input: use multmutex to increase concurrency for musig session manager

By using the multimutex here, we'll no longer rely on a single mutex for
the entire musig session set like we used to. Instead, we can use the
session ID to key into a map of mutexes and use those directly.

input/musig2_session_manager.go

@@ -3,12 +3,12 @@ package input
 import (
 	"crypto/sha256"
 	"fmt"
-	"sync"
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcec/v2/schnorr"
 	"github.com/btcsuite/btcd/btcec/v2/schnorr/musig2"
 	"github.com/lightningnetwork/lnd/keychain"
+	"github.com/lightningnetwork/lnd/multimutex"
 )
 
 // MuSig2State is a struct that holds on to the internal signing session state
@@ -34,10 +34,10 @@ type PrivKeyFetcher func(*keychain.KeyDescriptor) (*btcec.PrivateKey, error)
 // musig sessions. Each session is identified by a unique session ID which is
 // used by callers to interact with a given session.
 type MusigSessionManager struct {
-	sync.Mutex
-
 	keyFetcher PrivKeyFetcher
 
+	sessionMtx *multimutex.Mutex[MuSig2SessionID]
+
 	musig2Sessions map[MuSig2SessionID]*MuSig2State
 }
 
@@ -45,7 +45,9 @@ type MusigSessionManager struct {
 // fetcher.
 func NewMusigSessionManager(keyFetcher PrivKeyFetcher) *MusigSessionManager {
 	return &MusigSessionManager{
-		keyFetcher: keyFetcher,
+		keyFetcher:     keyFetcher,
+		musig2Sessions: make(map[MuSig2SessionID]*MuSig2State),
+		sessionMtx:     multimutex.NewMutex[MuSig2SessionID](),
 	}
 }
 
@@ -70,7 +72,7 @@ func (m *MusigSessionManager) MuSig2CreateSession(bipVersion MuSig2Version,
 		KeyLocator: keyLoc,
 	})
 	if err != nil {
-		return nil, fmt.Errorf("error deriving private key: %v", err)
+		return nil, fmt.Errorf("error deriving private key: %w", err)
 	}
 
 	// Create a signing context and session with the given private key and
@@ -98,7 +100,7 @@ func (m *MusigSessionManager) MuSig2CreateSession(bipVersion MuSig2Version,
 	// Register the new session.
 	combinedKey, err := musigContext.CombinedKey()
 	if err != nil {
-		return nil, fmt.Errorf("error getting combined key: %v", err)
+		return nil, fmt.Errorf("error getting combined key: %w", err)
 	}
 	session := &MuSig2State{
 		MuSig2SessionInfo: MuSig2SessionInfo{
@@ -120,7 +122,7 @@ func (m *MusigSessionManager) MuSig2CreateSession(bipVersion MuSig2Version,
 	if tweaks.HasTaprootTweak() {
 		internalKey, err := musigContext.TaprootInternalKey()
 		if err != nil {
-			return nil, fmt.Errorf("error getting internal key: %v",
+			return nil, fmt.Errorf("error getting internal key: %w",
 				err)
 		}
 		session.TaprootInternalKey = internalKey
@@ -129,9 +131,12 @@ func (m *MusigSessionManager) MuSig2CreateSession(bipVersion MuSig2Version,
 	// Since we generate new nonces for every session, there is no way that
 	// a session with the same ID already exists. So even if we call the API
 	// twice with the same signers, we still get a new ID.
-	m.Lock()
+	//
+	// We'll use just all zeroes as the session ID for the mutex, as this
+	// is a "global" action.
+	m.sessionMtx.Lock(MuSig2SessionID{})
 	m.musig2Sessions[session.SessionID] = session
-	m.Unlock()
+	m.sessionMtx.Unlock(MuSig2SessionID{})
 
 	return &session.MuSig2SessionInfo, nil
 }
@@ -149,8 +154,8 @@ func (m *MusigSessionManager) MuSig2Sign(sessionID MuSig2SessionID,
 	// We hold the lock during the whole operation, we don't want any
 	// interference with calls that might come through in parallel for the
 	// same session.
-	m.Lock()
-	defer m.Unlock()
+	m.sessionMtx.Lock(sessionID)
+	defer m.sessionMtx.Unlock(sessionID)
 
 	session, ok := m.musig2Sessions[sessionID]
 	if !ok {
@@ -190,8 +195,8 @@ func (m *MusigSessionManager) MuSig2CombineSig(sessionID MuSig2SessionID,
 	// We hold the lock during the whole operation, we don't want any
 	// interference with calls that might come through in parallel for the
 	// same session.
-	m.Lock()
-	defer m.Unlock()
+	m.sessionMtx.Lock(sessionID)
+	defer m.sessionMtx.Unlock(sessionID)
 
 	session, ok := m.musig2Sessions[sessionID]
 	if !ok {
@@ -237,8 +242,8 @@ func (m *MusigSessionManager) MuSig2Cleanup(sessionID MuSig2SessionID) error {
 	// We hold the lock during the whole operation, we don't want any
 	// interference with calls that might come through in parallel for the
 	// same session.
-	m.Lock()
-	defer m.Unlock()
+	m.sessionMtx.Lock(sessionID)
+	defer m.sessionMtx.Unlock(sessionID)
 
 	_, ok := m.musig2Sessions[sessionID]
 	if !ok {
@@ -259,8 +264,8 @@ func (m *MusigSessionManager) MuSig2RegisterNonces(sessionID MuSig2SessionID,
 	// We hold the lock during the whole operation, we don't want any
 	// interference with calls that might come through in parallel for the
 	// same session.
-	m.Lock()
-	defer m.Unlock()
+	m.sessionMtx.Lock(sessionID)
+	defer m.sessionMtx.Unlock(sessionID)
 
 	session, ok := m.musig2Sessions[sessionID]
 	if !ok {