Auto-refresh relay fees from conf can be disabled (#3260)

diff --git a/docs/release-notes/eclair-vnext.md b/docs/release-notes/eclair-vnext.md
index 006d8a9..4aed4af 100644
--- a/docs/release-notes/eclair-vnext.md
+++ b/docs/release-notes/eclair-vnext.md
@@ -9,6 +9,12 @@
 We remove the code used to support legacy channels that don't use anchor outputs or taproot.
 If you still have such channels, eclair won't start: you will need to close those channels, and will only be able to update eclair once they have been successfully closed.
 
+### Auto-refresh relay fees from configuration can be disabled
+
+There is a performance hit on restart if there is a large number (> 100s) of channels, so it can be disabled with a new `eclair.relay.fees.reset-existing-channels` setting.
+
+The default behavior is unchanged.
+
 ### Channel lifecyle events rework
 
 Eclair emits several events during a channel lifecycle, which can be received by plugins or through the websocket.
diff --git a/eclair-core/src/main/resources/reference.conf b/eclair-core/src/main/resources/reference.conf
index 2e456cf..d46515f 100644
--- a/eclair-core/src/main/resources/reference.conf
+++ b/eclair-core/src/main/resources/reference.conf
@@ -241,6 +241,10 @@ eclair {
         fee-base-msat = 1000
         fee-proportional-millionths = 100
       }
+      // By default, if the fees values are updated in configuration, they will automatically be applied to existing
+      // channels on restart (except if custom per-node settings have been defined, in which case they take precedence).
+      // There is a performance hit on restart if there is a large number (> 100s) of channels, so it can be disabled.
+      reset-existing-channels = true
       // Delay enforcement of channel fee updates
       enforcement-delay = 10 minutes
     }
diff --git a/eclair-core/src/main/scala/fr/acinq/eclair/NodeParams.scala b/eclair-core/src/main/scala/fr/acinq/eclair/NodeParams.scala
index be25030..f1e392b 100644
--- a/eclair-core/src/main/scala/fr/acinq/eclair/NodeParams.scala
+++ b/eclair-core/src/main/scala/fr/acinq/eclair/NodeParams.scala
@@ -638,6 +638,7 @@ object NodeParams extends Logging {
         publicChannelFees = getRelayFees(config.getConfig("relay.fees.public-channels")),
         privateChannelFees = getRelayFees(config.getConfig("relay.fees.private-channels")),
         minTrampolineFees = getRelayFees(config.getConfig("relay.fees.min-trampoline")),
+        resetExistingChannels = config.getBoolean("relay.fees.reset-existing-channels"),
         enforcementDelay = FiniteDuration(config.getDuration("relay.fees.enforcement-delay").getSeconds, TimeUnit.SECONDS),
         asyncPaymentsParams = AsyncPaymentsParams(asyncPaymentHoldTimeoutBlocks, asyncPaymentCancelSafetyBeforeTimeoutBlocks),
         peerReputationConfig = Reputation.Config(
diff --git a/eclair-core/src/main/scala/fr/acinq/eclair/channel/fsm/Channel.scala b/eclair-core/src/main/scala/fr/acinq/eclair/channel/fsm/Channel.scala
index 5ab4177..7b25860 100644
--- a/eclair-core/src/main/scala/fr/acinq/eclair/channel/fsm/Channel.scala
+++ b/eclair-core/src/main/scala/fr/acinq/eclair/channel/fsm/Channel.scala
@@ -452,13 +452,15 @@ class Channel(val nodeParams: NodeParams, val channelKeys: ChannelKeys, val wall
           goto(CLOSING) using closing
         case normal: DATA_NORMAL =>
           context.system.eventStream.publish(ShortChannelIdAssigned(self, normal.channelId, normal.lastAnnouncement_opt, normal.aliases, remoteNodeId))
-          // we check the configuration because the values for channel_update may have changed while eclair was down
-          val fees = getRelayFees(nodeParams, remoteNodeId, normal.commitments.announceChannel)
-          if (fees.feeBase != normal.channelUpdate.feeBaseMsat ||
-            fees.feeProportionalMillionths != normal.channelUpdate.feeProportionalMillionths ||
-            nodeParams.channelConf.expiryDelta != normal.channelUpdate.cltvExpiryDelta) {
-            log.debug("refreshing channel_update due to configuration changes")
-            self ! CMD_UPDATE_RELAY_FEE(ActorRef.noSender, fees.feeBase, fees.feeProportionalMillionths)
+          if (nodeParams.relayParams.resetExistingChannels) {
+            // we refresh the relay fee settings if eclair has been restarted with a new conf
+            val fees = getRelayFees(nodeParams, remoteNodeId, normal.commitments.announceChannel)
+            if (fees.feeBase != normal.channelUpdate.feeBaseMsat ||
+              fees.feeProportionalMillionths != normal.channelUpdate.feeProportionalMillionths ||
+              nodeParams.channelConf.expiryDelta != normal.channelUpdate.cltvExpiryDelta) {
+              log.debug("refreshing channel_update due to configuration changes")
+              self ! CMD_UPDATE_RELAY_FEE(ActorRef.noSender, fees.feeBase, fees.feeProportionalMillionths)
+            }
           }
           // we need to periodically re-send channel updates, otherwise channel will be considered stale and get pruned by network
           // we take into account the date of the last update so that we don't send superfluous updates when we restart the app
diff --git a/eclair-core/src/main/scala/fr/acinq/eclair/payment/relay/Relayer.scala b/eclair-core/src/main/scala/fr/acinq/eclair/payment/relay/Relayer.scala
index fe44843..06dec87 100644
--- a/eclair-core/src/main/scala/fr/acinq/eclair/payment/relay/Relayer.scala
+++ b/eclair-core/src/main/scala/fr/acinq/eclair/payment/relay/Relayer.scala
@@ -145,6 +145,7 @@ object Relayer extends Logging {
   case class RelayParams(publicChannelFees: RelayFees,
                          privateChannelFees: RelayFees,
                          minTrampolineFees: RelayFees,
+                         resetExistingChannels: Boolean,
                          enforcementDelay: FiniteDuration,
                          asyncPaymentsParams: AsyncPaymentsParams,
                          peerReputationConfig: Reputation.Config,
diff --git a/eclair-core/src/test/scala/fr/acinq/eclair/TestConstants.scala b/eclair-core/src/test/scala/fr/acinq/eclair/TestConstants.scala
index 3a43f34..0a8f0a0 100644
--- a/eclair-core/src/test/scala/fr/acinq/eclair/TestConstants.scala
+++ b/eclair-core/src/test/scala/fr/acinq/eclair/TestConstants.scala
@@ -180,6 +180,7 @@ object TestConstants {
         minTrampolineFees = RelayFees(
           feeBase = 548000 msat,
           feeProportionalMillionths = 30),
+        resetExistingChannels = true,
         enforcementDelay = 10 minutes,
         asyncPaymentsParams = AsyncPaymentsParams(1008, CltvExpiryDelta(144)),
         peerReputationConfig = Reputation.Config(enabled = true, 1 day, 10 minutes),
@@ -372,6 +373,7 @@ object TestConstants {
         minTrampolineFees = RelayFees(
           feeBase = 548000 msat,
           feeProportionalMillionths = 30),
+        resetExistingChannels = true,
         enforcementDelay = 10 minutes,
         asyncPaymentsParams = AsyncPaymentsParams(1008, CltvExpiryDelta(144)),
         peerReputationConfig = Reputation.Config(enabled = true, 2 day, 20 minutes),
diff --git a/eclair-core/src/test/scala/fr/acinq/eclair/channel/RestoreSpec.scala b/eclair-core/src/test/scala/fr/acinq/eclair/channel/RestoreSpec.scala
index db69a0d..1050912 100644
--- a/eclair-core/src/test/scala/fr/acinq/eclair/channel/RestoreSpec.scala
+++ b/eclair-core/src/test/scala/fr/acinq/eclair/channel/RestoreSpec.scala
@@ -124,11 +124,11 @@ class RestoreSpec extends TestKitBaseClass with FixtureAnyFunSuiteLike with Chan
       val newAlice: TestFSMRef[ChannelState, ChannelData, Channel] = TestFSMRef(new Channel(newConfig, Alice.channelKeys(), aliceWallet, Bob.nodeParams.nodeId, alice2blockchain.ref, alice2relayer.ref, FakeTxPublisherFactory(alice2blockchain)), alicePeer.ref)
       newAlice ! INPUT_RESTORED(oldStateData)
 
