cmd/lncli: add metadata kvpair flag

Add a new `metadata` string slice flag to lncli that allows the caller
to specify multiple key-value string pairs that should be appended to
the outgoing context.

cmd/lncli/main.go

@@ -24,6 +24,7 @@ import (
 	"golang.org/x/term"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/metadata"
 )
 
 const (
@@ -109,6 +110,12 @@ func getClientConn(ctx *cli.Context, skipMacaroons bool) *grpc.ClientConn {
 	// Create a dial options array.
 	opts := []grpc.DialOption{
 		grpc.WithTransportCredentials(creds),
+		grpc.WithUnaryInterceptor(
+			addMetadataUnaryInterceptor(profile.Metadata),
+		),
+		grpc.WithStreamInterceptor(
+			addMetaDataStreamInterceptor(profile.Metadata),
+		),
 	}
 
 	// Only process macaroon credentials if --no-macaroons isn't set and
@@ -141,16 +148,17 @@ func getClientConn(ctx *cli.Context, skipMacaroons bool) *grpc.ClientConn {
 		}
 
 		macConstraints := []macaroons.Constraint{
-			// We add a time-based constraint to prevent replay of the
-			// macaroon. It's good for 60 seconds by default to make up for
-			// any discrepancy between client and server clocks, but leaking
-			// the macaroon before it becomes invalid makes it possible for
-			// an attacker to reuse the macaroon. In addition, the validity
-			// time of the macaroon is extended by the time the server clock
-			// is behind the client clock, or shortened by the time the
+			// We add a time-based constraint to prevent replay of
+			// the macaroon. It's good for 60 seconds by default to
+			// make up for any discrepancy between client and server
+			// clocks, but leaking the macaroon before it becomes
+			// invalid makes it possible for an attacker to reuse
+			// the macaroon. In addition, the validity time of the
+			// macaroon is extended by the time the server clock is
+			// behind the client clock, or shortened by the time the
 			// server clock is ahead of the client clock (or invalid
-			// altogether if, in the latter case, this time is more than 60
-			// seconds).
+			// altogether if, in the latter case, this time is more
+			// than 60 seconds).
 			// TODO(aakselrod): add better anti-replay protection.
 			macaroons.TimeoutConstraint(profile.Macaroons.Timeout),
 
@@ -180,7 +188,9 @@ func getClientConn(ctx *cli.Context, skipMacaroons bool) *grpc.ClientConn {
 	// to connect to the grpc server.
 	if ctx.GlobalIsSet("socksproxy") {
 		socksProxy := ctx.GlobalString("socksproxy")
-		torDialer := func(_ context.Context, addr string) (net.Conn, error) {
+		torDialer := func(_ context.Context, addr string) (net.Conn,
+			error) {
+
 			return tor.Dial(
 				addr, socksProxy, false, false,
 				tor.DefaultConnTimeout,
@@ -204,6 +214,49 @@ func getClientConn(ctx *cli.Context, skipMacaroons bool) *grpc.ClientConn {
 	return conn
 }
 
+// addMetadataUnaryInterceptor returns a grpc client side interceptor that
+// appends any key-value metadata strings to the outgoing context of a grpc
+// unary call.
+func addMetadataUnaryInterceptor(
+	md map[string]string) grpc.UnaryClientInterceptor {
+
+	return func(ctx context.Context, method string, req, reply interface{},
+		cc *grpc.ClientConn, invoker grpc.UnaryInvoker,
+		opts ...grpc.CallOption) error {
+
+		outCtx := contextWithMetadata(ctx, md)
+		return invoker(outCtx, method, req, reply, cc, opts...)
+	}
+}
+
+// addMetaDataStreamInterceptor returns a grpc client side interceptor that
+// appends any key-value metadata strings to the outgoing context of a grpc
+// stream call.
+func addMetaDataStreamInterceptor(
+	md map[string]string) grpc.StreamClientInterceptor {
+
+	return func(ctx context.Context, desc *grpc.StreamDesc,
+		cc *grpc.ClientConn, method string, streamer grpc.Streamer,
+		opts ...grpc.CallOption) (grpc.ClientStream, error) {
+
+		outCtx := contextWithMetadata(ctx, md)
+		return streamer(outCtx, desc, cc, method, opts...)
+	}
+}
+
+// contextWithMetaData appends the given metadata key-value pairs to the given
+// context.
+func contextWithMetadata(ctx context.Context,
+	md map[string]string) context.Context {
+
+	kvPairs := make([]string, 0, 2*len(md))
+	for k, v := range md {
+		kvPairs = append(kvPairs, k, v)
+	}
+
+	return metadata.AppendToOutgoingContext(ctx, kvPairs...)
+}
+
 // extractPathArgs parses the TLS certificate and macaroon paths from the
 // command.
 func extractPathArgs(ctx *cli.Context) (string, string, error) {
@@ -349,6 +402,14 @@ func main() {
 				"macaroon jar instead of the default one. " +
 				"Can only be used if profiles are defined.",
 		},
+		cli.StringSliceFlag{
+			Name: "metadata",
+			Usage: "This flag can be used to specify a key-value " +
+				"pair that should be appended to the " +
+				"outgoing context before the request is sent " +
+				"to lnd. This flag may be specified multiple " +
+				"times. The format is: \"key:value\".",
+		},
 	}
 	app.Commands = []cli.Command{
 		createCommand,