bug and want to stop the frequent error logs.
### Tasks
- [x] Initial cache implementation in `BeaconState`
- [x] Perform checks in fork choice to compare the progressive balances cache against results from `ParticipationCache`
- [x] Add CLI flag, and disable the optimization by default
- [x] Testing on Goerli & Benchmarking
- [x] Move caching logic from state processing to the `ProgressiveBalancesCache` (see [this comment](https://github.com/sigp/lighthouse/pull/4362#discussion_r1230877001))
- [x] Add attesting balance metrics
Co-authored-by: Jimmy Chen <jimmy@sigmaprime.io>
## Proposed Changes
Remove `max-skip-slots` checks when processing blocks.
This was legacy code which was previously used in the Medalla testnet to sync to the correct fork.
With the addition of checkpoint sync which allows us to sync to any arbitrary fork, this is no longer a necessary feature, so it has been removed for simplicity.
## Additional Notes
The CLI flag and checks for attestation processing have been retained as it still may have uses in DoS protection.
Done in different PRs so that they can reviewed independently, as it's likely this won't be merged before I leave
Includes resolution for #4080
- [ ] #4299
- [ ] #4318
- [ ] #4320
Co-authored-by: Diva M <divma@protonmail.com>
Co-authored-by: Age Manning <Age@AgeManning.com>
## Issue Addressed
This PR addresses issue https://github.com/sigp/lighthouse/issues/4350
## Proposed Changes
This change will enable slasher broadcast in the following cases:
No flag is passed,
`--slasher-broadcast` is passed and,
`--slasher-broadcast=true` is passed.
Only when an explicit false value is passed the slasher does not broadcast.(`--slasher-broadcast=false`).
## Additional Info
TODO
- [x] Modify CLI parsing logic
- [x] Write test
Refer to #4353
Co-authored-by: Rahul Dogra <rahulcooldogra@gmail.com>
Co-authored-by: Gua00va <105484243+Gua00va@users.noreply.github.com>
## Issue Addressed
Closes#4354Closes#3987
Replaces #4305, #4283
## Proposed Changes
This switches the default slasher backend _back_ to LMDB.
If an MDBX database exists and the MDBX backend is enabled then MDBX will continue to be used. Our release binaries and Docker images will continue to include MDBX for as long as it is practical, so users of these should not notice any difference.
The main benefit is to users compiling from source and devs running tests. These users no longer have to struggle to compile MDBX and deal with the compatibility issues that arises. Similarly, devs don't need to worry about toggling feature flags in tests or risk forgetting to run the slasher tests due to backend issues.
## Issue Addressed
On deneb devnetv5, lighthouse keeps rate limiting peers which makes it harder to bootstrap new nodes as there are very few peers in the network. This PR adds an option to disable the inbound rate limiter for testnets.
Added an option to configure inbound rate limits as well.
Co-authored-by: Diva M <divma@protonmail.com>
This PR adds the ability to read the Lighthouse logs from the HTTP API for both the BN and the VC.
This is done in such a way to as minimize any kind of performance hit by adding this feature.
The current design creates a tokio broadcast channel and mixes is into a form of slog drain that combines with our main global logger drain, only if the http api is enabled.
The drain gets the logs, checks the log level and drops them if they are below INFO. If they are INFO or higher, it sends them via a broadcast channel only if there are users subscribed to the HTTP API channel. If not, it drops the logs.
If there are more than one subscriber, the channel clones the log records and converts them to json in their independent HTTP API tasks.
Co-authored-by: Michael Sproul <micsproul@gmail.com>
## Issue Addressed
NA
## Proposed Changes
Adds a flag to store invalid blocks on disk for teh debugz. Only *some* invalid blocks are stored, those which:
- Were received via gossip (rather than RPC, for instance)
- This keeps things simple to start with and should capture most blocks.
- Passed gossip verification
- This reduces the ability for random people to fill up our disk. A proposer signature is required to write something to disk.
## Additional Info
It's possible that we'll store blocks that aren't necessarily invalid, but we had an internal error during verification. Those blocks seem like they might be useful sometimes.
## Limit Backfill Sync
This PR transitions Lighthouse from syncing all the way back to genesis to only syncing back to the weak subjectivity point (~ 5 months) when syncing via a checkpoint sync.
There are a number of important points to note with this PR:
- Firstly and most importantly, this PR fundamentally shifts the default security guarantees of checkpoint syncing in Lighthouse. Prior to this PR, Lighthouse could verify the checkpoint of any given chain by ensuring the chain eventually terminates at the corresponding genesis. This guarantee can still be employed via the new CLI flag --genesis-backfill which will prompt lighthouse to the old behaviour of downloading all blocks back to genesis. The new behaviour only checks the proposer signatures for the last 5 months of blocks but cannot guarantee the chain matches the genesis chain.
