channeldb: cache htlc attempt session key

channeldb/mp_payment.go

@@ -26,6 +26,11 @@ type HTLCAttemptInfo struct {
 	// EC operations used by btcec.PrivKeyFromBytes.
 	sessionKey [btcec.PrivKeyBytesLen]byte
 
+	// cachedSessionKey is our fully deserialized sesionKey. This value
+	// may be nil if the attempt has just been read from disk and its
+	// session key has not been used yet.
+	cachedSessionKey *btcec.PrivateKey
+
 	// Route is the route attempted to send the HTLC.
 	Route route.Route
 
@@ -49,19 +54,25 @@ func NewHtlcAttemptInfo(attemptID uint64, sessionKey *btcec.PrivateKey,
 	copy(scratch[:], sessionKey.Serialize())
 
 	return &HTLCAttemptInfo{
-		AttemptID:   attemptID,
-		sessionKey:  scratch,
-		Route:       route,
-		AttemptTime: attemptTime,
-		Hash:        hash,
+		AttemptID:        attemptID,
+		sessionKey:       scratch,
+		cachedSessionKey: sessionKey,
+		Route:            route,
+		AttemptTime:      attemptTime,
+		Hash:             hash,
 	}
 }
 
 // SessionKey returns the ephemeral key used for a htlc attempt. This function
-// performs expensive ec-ops to obtain the session key.
+// performs expensive ec-ops to obtain the session key if it is not cached.
 func (h *HTLCAttemptInfo) SessionKey() *btcec.PrivateKey {
-	priv, _ := btcec.PrivKeyFromBytes(btcec.S256(), h.sessionKey[:])
-	return priv
+	if h.cachedSessionKey == nil {
+		h.cachedSessionKey, _ = btcec.PrivKeyFromBytes(
+			btcec.S256(), h.sessionKey[:],
+		)
+	}
+
+	return h.cachedSessionKey
 }
 
 // HTLCAttempt contains information about a specific HTLC attempt for a given

channeldb/payments_test.go

@@ -120,6 +120,10 @@ func TestSentPaymentSerialization(t *testing.T) {
 	newWireInfo.Route = route.Route{}
 	s.Route = route.Route{}
 
+	// Call session key method to set our cached session key so we can use
+	// DeepEqual, and assert that our key equals the original key.
+	require.Equal(t, s.cachedSessionKey, newWireInfo.SessionKey())
+
 	if !reflect.DeepEqual(s, newWireInfo) {
 		t.Fatalf("Payments do not match after "+
 			"serialization/deserialization %v vs %v",