-      val u1 = channelUpdateListener.expectMsgType[ChannelUpdateParametersChanged]
-      assert(!Announcements.areSameRelayParams(u1.channelUpdate, oldStateData.channelUpdate))
-      assert(u1.channelUpdate.feeBaseMsat == newConfig.relayParams.privateChannelFees.feeBase)
-      assert(u1.channelUpdate.feeProportionalMillionths == newConfig.relayParams.privateChannelFees.feeProportionalMillionths)
-      assert(u1.channelUpdate.cltvExpiryDelta == newConfig.channelConf.expiryDelta)
+      val cup = channelUpdateListener.expectMsgType[ChannelUpdateParametersChanged]
+      assert(!Announcements.areSameRelayParams(cup.channelUpdate, oldStateData.channelUpdate))
+      assert(cup.channelUpdate.feeBaseMsat == newConfig.relayParams.privateChannelFees.feeBase)
+      assert(cup.channelUpdate.feeProportionalMillionths == newConfig.relayParams.privateChannelFees.feeProportionalMillionths)
+      assert(cup.channelUpdate.cltvExpiryDelta == newConfig.channelConf.expiryDelta)
 
       newAlice ! INPUT_RECONNECTED(alice2bob.ref, aliceInit, bobInit)
       bob ! INPUT_RECONNECTED(bob2alice.ref, bobInit, aliceInit)
@@ -136,7 +136,9 @@ class RestoreSpec extends TestKitBaseClass with FixtureAnyFunSuiteLike with Chan
       bob2alice.expectMsgType[ChannelReestablish]
       alice2bob.forward(bob)
       bob2alice.forward(newAlice)
-      alice2bob.expectMsgType[ChannelUpdate]
+      val u1 = alice2bob.expectMsgType[ChannelUpdate]
+      assert(cup.channelUpdate.feeBaseMsat == u1.feeBaseMsat)
+      assert(cup.channelUpdate.feeProportionalMillionths == u1.feeProportionalMillionths)
       bob2alice.expectMsgType[ChannelUpdate]
       alice2bob.expectNoMessage()
       bob2alice.expectNoMessage()