lnd.xprv/feature/deps.go
2019-12-16 13:06:30 -08:00

125 lines
4.1 KiB
Go

package feature
import (
"fmt"
"github.com/lightningnetwork/lnd/lnwire"
)
type (
// featureSet contains a set of feature bits.
featureSet map[lnwire.FeatureBit]struct{}
// supportedFeatures maps the feature bit from a feature vector to a
// boolean indicating if this features dependencies have already been
// verified. This allows us to short circuit verification if multiple
// features have common dependencies, or map traversal starts verifying
// from the bottom up.
supportedFeatures map[lnwire.FeatureBit]bool
// depDesc maps a features to its set of dependent features, which must
// also be present for the vector to be valid. This can be used to
// recursively check the dependency chain for features in a feature
// vector.
depDesc map[lnwire.FeatureBit]featureSet
)
// ErrMissingFeatureDep is an error signaling that a transitive dependency in a
// feature vector is not set properly.
type ErrMissingFeatureDep struct {
dep lnwire.FeatureBit
}
// Error returns a human-readable description of the missing dep error.
func (e ErrMissingFeatureDep) Error() string {
return fmt.Sprintf("missing feature dependency: %v", e.dep)
}
// deps is the default set of dependencies for assigned feature bits. If a
// feature is not present in the depDesc it is assumed to have no dependencies.
//
// NOTE: For proper functioning, only the optional variant of feature bits
// should be used in the following descriptor. In the future it may be necessary
// to distinguish the dependencies for optional and required bits, but for now
// the validation code maps required bits to optional ones since it simplifies
// the number of constraints.
var deps = depDesc{
lnwire.PaymentAddrOptional: {
lnwire.TLVOnionPayloadOptional: {},
},
lnwire.MPPOptional: {
lnwire.PaymentAddrOptional: {},
},
}
// ValidateDeps asserts that a feature vector sets all features and their
// transitive dependencies properly. It assumes that the dependencies between
// optional and required features are identical, e.g. if a feature is required
// but its dependency is optional, that is sufficient.
func ValidateDeps(fv *lnwire.FeatureVector) error {
features := fv.Features()
supported := initSupported(features)
return validateDeps(features, supported)
}
// 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 {
for bit := range features {
// Convert any required bits to optional.
bit = mapToOptional(bit)
// If the supported features doesn't contain the dependency, this
// vector is invalid.
checked, ok := supported[bit]
if !ok {
return ErrMissingFeatureDep{bit}
}
// Alternatively, if we know that this depdendency is valid, we
// can short circuit and continue verifying other bits.
if checked {
continue
}
// Recursively validate dependencies, since this method ranges
// over the subDeps. This method will return true even if
// subDeps is nil.
subDeps := deps[bit]
if err := validateDeps(subDeps, supported); err != nil {
return err
}
// Once we've confirmed that this feature's dependencies, if
// any, are sound, we record this so other paths taken through
// `bit` return early when inspecting the supported map.
supported[bit] = true
}
return nil
}
// initSupported sets all bits from the feature vector as supported but not
// checked. This signals that the validity of their dependencies has not been
// verified. All required bits are mapped to optional to simplify the DAG.
func initSupported(features featureSet) supportedFeatures {
supported := make(supportedFeatures)
for bit := range features {
bit = mapToOptional(bit)
supported[bit] = false
}
return supported
}
// mapToOptional returns the optional variant of a given feature bit pair. Our
// dependendency graph is described using only optional feature bits, which
// reduces the number of constraints we need to express in the descriptor.
func mapToOptional(bit lnwire.FeatureBit) lnwire.FeatureBit {
if bit.IsRequired() {
bit ^= 0x01
}
return bit
}