lsat: extract runtime info into interceptContext

To make the code easier to be reused for the upcoming stream
interceptor, all generic information about the interceptor's context
is extracted into a struct and the helper methods are adapted to
work with that struct.

lsat/interceptor.go

@@ -89,6 +89,15 @@ func NewInterceptor(lnd *lndclient.LndServices, store Store,
 	}
 }
 
+// interceptContext is a struct that contains all information about a call that
+// is intercepted by the interceptor.
+type interceptContext struct {
+	mainCtx  context.Context
+	opts     []grpc.CallOption
+	metadata *metadata.MD
+	token    *Token
+}
+
 // UnaryInterceptor is an interceptor method that can be used directly by gRPC
 // for unary calls. If the store contains a token, it is attached as credentials
 // to every call before patching it through. The response error is also
@@ -105,21 +114,54 @@ func (i *Interceptor) UnaryInterceptor(ctx context.Context, method string,
 	i.lock.Lock()
 	defer i.lock.Unlock()
 
-	addLsatCredentials := func(token *Token) error {
-		macaroon, err := token.PaidMacaroon()
-		if err != nil {
-			return err
-		}
-		opts = append(opts, grpc.PerRPCCredentials(
-			macaroons.NewMacaroonCredential(macaroon),
-		))
-		return nil
+	// Create the context that we'll use to initiate the real request. This
+	// contains the means to extract response headers and possibly also an
+	// auth token, if we already have paid for one.
+	iCtx, err := i.newInterceptContext(ctx, opts)
+	if err != nil {
+		return err
+	}
+
+	// Try executing the call now. If anything goes wrong, we only handle
+	// the LSAT error message that comes in the form of a gRPC status error.
+	rpcCtx, cancel := context.WithTimeout(ctx, i.callTimeout)
+	defer cancel()
+	err = invoker(rpcCtx, method, req, reply, cc, iCtx.opts...)
+	if !isPaymentRequired(err) {
+		return err
+	}
+
+	// Find out if we need to pay for a new token or perhaps resume
+	// a previously aborted payment.
+	err = i.handlePayment(iCtx)
+	if err != nil {
+		return err
+	}
+
+	// Execute the same request again, now with the LSAT
+	// token added as an RPC credential.
+	rpcCtx2, cancel2 := context.WithTimeout(ctx, i.callTimeout)
+	defer cancel2()
+	return invoker(rpcCtx2, method, req, reply, cc, iCtx.opts...)
+}
+
+// newInterceptContext creates the initial intercept context that can capture
+// metadata from the server and sends the local token to the server if one
+// already exists.
+func (i *Interceptor) newInterceptContext(ctx context.Context,
+	opts []grpc.CallOption) (*interceptContext, error) {
+
+	iCtx := &interceptContext{
+		mainCtx:  ctx,
+		opts:     opts,
+		metadata: &metadata.MD{},
 	}
 
 	// Let's see if the store already contains a token and what state it
 	// might be in. If a previous call was aborted, we might have a pending
 	// token that needs to be handled separately.
-	token, err := i.store.CurrentToken()
+	var err error
+	iCtx.token, err = i.store.CurrentToken()
 	switch {
 	// If there is no token yet, nothing to do at this point.
 	case err == ErrNoToken:
@@ -127,16 +169,18 @@ func (i *Interceptor) UnaryInterceptor(ctx context.Context, method string,
 	// Some other error happened that we have to surface.
 	case err != nil:
 		log.Errorf("Failed to get token from store: %v", err)
-		return fmt.Errorf("getting token from store failed: %v", err)
+		return nil, fmt.Errorf("getting token from store failed: %v",
+			err)
 
 	// Only if we have a paid token append it. We don't resume a pending
 	// payment just yet, since we don't even know if a token is required for
 	// this call. We also never send a pending payment to the server since
 	// we know it's not valid.
-	case !token.isPending():
-		if err = addLsatCredentials(token); err != nil {
+	case !iCtx.token.isPending():
+		if err = i.addLsatCredentials(iCtx); err != nil {
 			log.Errorf("Adding macaroon to request failed: %v", err)
-			return fmt.Errorf("adding macaroon failed: %v", err)
+			return nil, fmt.Errorf("adding macaroon failed: %v",
+				err)
 		}
 	}
 
