multi: adapt fee floor to bitcoind version

Introduces dynamic detection of the Bitcoin Core fee floor
based on the connected backend version. Previously, PeerSwap enforced
a hardcoded floor of 253 sat/kw (approx. 1 sat/vB).

With Bitcoin Core v29.2 lowering the default minimum relay fee, this
change allows PeerSwap to utilize cheaper fee rates when supported.

Added `DetermineFeeFloor` to select between legacy (253 sat/kw)
and modern (25 sat/kw) floors based on version parsing.

Author: YusukeShimizu

.devcontainer/devcontainer.json

@@ -3,6 +3,9 @@
 {
     "name": "peerswap",
     "image": "ghcr.io/xtruder/nix-devcontainer:v1",
+    "runArgs": [
+        "--platform=linux/amd64"
+    ],
     "onCreateCommand": "nix-env -iA cachix -f https://cachix.org/api/v1/install && cachix use peerswap && nix develop --command echo 'Nix environment ready'",
     "postCreateCommand": "sudo chmod a+w /usr/local/bin/ && nix develop --command bash -c 'GOBIN=/usr/local/bin go install github.com/bufbuild/buf/cmd/buf@v1.50.0'",
     "customizations": {
@@ -12,7 +15,8 @@
                 "jnoortheen.nix-ide",
                 "mkhl.direnv",
                 "golang.go",
-                "bufbuild.vscode-buf"
+                "bufbuild.vscode-buf",
+                "openai.chatgpt"
             ]
         }
     },

cmd/peerswap-plugin/main.go

@@ -244,6 +244,7 @@ func run(ctx context.Context, lightningPlugin *clightning.ClightningClient) erro
 		return err
 	}
 
+	bitcoinFeeFloor := onchain.LegacyFeeFloorSatPerKw
 	var bitcoinTxWatcher *txwatcher.BlockchainRpcTxWatcher
 	var bitcoinOnChainService *onchain.BitcoinOnChain
 	var bitcoinEnabled bool
@@ -253,6 +254,14 @@ func run(ctx context.Context, lightningPlugin *clightning.ClightningClient) erro
 		bitcoinEnabled = true
 		bitcoinTxWatcher = txwatcher.NewBlockchainRpcTxWatcher(ctx, txwatcher.NewBitcoinRpc(bitcoinCli), onchain.BitcoinMinConfs, onchain.BitcoinCsv)
 
+		floor, detectedVersion, floorErr := determineBitcoinFeeFloor(bitcoinCli)
+		if floorErr != nil {
+			log.Infof("Unable to detect Bitcoin Core version, defaulting fee floor to %d sat/kw: %v", floor, floorErr)
+		} else {
+			log.Infof("Detected Bitcoin Core version %s, using fee floor %d sat/kw", detectedVersion, floor)
+		}
+		bitcoinFeeFloor = floor
+
 		// We set the default Estimator to the static regtest estimator.
 		var bitcoinEstimator onchain.Estimator
 		bitcoinEstimator, _ = onchain.NewRegtestFeeEstimator()
@@ -271,6 +280,7 @@ func run(ctx context.Context, lightningPlugin *clightning.ClightningClient) erro
 				bitcoinCli,
 				"ECONOMICAL",
 				fallbackFeeRateSatPerKw,
+				bitcoinFeeFloor,
 			)
 			if err != nil {
 				return err
@@ -281,14 +291,12 @@ func run(ctx context.Context, lightningPlugin *clightning.ClightningClient) erro
 			return err
 		}
 
-		// Create the bitcoin onchain service with a fallback fee rate of
-		// 253 sat/kw. (This should be useless in this case).
-		// TODO: This fee rate does not matter right now but we might want to
-		// add a config flag to set this higher than the assumed floor fee rate
-		// of 275 sat/kw (1.1 sat/vb).
+		// Create the bitcoin onchain service while keeping the same fee floor for
+		// estimator and fallback paths.
 		bitcoinOnChainService = onchain.NewBitcoinOnChain(
 			bitcoinEstimator,
-			btcutil.Amount(253),
+			bitcoinFeeFloor,
+			bitcoinFeeFloor,
 			chain,
 		)
 	} else {
@@ -508,6 +516,30 @@ func getBitcoinClient(li *glightning.Lightning, pluginConfig *clightning.Config)
 	return bitcoin, nil
 }
 
+type getNetworkInfoRequest struct{}
+
+func (r *getNetworkInfoRequest) Name() string {
+	return "getnetworkinfo"
+}
+
+type bitcoindNetworkInfo struct {
+	Subversion string `json:"subversion"`
+}
+
+func determineBitcoinFeeFloor(bitcoinCli *gbitcoin.Bitcoin) (btcutil.Amount, string, error) {
+	var networkInfo bitcoindNetworkInfo
+	if err := bitcoinCli.Request(&getNetworkInfoRequest{}, &networkInfo); err != nil {
+		return onchain.LegacyFeeFloorSatPerKw, "", err
+	}
+
+	floor, normalized := onchain.DetermineFeeFloor(networkInfo.Subversion)
+	if normalized == "" {
+		return floor, "", fmt.Errorf("unable to parse bitcoin core subversion %q", networkInfo.Subversion)
+	}
+
+	return floor, normalized, nil
+}
+
 func checkClnVersion(network string, fullVersionString string) (bool, error) {
 	// skip version check if running signet as it needs a custom build
 	// ? Can someone explain why we need this here?

cmd/peerswaplnd/peerswapd/main.go

@@ -26,7 +26,6 @@ import (
 	"github.com/elementsproject/peerswap/version"
 	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
 
-	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/elementsproject/glightning/gelements"
 	"github.com/elementsproject/peerswap/cmd/peerswaplnd"
@@ -164,10 +163,12 @@ func run() error {
 			return err
 		}
 
+		bitcoinFeeFloor := onchain.LegacyFeeFloorSatPerKw
+
 		// Start the LndEstimator.
 		lndEstimator, err := onchain.NewLndEstimator(
 			walletrpc.NewWalletKitClient(cc),
-			btcutil.Amount(253),
+			bitcoinFeeFloor,
 			10*time.Minute,
 		)
 		if err != nil {
@@ -177,14 +178,11 @@ func run() error {
 			return err
 		}
 
-		// Create the bitcoin onchain service with a fallback fee rate of
-		// 253 sat/kw.
-		// TODO: This fee rate does not matter right now but we might want to
-		// add a config flag to set this higher than the assumed floor fee rate
-		// of 275 sat/kw (1.1 sat/vb).
+		// Keep estimator and fallback paths aligned by reusing the same fee floor.
 		bitcoinOnChainService = onchain.NewBitcoinOnChain(
 			lndEstimator,
-			btcutil.Amount(253),
+			bitcoinFeeFloor,
+			bitcoinFeeFloor,
 			chain,
 		)
 		log.Infof("Bitcoin swaps enabled on network %s", chain.Name)