Add delay before setting watches at startup (#1489)

Watching the blockchain is an asynchronous task, so it is "always late"
and it doesn't matter if we don't synchronously set the watch back when
the node is restarted.

It allows us to smoothen the load if needed at startup, because setting
tens of thousands of `watch-spent` all at once at startup is pretty
expensive.

Co-authored-by: Bastien Teinturier <31281497+t-bast@users.noreply.github.com>

eclair-core/src/main/resources/reference.conf

@@ -20,6 +20,7 @@ eclair {
   # backup-notify-script = "/absolute/path/to/script.sh"
 
   watcher-type = "bitcoind" // other *experimental* values include "electrum"
+  watch-spent-window = 1 minute // at startup watches will be put back within that window to reduce herd effect; must be > 0s
 
   bitcoind {
     host = "localhost"

eclair-core/src/main/scala/fr/acinq/eclair/NodeParams.scala

@@ -36,7 +36,7 @@ import fr.acinq.eclair.tor.Socks5ProxyParams
 import fr.acinq.eclair.wire.{Color, EncodingType, NodeAddress}
 import scodec.bits.ByteVector
 
-import scala.concurrent.duration.FiniteDuration
+import scala.concurrent.duration._
 import scala.jdk.CollectionConverters._
 
 /**
@@ -79,6 +79,7 @@ case class NodeParams(keyManager: KeyManager,
                       chainHash: ByteVector32,
                       channelFlags: Byte,
                       watcherType: WatcherType,
+                      watchSpentWindow: FiniteDuration,
                       paymentRequestExpiry: FiniteDuration,
                       multiPartPaymentExpiry: FiniteDuration,
                       minFundingSatoshis: Satoshi,
@@ -167,6 +168,9 @@ object NodeParams {
       case _ => BITCOIND
     }
 
+    val watchSpentWindow = FiniteDuration(config.getDuration("watch-spent-window").getSeconds, TimeUnit.SECONDS)
+    require(watchSpentWindow > 0.seconds, "watch-spent-window must be strictly greater than 0")
+
     val dustLimitSatoshis = Satoshi(config.getLong("dust-limit-satoshis"))
     if (chainHash == Block.LivenetGenesisBlock.hash) {
       require(dustLimitSatoshis >= Channel.MIN_DUSTLIMIT, s"dust limit must be greater than ${Channel.MIN_DUSTLIMIT}")
@@ -282,6 +286,7 @@ object NodeParams {
       chainHash = chainHash,
       channelFlags = config.getInt("channel-flags").toByte,
       watcherType = watcherType,
+      watchSpentWindow = watchSpentWindow,
       paymentRequestExpiry = FiniteDuration(config.getDuration("payment-request-expiry").getSeconds, TimeUnit.SECONDS),
       multiPartPaymentExpiry = FiniteDuration(config.getDuration("multi-part-payment-expiry").getSeconds, TimeUnit.SECONDS),
       minFundingSatoshis = Satoshi(config.getLong("min-funding-satoshis")),

eclair-core/src/main/scala/fr/acinq/eclair/router/Router.scala

@@ -44,7 +44,7 @@ import kamon.context.Context
 import scala.collection.immutable.SortedMap
 import scala.concurrent.duration._
 import scala.concurrent.{ExecutionContext, Promise}
-import scala.util.Try
+import scala.util.{Random, Try}
 
 /**
  * Created by PM on 24/05/2016.
@@ -90,7 +90,10 @@ class Router(val nodeParams: NodeParams, watcher: ActorRef, initialized: Option[
       val txid = pc.fundingTxid
       val TxCoordinates(_, _, outputIndex) = ShortChannelId.coordinates(pc.ann.shortChannelId)
       val fundingOutputScript = write(pay2wsh(Scripts.multiSig2of2(pc.ann.bitcoinKey1, pc.ann.bitcoinKey2)))
-      watcher ! WatchSpentBasic(self, txid, outputIndex, fundingOutputScript, BITCOIN_FUNDING_EXTERNAL_CHANNEL_SPENT(pc.ann.shortChannelId))
+      // avoid herd effect at startup because watch-spent are intensive in terms of rpc calls to bitcoind
+      context.system.scheduler.scheduleOnce(Random.nextLong(nodeParams.watchSpentWindow.toSeconds).seconds) {
+        watcher ! WatchSpentBasic(self, txid, outputIndex, fundingOutputScript, BITCOIN_FUNDING_EXTERNAL_CHANNEL_SPENT(pc.ann.shortChannelId))
+      }
     }
 
     // on restart we update our node announcement

eclair-core/src/test/scala/fr/acinq/eclair/TestConstants.scala

@@ -179,6 +179,7 @@ object TestConstants {
       chainHash = Block.RegtestGenesisBlock.hash,
       channelFlags = 1,
       watcherType = BITCOIND,
+      watchSpentWindow = 1 second,
       paymentRequestExpiry = 1 hour,
       multiPartPaymentExpiry = 30 seconds,
       minFundingSatoshis = 1000 sat,
@@ -267,6 +268,7 @@ object TestConstants {
       chainHash = Block.RegtestGenesisBlock.hash,
       channelFlags = 1,
       watcherType = BITCOIND,
+      watchSpentWindow = 1 second,
       paymentRequestExpiry = 1 hour,
       multiPartPaymentExpiry = 30 seconds,
       minFundingSatoshis = 1000 sat,