@@ -145,60 +189,59 @@ func (i *Interceptor) UnaryInterceptor(ctx context.Context, method string,
 	// option. We execute the request and inspect the error. If it's the
 	// LSAT specific payment required error, we might execute the same
 	// method again later with the paid LSAT token.
-	trailerMetadata := &metadata.MD{}
-	opts = append(opts, grpc.Trailer(trailerMetadata))
-	rpcCtx, cancel := context.WithTimeout(ctx, i.callTimeout)
-	defer cancel()
-	err = invoker(rpcCtx, method, req, reply, cc, opts...)
-
-	// Only handle the LSAT error message that comes in the form of
-	// a gRPC status error.
-	if isPaymentRequired(err) {
-		paidToken, err := i.handlePayment(ctx, token, trailerMetadata)
-		if err != nil {
-			return err
-		}
-		if err = addLsatCredentials(paidToken); err != nil {
-			log.Errorf("Adding macaroon to request failed: %v", err)
-			return fmt.Errorf("adding macaroon failed: %v", err)
-		}
-
-		// Execute the same request again, now with the LSAT
-		// token added as an RPC credential.
-		rpcCtx2, cancel2 := context.WithTimeout(ctx, i.callTimeout)
-		defer cancel2()
-		return invoker(rpcCtx2, method, req, reply, cc, opts...)
-	}
-	return err
+	iCtx.opts = append(iCtx.opts, grpc.Trailer(iCtx.metadata))
+	return iCtx, nil
 }
 
 // handlePayment tries to obtain a valid token by either tracking the payment
 // status of a pending token or paying for a new one.
-func (i *Interceptor) handlePayment(ctx context.Context, token *Token,
-	md *metadata.MD) (*Token, error) {
-
+func (i *Interceptor) handlePayment(iCtx *interceptContext) error {
 	switch {
 	// Resume/track a pending payment if it was interrupted for some reason.
-	case token != nil && token.isPending():
+	case iCtx.token != nil && iCtx.token.isPending():
 		log.Infof("Payment of LSAT token is required, resuming/" +
 			"tracking previous payment from pending LSAT token")
-		err := i.trackPayment(ctx, token)
+		err := i.trackPayment(iCtx.mainCtx, iCtx.token)
 		if err != nil {
-			return nil, err
+			return err
 		}
-		return token, nil
 
 	// We don't have a token yet, try to get a new one.
-	case token == nil:
+	case iCtx.token == nil:
 		// We don't have a token yet, get a new one.
 		log.Infof("Payment of LSAT token is required, paying invoice")
-		return i.payLsatToken(ctx, md)
+		var err error
+		iCtx.token, err = i.payLsatToken(iCtx.mainCtx, iCtx.metadata)
+		if err != nil {
+			return err
+		}
 
 	// We have a token and it's valid, nothing more to do here.
 	default:
 		log.Debugf("Found valid LSAT token to add to request")
-		return token, nil
 	}
+
+	if err := i.addLsatCredentials(iCtx); err != nil {
+		log.Errorf("Adding macaroon to request failed: %v", err)
+		return fmt.Errorf("adding macaroon failed: %v", err)
+	}
+	return nil
+}
+
+// addLsatCredentials adds an LSAT token to the given intercept context.
+func (i *Interceptor) addLsatCredentials(iCtx *interceptContext) error {
+	if iCtx.token == nil {
+		return fmt.Errorf("cannot add nil token to context")
+	}
+
+	macaroon, err := iCtx.token.PaidMacaroon()
+	if err != nil {
+		return err
+	}
+	iCtx.opts = append(iCtx.opts, grpc.PerRPCCredentials(
+		macaroons.NewMacaroonCredential(macaroon),
+	))
+	return nil
 }
 
 // payLsatToken reads the payment challenge from the response metadata and tries