- I have not modified any of the peer scoring or RPC responses. Clients syncing from gensis, will downscore new Lighthouse peers that do not possess blocks prior to the WSP. This is by design, as Lighthouse nodes of this form, need a mechanism to sort through peers in order to find useful peers in order to complete their genesis sync. We therefore do not discriminate between empty/error responses for blocks prior or post the local WSP. If we request a block that a peer does not posses, then fundamentally that peer is less useful to us than other peers.
- This will make a radical shift in that the majority of nodes will no longer store the full history of the chain. In the future we could add a pruning mechanism to remove old blocks from the db also.
Co-authored-by: Paul Hauner <paul@paulhauner.com>
## Issue Addressed
#3873
## Proposed Changes
add a cache to optimise historical state lookup.
## Additional Info
N/A
Co-authored-by: Michael Sproul <micsproul@gmail.com>
## Issue Addressed
#4150
## Proposed Changes
Maintain trusted peers in the pruning logic. ~~In principle the changes here are not necessary as a trusted peer has a max score (100) and all other peers can have at most 0 (because we don't implement positive scores). This means that we should never prune trusted peers unless we have more trusted peers than the target peer count.~~
This change shifts this logic to explicitly never prune trusted peers which I expect is the intuitive behaviour.
~~I suspect the issue in #4150 arises when a trusted peer disconnects from us for one reason or another and then we remove that peer from our peerdb as it becomes stale. When it re-connects at some large time later, it is no longer a trusted peer.~~
Currently we do disconnect trusted peers, and this PR corrects this to maintain trusted peers in the pruning logic.
As suggested in #4150 we maintain trusted peers in the db and thus we remember them even if they disconnect from us.
It is a well-known fact that IP addresses for beacon nodes used by specific validators can be de-anonymized. There is an assumed risk that a malicious user may attempt to DOS validators when producing blocks to prevent chain growth/liveness.
Although there are a number of ideas put forward to address this, there a few simple approaches we can take to mitigate this risk.
Currently, a Lighthouse user is able to set a number of beacon-nodes that their validator client can connect to. If one beacon node is taken offline, it can fallback to another. Different beacon nodes can use VPNs or rotate IPs in order to mask their IPs.
This PR provides an additional setup option which further mitigates attacks of this kind.
This PR introduces a CLI flag --proposer-only to the beacon node. Setting this flag will configure the beacon node to run with minimal peers and crucially will not subscribe to subnets or sync committees. Therefore nodes of this kind should not be identified as nodes connected to validators of any kind.
It also introduces a CLI flag --proposer-nodes to the validator client. Users can then provide a number of beacon nodes (which may or may not run the --proposer-only flag) that the Validator client will use for block production and propagation only. If these nodes fail, the validator client will fallback to the default list of beacon nodes.
Users are then able to set up a number of beacon nodes dedicated to block proposals (which are unlikely to be identified as validator nodes) and point their validator clients to produce blocks on these nodes and attest on other beacon nodes. An attack attempting to prevent liveness on the eth2 network would then need to preemptively find and attack the proposer nodes which is significantly more difficult than the default setup.
This is a follow on from: #3328
Co-authored-by: Michael Sproul <michael@sigmaprime.io>
Co-authored-by: Paul Hauner <paul@paulhauner.com>
## Issue Addressed
Closes#4185
## Proposed Changes
- Set user agent to `Lighthouse/vX.Y.Z-<commit hash>` by default
- Allow tweaking user agent via `--builder-user-agent "agent"`
## Proposed Changes
This change attempts to prevent failed re-orgs by:
1. Lowering the re-org cutoff from 2s to 1s. This is informed by a failed re-org attempted by @yorickdowne's node. The failed block was requested in the 1.5-2s window due to a Vouch failure, and failed to propagate to the majority of the network before the attestation deadline at 4s.
2. Allow users to adjust their re-org cutoff depending on observed network conditions and their risk profile. The static 2 second cutoff was too rigid.
3. Add a `--proposer-reorg-disallowed-offsets` flag which can be used to prohibit reorgs at certain slots. This is intended to help workaround an issue whereby reorging blocks at slot 1 are currently taking ~1.6s to propagate on gossip rather than ~500ms. This is suspected to be due to a cache miss in current versions of Prysm, which should be fixed in their next release.
