lndclient: add new macaroonPouch struct

In this commit, we add a new struct, the `macaroonPouch`. This struct
bundles all the macaroons we need for each sub-server. Along the way, we
also export the set of default* paths for lnd, and add a new set of
variables that store the default file names of each of the macaroons for
each sub-server.

The `WithMacaroonAuth` method on the `serializedMacaroon` type will
allow us to attach a macaroon for each call rather than attaching it at
the connection level. This slight change will allow us to use multiple
macaroons over a single RPC connection, rather than a single global
macaroon.

lndclient/lnd_services.go

@@ -122,11 +122,17 @@ var (
 	defaultRPCPort         = "10009"
 	defaultLndDir          = btcutil.AppDataDir("lnd", false)
 	defaultTLSCertFilename = "tls.cert"
-	defaultTLSCertPath     = filepath.Join(defaultLndDir,
-		defaultTLSCertFilename)
-	defaultDataDir          = "data"
-	defaultChainSubDir      = "chain"
-	defaultMacaroonFilename = "admin.macaroon"
+	defaultTLSCertPath     = filepath.Join(
+		defaultLndDir, defaultTLSCertFilename,
+	)
+	defaultDataDir     = "data"
+	defaultChainSubDir = "chain"
+
+	defaultAdminMacaroonFilename     = "admin.macaroon"
+	defaultInvoiceMacaroonFilename   = "invoices.macaroon"
+	defaultChainMacaroonFilename     = "chainnotifier.macaroon"
+	defaultWalletKitMacaroonFilename = "walletkit.macaroon"
+	defaultSignerFilename            = "signer.macaroon"
 )
 
 func getClientConn(address string, network string, macPath, tlsPath string) (
@@ -151,7 +157,7 @@ func getClientConn(address string, network string, macPath, tlsPath string) (
 	if macPath == "" {
 		macPath = filepath.Join(
 			defaultLndDir, defaultDataDir, defaultChainSubDir,
-			"bitcoin", network, defaultMacaroonFilename,
+			"bitcoin", network, defaultAdminMacaroonFilename,
 		)
 	}
 

lndclient/macaroon_pouch.go

@@ -0,0 +1,98 @@
+package lndclient
+
+import (
+	"context"
+	"encoding/hex"
+	"io/ioutil"
+	"path/filepath"
+
+	"google.golang.org/grpc/metadata"
+)
+
+// serializedMacaroon is a type that represents a hex-encoded macaroon. We'll
+// use this primarily vs the raw binary format as the gRPC metadata feature
+// requires that all keys and values be strings.
+type serializedMacaroon string
+
+// newSerializedMacaroon reads a new serializedMacaroon from that target
+// macaroon path. If the file can't be found, then an error is returned.
+func newSerializedMacaroon(macaroonPath string) (serializedMacaroon, error) {
+	macBytes, err := ioutil.ReadFile(macaroonPath)
+	if err != nil {
+		return "", err
+	}
+
+	return serializedMacaroon(hex.EncodeToString(macBytes)), nil
+}
+
+// WithMacaroonAuth modifies the passed context to include the macaroon KV
+// metadata of the target macaroon. This method can be used to add the macaroon
+// at call time, rather than when the connection to the gRPC server is created.
+func (s serializedMacaroon) WithMacaroonAuth(ctx context.Context) context.Context {
+	return metadata.AppendToOutgoingContext(ctx, "macaroon", string(s))
+}
+
+// macaroonPouch holds the set of macaroons we need to interact with lnd for
+// Loop. Each sub-server has its own macaroon, and for the remaining temporary
+// calls that directly hit lnd, we'll use the admin macaroon.
+type macaroonPouch struct {
+	// invoiceMac is the macaroon for the invoices sub-server.
+	invoiceMac serializedMacaroon
+
+	// chainMac is the macaroon for the ChainNotifier sub-server.
+	chainMac serializedMacaroon
+
+	// signerMac is the macaroon for the Signer sub-server.
+	signerMac serializedMacaroon
+
+	// walletKitMac is the macaroon for the WalletKit sub-server.
+	walletKitMac serializedMacaroon
+
+	// adminMac is the primary admin macaroon for lnd.
+	adminMac serializedMacaroon
+}
+
+// newMacaroonPouch returns a new instance of a fully populated macaroonPouch
+// given the directory where all the macaroons are stored.
+func newMacaroonPouch(macaroonDir string) (*macaroonPouch, error) {
+	m := &macaroonPouch{}
+
+	var err error
+
+	m.invoiceMac, err = newSerializedMacaroon(
+		filepath.Join(macaroonDir, defaultInvoiceMacaroonFilename),
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	m.chainMac, err = newSerializedMacaroon(
+		filepath.Join(macaroonDir, defaultChainMacaroonFilename),
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	m.signerMac, err = newSerializedMacaroon(
+		filepath.Join(macaroonDir, defaultSignerFilename),
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	m.walletKitMac, err = newSerializedMacaroon(
+		filepath.Join(macaroonDir, defaultWalletKitMacaroonFilename),
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	m.adminMac, err = newSerializedMacaroon(
+		filepath.Join(macaroonDir, defaultAdminMacaroonFilename),
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	return m, nil
+}