feature: add new feature pkg to manage feature sets

This commit introduces a feature.Manager, which derives feature vectors
for various contexts within the daemon. The sets can be described via a
staticly compiled format, which makes any runtime adjustments to the
feature sets when the manager is initialized.

feature/manager.go

@@ -0,0 +1,97 @@
+package feature
+
+import (
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+// Config houses any runtime modifications to the default set descriptors. For
+// our purposes, this typically means disabling certain features to test legacy
+// protocol interoperability or functionality.
+type Config struct {
+	// NoTLVOnion unsets any optional or required TLVOnionPaylod bits from
+	// all feature sets.
+	NoTLVOnion bool
+
+	// NoStaticRemoteKey unsets any optional or required StaticRemoteKey
+	// bits from all feature sets.
+	NoStaticRemoteKey bool
+}
+
+// Manager is responsible for generating feature vectors for different requested
+// feature sets.
+type Manager struct {
+	// fsets is a static map of feature set to raw feature vectors. Requests
+	// are fulfilled by cloning these interal feature vectors.
+	fsets map[Set]*lnwire.RawFeatureVector
+}
+
+// NewManager creates a new feature Manager, applying any custom modifications
+// to its feature sets before returning.
+func NewManager(cfg Config) (*Manager, error) {
+	return newManager(cfg, defaultSetDesc)
+}
+
+// newManager creates a new feeature Manager, applying any custom modifications
+// to its feature sets before returning. This method accepts the setDesc as its
+// own parameter so that it can be unit tested.
+func newManager(cfg Config, desc setDesc) (*Manager, error) {
+	// First build the default feature vector for all known sets.
+	fsets := make(map[Set]*lnwire.RawFeatureVector)
+	for bit, sets := range desc {
+		for set := range sets {
+			// Fetch the feature vector for this set, allocating a
+			// new one if it doesn't exist.
+			fv, ok := fsets[set]
+			if !ok {
+				fv = lnwire.NewRawFeatureVector()
+			}
+
+			// Set the configured bit on the feature vector,
+			// ensuring that we don't set two feature bits for the
+			// same pair.
+			err := fv.SafeSet(bit)
+			if err != nil {
+				return nil, fmt.Errorf("unable to set "+
+					"%v in %v: %v", bit, set, err)
+			}
+
+			// Write the updated feature vector under its set.
+			fsets[set] = fv
+		}
+	}
+
+	// Now, remove any features as directed by the config.
+	for _, fv := range fsets {
+		if cfg.NoTLVOnion {
+			fv.Unset(lnwire.TLVOnionPayloadOptional)
+			fv.Unset(lnwire.TLVOnionPayloadRequired)
+		}
+		if cfg.NoStaticRemoteKey {
+			fv.Unset(lnwire.StaticRemoteKeyOptional)
+			fv.Unset(lnwire.StaticRemoteKeyRequired)
+		}
+	}
+
+	return &Manager{
+		fsets: fsets,
+	}, nil
+}
+
+// GetRaw returns a raw feature vector for the passed set. If no set is known,
+// an empty raw feature vector is returned.
+func (m *Manager) GetRaw(set Set) *lnwire.RawFeatureVector {
+	if fv, ok := m.fsets[set]; ok {
+		return fv.Clone()
+	}
+
+	return lnwire.NewRawFeatureVector()
+}
+
+// Get returns a feature vector for the passed set. If no set is known, an empty
+// feature vector is returned.
+func (m *Manager) Get(set Set) *lnwire.FeatureVector {
+	raw := m.GetRaw(set)
+	return lnwire.NewFeatureVector(raw, lnwire.Features)
+}