## Additional Info
I'm of two minds about removing the `shuffling_stable` check which checks for blocks at slot 0 in the epoch. If we removed it users would be able to configure Lighthouse to try reorging at slot 0, which likely wouldn't work very well due to interactions with the proposer index cache. I think we could leave it for now and revisit it later.
## Issue Addressed
N/A
## Proposed Changes
Adds a flag for disabling peer scoring. This is useful for local testing and testing small networks for new features.
## Issue Addressed
#3212
## Proposed Changes
- Introduce a new `rate_limiting_backfill_queue` - any new inbound backfill work events gets immediately sent to this FIFO queue **without any processing**
- Spawn a `backfill_scheduler` routine that pops a backfill event from the FIFO queue at specified intervals (currently halfway through a slot, or at 6s after slot start for 12s slots) and sends the event to `BeaconProcessor` via a `scheduled_backfill_work_tx` channel
- This channel gets polled last in the `InboundEvents`, and work event received is wrapped in a `InboundEvent::ScheduledBackfillWork` enum variant, which gets processed immediately or queued by the `BeaconProcessor` (existing logic applies from here)
Diagram comparing backfill processing with / without rate-limiting:
https://github.com/sigp/lighthouse/issues/3212#issuecomment-1386249922
See this comment for @paulhauner's explanation and solution: https://github.com/sigp/lighthouse/issues/3212#issuecomment-1384674956
## Additional Info
I've compared this branch (with backfill processing rate limited to to 1 and 3 batches per slot) against the latest stable version. The CPU usage during backfill sync is reduced by ~5% - 20%, more details on this page:
https://hackmd.io/@jimmygchen/SJuVpJL3j
The above testing is done on Goerli (as I don't currently have hardware for Mainnet), I'm guessing the differences are likely to be bigger on mainnet due to block size.
### TODOs
- [x] Experiment with processing multiple batches per slot. (need to think about how to do this for different slot durations)
- [x] Add option to disable rate-limiting, enabed by default.
- [x] (No longer required now we're reusing the reprocessing queue) Complete the `backfill_scheduler` task when backfill sync is completed or not required
## Issue Addressed
NA
## Proposed Changes
- Implements https://github.com/ethereum/consensus-specs/pull/3290/
- Bumps `ef-tests` to [v1.3.0-rc.4](https://github.com/ethereum/consensus-spec-tests/releases/tag/v1.3.0-rc.4).
The `CountRealizedFull` concept has been removed and the `--count-unrealized-full` and `--count-unrealized` BN flags now do nothing but log a `WARN` when used.
## Database Migration Debt
This PR removes the `best_justified_checkpoint` from fork choice. This field is persisted on-disk and the correct way to go about this would be to make a DB migration to remove the field. However, in this PR I've simply stubbed out the value with a junk value. I've taken this approach because if we're going to do a DB migration I'd love to remove the `Option`s around the justified and finalized checkpoints on `ProtoNode` whilst we're at it. Those options were added in #2822 which was included in Lighthouse v2.1.0. The options were only put there to handle the migration and they've been set to `Some` ever since v2.1.0. There's no reason to keep them as options anymore.
I started adding the DB migration to this branch but I started to feel like I was bloating this rather critical PR with nice-to-haves. I've kept the partially-complete migration [over in my repo](https://github.com/paulhauner/lighthouse/tree/fc-pr-18-migration) so we can pick it up after this PR is merged.
This PR enables the user to adjust the shuffling cache size.
This is useful for some HTTP API requests which require re-computing old shufflings. This PR currently optimizes the
beacon/states/{state_id}/committees HTTP API by first checking the cache before re-building shuffling.
If the shuffling is set to a non-default value, then the HTTP API request will also fill the cache when as it constructs new shufflings.
If the CLI flag is not present or the value is set to the default of 16 the default behaviour is observed.
Co-authored-by: Michael Sproul <michael@sigmaprime.io>
## Issue Addressed
Add support for ipv6 and dual stack in lighthouse.
## Proposed Changes
From an user perspective, now setting an ipv6 address, optionally configuring the ports should feel exactly the same as using an ipv4 address. If listening over both ipv4 and ipv6 then the user needs to:
- use the `--listen-address` two times (ipv4 and ipv6 addresses)
- `--port6` becomes then required
- `--discovery-port6` can now be used to additionally configure the ipv6 udp port
### Rough list of code changes
- Discovery:
- Table filter and ip mode set to match the listening config.
- Ipv6 address, tcp port and udp port set in the ENR builder
- Reported addresses now check which tcp port to give to libp2p
- LH Network Service:
- Can listen over Ipv6, Ipv4, or both. This uses two sockets. Using mapped addresses is disabled from libp2p and it's the most compatible option.
