Added possibility to have a secondary el on the host (#219)

This change, relatively simple and backward compatible, enables syncing
an external execution client from the host itself instead of relying on
running within Docker. We tested it and to succesfully sync with the
inner el peer, one further change is needed which is to expose the P2P
address to the public interface (instead of only within the host), PR
incoming for this.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: maoueh

README.md

@@ -49,7 +49,7 @@ Flags:
 - `--latest-fork`: Enable the latest fork at startup
 - `--block-time`: Change the default block time (`12s`), to be provided in duration format (e.g. `--block-time=1s`)
 - `--use-reth-for-validation`: Use Reth EL for block validation in mev-boost.
-- `--secondary-el`: Port to use for a secondary el (enables the internal cl-proxy proxy)
+- `--secondary-el`: Host or port to use for a secondary el (enables the internal cl-proxy proxy). Can be a port number (e.g., '8551') in which case the full URL is derived as `http://localhost:<port>` or a complete URL (e.g., `http://remote-host:8551`), use `http://host.docker.internal:<port>` to reach a secondary execution client that runs on your host and not within Docker.
 - `--use-native-reth`: Run the Reth EL binary on the host instead of docker (recommended to bind to the Reth DB)
 - `--use-separate-mev-boost`: Spins a seperate service for mev-boost in addition with mev-boost-relay
 

playground/recipe_l1.go

@@ -2,6 +2,7 @@ package playground
 
 import (
 	"fmt"
+	"regexp"
 	"time"
 
 	flag "github.com/spf13/pflag"
@@ -20,9 +21,12 @@ type L1Recipe struct {
 	// useRethForValidation signals mev-boost to use the Reth EL node for block validation
 	useRethForValidation bool
 
-	// secondaryELPort enables the use of a secondary EL connected to the validator beacon node
-	// It is enabled through the use of the cl-proxy service
-	secondaryELPort uint64
+	// secondaryEL enables the use of a secondary EL connected to the validator beacon node
+	// It is enabled through the use of the cl-proxy service. If the input is a plain number, it is assumed
+	// to be a port number and the secondary EL is assumed to be running on localhost at that port.
+	// Otherwise, it is assumed to be a full address (e.g http://some-el:8551) where to reach the secondary EL,
+	// use http://host.docker.internal:<port> to reach the host machine from within docker.
+	secondaryEL string
 
 	// if useNativeReth is set to true, the Reth EL execution client for the validator beacon node
 	// will run on the host machine. This is useful if you want to bind to the Reth database and you
@@ -45,7 +49,7 @@ func (l *L1Recipe) Flags() *flag.FlagSet {
 	flags.BoolVar(&l.latestFork, "latest-fork", false, "use the latest fork")
 	flags.DurationVar(&l.blockTime, "block-time", time.Duration(defaultL1BlockTimeSeconds)*time.Second, "Block time to use for the L1")
 	flags.BoolVar(&l.useRethForValidation, "use-reth-for-validation", false, "use reth for validation")
-	flags.Uint64Var(&l.secondaryELPort, "secondary-el", 0, "port to use for the secondary builder")
+	flags.StringVar(&l.secondaryEL, "secondary-el", "", "Address or port to use for the secondary EL (execution layer); Can be a port number (e.g., '8551') in which case the full URL is derived as `http://localhost:<port>` or a complete URL (e.g., `http://docker-container-name:8551`), use `http://host.docker.internal:<port>` to reach a secondary execution client that runs on your host and not within Docker.")
 	flags.BoolVar(&l.useNativeReth, "use-native-reth", false, "use the native reth binary")
 	flags.BoolVar(&l.useSeparateMevBoost, "use-separate-mev-boost", false, "use separate mev-boost and mev-boost-relay services")
 	return flags
@@ -59,20 +63,27 @@ func (l *L1Recipe) Artifacts() *ArtifactsBuilder {
 	return builder
 }
 
+var looksLikePortRegex = regexp.MustCompile(`^\d{2,5}$`)
+
 func (l *L1Recipe) Apply(svcManager *Manifest) {
 	svcManager.AddService(&RethEL{
 		UseRethForValidation: l.useRethForValidation,
 		UseNativeReth:        l.useNativeReth,
 	})
 
 	var elService string
-	if l.secondaryELPort != 0 {
+	if l.secondaryEL != "" {
+		address := l.secondaryEL
+		if looksLikePortRegex.MatchString(l.secondaryEL) {
+			address = fmt.Sprintf("http://localhost:%s", l.secondaryEL)
+		}
+
 		// we are going to use the cl-proxy service to connect the beacon node to two builders
 		// one the 'el' builder and another one the remote one
 		elService = "cl-proxy"
 		svcManager.AddService(&ClProxy{
 			PrimaryBuilder:   "el",
-			SecondaryBuilder: fmt.Sprintf("http://localhost:%d", l.secondaryELPort),
+			SecondaryBuilder: address,
 		})
 	} else {
 		elService = "el"