feature: add SetBit helper to set dependent bits

This commit adds a new SetBit helper function in the features package
along with a test for it.  SetBit sets the given feature bit on the
given feature bit vector along with any of its dependencies. If the bit
is required, then all the dependencies are also set to required,
otherwise, the optional dependency bits are set. Existing bits are only
upgraded from optional to required but never downgraded from required to
optional.

feature/deps.go

@@ -98,6 +98,58 @@ func ValidateDeps(fv *lnwire.FeatureVector) error {
 	return validateDeps(features, supported)
 }
 
+// SetBit sets the given feature bit on the given feature bit vector along with
+// any of its dependencies. If the bit is required, then all the dependencies
+// are also set to required, otherwise, the optional dependency bits are set.
+// Existing bits are only upgraded from optional to required but never
+// downgraded from required to optional.
+func SetBit(vector *lnwire.FeatureVector,
+	bit lnwire.FeatureBit) *lnwire.FeatureVector {
+
+	fv := vector.Clone()
+
+	// Get the optional version of the bit since that is what the deps map
+	// uses.
+	optBit := mapToOptional(bit)
+
+	// If the bit we are setting is optional, then we set it (in its
+	// optional form) and also set all its dependents as optional if they
+	// are not already set (they may already be set in a required form in
+	// which case they should not be overridden).
+	if !bit.IsRequired() {
+		// Set the bit itself if it does not already exist. We use
+		// SafeSet here so that if the bit already exists in the
+		// required form, then this is not overwritten.
+		_ = fv.SafeSet(bit)
+
+		// Do the same for all the dependent bits.
+		for depBit := range deps[optBit] {
+			fv = SetBit(fv, depBit)
+		}
+
+		return fv
+	}
+
+	// The bit is required. In this case, we do want to override any
+	// existing optional bit for both the bit itself and for the dependent
+	// bits.
+	fv.Unset(optBit)
+	fv.Set(bit)
+
+	// Do the same for all the dependent bits.
+	for depBit := range deps[optBit] {
+		// The deps map only contains the optional versions of bits, so
+		// there is no need to first map the bit to the optional
+		// version.
+		fv.Unset(depBit)
+
+		// Set the required version of the bit instead.
+		fv = SetBit(fv, mapToRequired(depBit))
+	}
+
+	return fv
+}
+
 // validateDeps is a subroutine that recursively checks that the passed features
 // have all of their associated dependencies in the supported map.
 func validateDeps(features featureSet, supported supportedFeatures) error {
@@ -157,3 +209,13 @@ func mapToOptional(bit lnwire.FeatureBit) lnwire.FeatureBit {
 	}
 	return bit
 }
+
+// mapToRequired returns the required variant of a given feature bit pair.
+func mapToRequired(bit lnwire.FeatureBit) lnwire.FeatureBit {
+	if bit.IsRequired() {
+		return bit
+	}
+	bit ^= 0x01
+
+	return bit
+}