- NetworkGlobals:
- No longer stores udp port since was not used at all. Instead, stores the Ipv4 and Ipv6 TCP ports.
- NetworkConfig:
- Update names to make it clear that previous udp and tcp ports in ENR were Ipv4
- Add fields to configure Ipv6 udp and tcp ports in the ENR
- Include advertised enr Ipv6 address.
- Add type to model Listening address that's either Ipv4, Ipv6 or both. A listening address includes the ip, udp port and tcp port.
- UPnP:
- Kept only for ipv4
- Cli flags:
- `--listen-addresses` now can take up to two values
- `--port` will apply to ipv4 or ipv6 if only one listening address is given. If two listening addresses are given it will apply only to Ipv4.
- `--port6` New flag required when listening over ipv4 and ipv6 that applies exclusively to Ipv6.
- `--discovery-port` will now apply to ipv4 and ipv6 if only one listening address is given.
- `--discovery-port6` New flag to configure the individual udp port of ipv6 if listening over both ipv4 and ipv6.
- `--enr-udp-port` Updated docs to specify that it only applies to ipv4. This is an old behaviour.
- `--enr-udp6-port` Added to configure the enr udp6 field.
- `--enr-tcp-port` Updated docs to specify that it only applies to ipv4. This is an old behaviour.
- `--enr-tcp6-port` Added to configure the enr tcp6 field.
- `--enr-addresses` now can take two values.
- `--enr-match` updated behaviour.
- Common:
- rename `unused_port` functions to specify that they are over ipv4.
- add functions to get unused ports over ipv6.
- Testing binaries
- Updated code to reflect network config changes and unused_port changes.
## Additional Info
TODOs:
- use two sockets in discovery. I'll get back to this and it's on https://github.com/sigp/discv5/pull/160
- lcli allow listening over two sockets in generate_bootnodes_enr
- add at least one smoke flag for ipv6 (I have tested this and works for me)
- update the book
## Issue Addressed
#4040
## Proposed Changes
- Add the `always_prefer_builder_payload` field to `Config` in `beacon_node/client/src/config.rs`.
- Add that same field to `Inner` in `beacon_node/execution_layer/src/lib.rs`
- Modify the logic for picking the payload in `beacon_node/execution_layer/src/lib.rs`
- Add the `always-prefer-builder-payload` flag to the beacon node CLI
- Test the new flags in `lighthouse/tests/beacon_node.rs`
Co-authored-by: Paul Hauner <paul@paulhauner.com>
## Issue Addressed
Closes#3896Closes#3998Closes#3700
## Proposed Changes
- Optimise the calculation of withdrawals for payload attributes by avoiding state clones, avoiding unnecessary state advances and reading from the snapshot cache if possible.
- Use the execution layer's payload attributes cache to avoid re-calculating payload attributes. I actually implemented a new LRU cache just for withdrawals but it had the exact same key and most of the same data as the existing payload attributes cache, so I deleted it.
- Add a new SSE event that fires when payloadAttributes are calculated. This is useful for block builders, a la https://github.com/ethereum/beacon-APIs/issues/244.
- Add a new CLI flag `--always-prepare-payload` which forces payload attributes to be sent with every fcU regardless of connected proposers. This is intended for use by builders/relays.
For maximum effect, the flags I've been using to run Lighthouse in "payload builder mode" are:
```
--always-prepare-payload \
--prepare-payload-lookahead 12000 \
--suggested-fee-recipient 0x0000000000000000000000000000000000000000
```
The fee recipient is required so Lighthouse has something to pack in the payload attributes (it can be ignored by the builder). The lookahead causes fcU to be sent at the start of every slot rather than at 8s. As usual, fcU will also be sent after each change of head block. I think this combination is sufficient for builders to build on all viable heads. Often there will be two fcU (and two payload attributes) sent for the same slot: one sent at the start of the slot with the head from `n - 1` as the parent, and one sent after the block arrives with `n` as the parent.
Example usage of the new event stream:
```bash
curl -N "http://localhost:5052/eth/v1/events?topics=payload_attributes"
```
## Additional Info
- [x] Tests added by updating the proposer re-org tests. This has the benefit of testing the proposer re-org code paths with withdrawals too, confirming that the new changes don't interact poorly.
- [ ] Benchmarking with `blockdreamer` on devnet-7 showed promising results but I'm yet to do a comparison to `unstable`.
Co-authored-by: Michael Sproul <micsproul@gmail.com>
