Configure the validator/register_validator batch size via the CLI (#4399)

## Issue Addressed

NA

## Proposed Changes

Adds the `--validator-registration-batch-size` flag to the VC to allow runtime configuration of the number of validators POSTed to the [`validator/register_validator`](https://ethereum.github.io/beacon-APIs/?urls.primaryName=dev#/Validator/registerValidator) endpoint.

There are builders (Agnostic and Eden) that are timing out with `regsiterValidator` requests with ~400 validators, even with a 9 second timeout. Exposing the batch size will help tune batch sizes to (hopefully) avoid this.

This PR should not change the behavior of Lighthouse when the new flag is not provided (i.e., the same default value is used).

## Additional Info

NA

lighthouse/tests/validator_client.rs

@@ -499,3 +499,24 @@ fn latency_measurement_service() {
             assert!(!config.enable_latency_measurement_service);
         });
 }
+
+#[test]
+fn validator_registration_batch_size() {
+    CommandLineTest::new().run().with_config(|config| {
+        assert_eq!(config.validator_registration_batch_size, 500);
+    });
+    CommandLineTest::new()
+        .flag("validator-registration-batch-size", Some("100"))
+        .run()
+        .with_config(|config| {
+            assert_eq!(config.validator_registration_batch_size, 100);
+        });
+}
+
+#[test]
+#[should_panic]
+fn validator_registration_batch_size_zero_value() {
+    CommandLineTest::new()
+        .flag("validator-registration-batch-size", Some("0"))
+        .run();
+}

validator_client/src/cli.rs

@@ -333,6 +333,16 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .default_value("true")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("validator-registration-batch-size")
+                .long("validator-registration-batch-size")
+                .value_name("INTEGER")
+                .help("Defines the number of validators per \
+                    validator/register_validator request sent to the BN. This value \
+                    can be reduced to avoid timeouts from builders.")
+                .default_value("500")
+                .takes_value(true),
+        )
         /*
          * Experimental/development options.
          */

validator_client/src/config.rs

@@ -77,6 +77,8 @@ pub struct Config {
     pub disable_run_on_all: bool,
     /// Enables a service which attempts to measure latency between the VC and BNs.
     pub enable_latency_measurement_service: bool,
+    /// Defines the number of validators per `validator/register_validator` request sent to the BN.
+    pub validator_registration_batch_size: usize,
 }
 
 impl Default for Config {
@@ -117,6 +119,7 @@ impl Default for Config {
             gas_limit: None,
             disable_run_on_all: false,
             enable_latency_measurement_service: true,
+            validator_registration_batch_size: 500,
         }
     }
 }
@@ -380,6 +383,12 @@ impl Config {
         config.enable_latency_measurement_service =
             parse_optional(cli_args, "latency-measurement-service")?.unwrap_or(true);
 
+        config.validator_registration_batch_size =
+            parse_required(cli_args, "validator-registration-batch-size")?;
+        if config.validator_registration_batch_size == 0 {
+            return Err("validator-registration-batch-size cannot be 0".to_string());
+        }
+
         /*
          * Experimental
          */

validator_client/src/lib.rs

@@ -487,6 +487,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
             .beacon_nodes(beacon_nodes.clone())
             .runtime_context(context.service_context("preparation".into()))
             .builder_registration_timestamp_override(config.builder_registration_timestamp_override)
+            .validator_registration_batch_size(config.validator_registration_batch_size)
             .build()?;
 
         let sync_committee_service = SyncCommitteeService::new(

validator_client/src/preparation_service.rs

@@ -23,9 +23,6 @@ const PROPOSER_PREPARATION_LOOKAHEAD_EPOCHS: u64 = 2;
 /// Number of epochs to wait before re-submitting validator registration.
 const EPOCHS_PER_VALIDATOR_REGISTRATION_SUBMISSION: u64 = 1;
 
-/// The number of validator registrations to include per request to the beacon node.
-const VALIDATOR_REGISTRATION_BATCH_SIZE: usize = 500;
-
 /// Builds an `PreparationService`.
 pub struct PreparationServiceBuilder<T: SlotClock + 'static, E: EthSpec> {
     validator_store: Option<Arc<ValidatorStore<T, E>>>,
@@ -33,6 +30,7 @@ pub struct PreparationServiceBuilder<T: SlotClock + 'static, E: EthSpec> {
     beacon_nodes: Option<Arc<BeaconNodeFallback<T, E>>>,
     context: Option<RuntimeContext<E>>,
     builder_registration_timestamp_override: Option<u64>,
+    validator_registration_batch_size: Option<usize>,
 }
 
 impl<T: SlotClock + 'static, E: EthSpec> PreparationServiceBuilder<T, E> {
@@ -43,6 +41,7 @@ impl<T: SlotClock + 'static, E: EthSpec> PreparationServiceBuilder<T, E> {
             beacon_nodes: None,
             context: None,
             builder_registration_timestamp_override: None,
+            validator_registration_batch_size: None,
         }
     }
 
@@ -74,6 +73,14 @@ impl<T: SlotClock + 'static, E: EthSpec> PreparationServiceBuilder<T, E> {
         self
     }
 
+    pub fn validator_registration_batch_size(
+        mut self,
+        validator_registration_batch_size: usize,
+    ) -> Self {
+        self.validator_registration_batch_size = Some(validator_registration_batch_size);
+        self
+    }
+
     pub fn build(self) -> Result<PreparationService<T, E>, String> {
         Ok(PreparationService {
             inner: Arc::new(Inner {
@@ -91,6 +98,9 @@ impl<T: SlotClock + 'static, E: EthSpec> PreparationServiceBuilder<T, E> {
                     .ok_or("Cannot build PreparationService without runtime_context")?,
                 builder_registration_timestamp_override: self
                     .builder_registration_timestamp_override,
+                validator_registration_batch_size: self.validator_registration_batch_size.ok_or(
+                    "Cannot build PreparationService without validator_registration_batch_size",
+                )?,
                 validator_registration_cache: RwLock::new(HashMap::new()),
             }),
         })
@@ -107,6 +117,7 @@ pub struct Inner<T, E: EthSpec> {
     // Used to track unpublished validator registration changes.
     validator_registration_cache:
         RwLock<HashMap<ValidatorRegistrationKey, SignedValidatorRegistrationData>>,
+    validator_registration_batch_size: usize,
 }
 
 #[derive(Hash, Eq, PartialEq, Debug, Clone)]
@@ -447,7 +458,7 @@ impl<T: SlotClock + 'static, E: EthSpec> PreparationService<T, E> {
         }
 
         if !signed.is_empty() {
-            for batch in signed.chunks(VALIDATOR_REGISTRATION_BATCH_SIZE) {
+            for batch in signed.chunks(self.validator_registration_batch_size) {
                 match self
                     .beacon_nodes
                     .first_success(
@@ -462,7 +473,7 @@ impl<T: SlotClock + 'static, E: EthSpec> PreparationService<T, E> {
                     Ok(()) => info!(
                         log,
                         "Published validator registrations to the builder network";
-                        "count" => registration_data_len,
+                        "count" => batch.len(),
                     ),
                     Err(e) => warn!(
                         log,