fix(frontend/config): non-depth link selectors should not be fused af…

…ter rejection

pkg/frontend/reader/merge/merge.go

@@ -110,8 +110,8 @@ func (fl *followLinkPool[M]) scheduleFrom(obj *object[M], followLimit *atomic.In
 			parentKey, childKey, parentIsSource = link.Key, obj.key, false
 		}
 
-		subSelector := linkSelector.Admit(parentKey, childKey, parentIsSource, link.Class)
-		if subSelector != nil {
+		isAdmitted, subSelector := linkSelector.Admit(parentKey, childKey, parentIsSource, link.Class)
+		if isAdmitted {
 			admittedLinks = append(admittedLinks, link)
 			fl.knownKeys.Insert(link.Key)
 			fl.schedule(link.Key, subSelector, followLimit, int32(fl.endTime.Sub(fl.startTime)/(time.Minute*30)))

pkg/frontend/tf/config/link_selector.go

@@ -19,9 +19,19 @@ import utilobject "github.com/kubewharf/kelemetry/pkg/util/object"
 type LinkSelector interface {
 	// Whether to follow the given link.
 	//
-	// If link should be followed, return a non-nil LinkSelector.
-	// The returned object will be used to recursively follow links in the linked object.
-	Admit(parent utilobject.Key, child utilobject.Key, parentIsSource bool, linkClass string) LinkSelector
+	// The first output indicates whether the selector admits this link.
+	// The second output is the selector state that transitive links should use for admission.
+	// The second output may still be useful even if the first output is false when it is part of a UnionLinkSelector.
+	// A nil state "fuses" the selector, i.e. always return false for further transitive links.
+	Admit(parent utilobject.Key, child utilobject.Key, parentIsSource bool, linkClass string) (_admit bool, _nextState LinkSelector)
+}
+
+func nilableLinkSelectorAdmit(selector LinkSelector, parent utilobject.Key, child utilobject.Key, parentIsSource bool, linkClass string) (bool, LinkSelector) {
+	if selector == nil {
+		return false, nil
+	}
+
+	return selector.Admit(parent, child, parentIsSource, linkClass)
 }
 
 type ConstantLinkSelector bool
@@ -31,12 +41,12 @@ func (selector ConstantLinkSelector) Admit(
 	child utilobject.Key,
 	parentIsSource bool,
 	linkClass string,
-) LinkSelector {
+) (_admit bool, _nextState LinkSelector) {
 	if selector {
-		return selector
+		return true, selector
 	}
 
-	return nil
+	return false, selector
 }
 
 type IntersectLinkSelector []LinkSelector
@@ -46,17 +56,17 @@ func (selector IntersectLinkSelector) Admit(
 	childKey utilobject.Key,
 	parentIsSource bool,
 	linkClass string,
-) LinkSelector {
-	newChildren := make([]LinkSelector, len(selector))
-
-	for i, child := range selector {
-		newChildren[i] = child.Admit(parentKey, childKey, parentIsSource, linkClass)
-		if newChildren[i] == nil {
-			return nil
-		}
+) (_admit bool, _nextState LinkSelector) {
+	admit := true
+	nextMemberStates := make([]LinkSelector, len(selector))
+
+	for i, member := range selector {
+		memberAdmit, nextMemberState := nilableLinkSelectorAdmit(member, parentKey, childKey, parentIsSource, linkClass)
+		admit = admit && memberAdmit
+		nextMemberStates[i] = nextMemberState
 	}
 
-	return IntersectLinkSelector(newChildren)
+	return admit, IntersectLinkSelector(nextMemberStates)
 }
 
 type UnionLinkSelector []LinkSelector
@@ -66,22 +76,15 @@ func (selector UnionLinkSelector) Admit(
 	childKey utilobject.Key,
 	parentIsSource bool,
 	linkClass string,
-) LinkSelector {
-	newChildren := make([]LinkSelector, len(selector))
-
-	ok := false
-	for i, child := range selector {
-		if child != nil {
-			newChildren[i] = child.Admit(parentKey, childKey, parentIsSource, linkClass)
-			if newChildren[i] != nil {
-				ok = true
-			}
-		}
-	}
+) (_admit bool, _nextState LinkSelector) {
+	admit := false
+	nextMemberStates := make([]LinkSelector, len(selector))
 
-	if ok {
-		return UnionLinkSelector(newChildren)
+	for i, member := range selector {
+		memberAdmit, nextMemberState := nilableLinkSelectorAdmit(member, parentKey, childKey, parentIsSource, linkClass)
+		admit = admit || memberAdmit
+		nextMemberStates[i] = nextMemberState
 	}
 
-	return nil
+	return admit, UnionLinkSelector(nextMemberStates)
 }

pkg/frontend/tf/defaults/modifier/link_selector.go

@@ -146,29 +146,27 @@ func (s denySiblingsLinkSelector) Admit(
 	child utilobject.Key,
 	isFromParent bool,
 	linkClass string,
-) tfconfig.LinkSelector {
+) (_admit bool, _nextState tfconfig.LinkSelector) {
 	if !s.hasFirst {
-		return denySiblingsLinkSelector{hasFirst: true, firstIsFromParent: isFromParent}
+		return true, denySiblingsLinkSelector{hasFirst: true, firstIsFromParent: isFromParent}
 	}
-	if s.firstIsFromParent != isFromParent {
-		return nil
-	}
-	return s
+
+	return s.firstIsFromParent == isFromParent, s
 }
 
 // The path from queried object to any other object in the tree must only contain links matching this pattern.
 type patternLinkSelector struct {
 	patterns []LinkPattern
 }
 
-func (s patternLinkSelector) Admit(parent utilobject.Key, child utilobject.Key, isFromParent bool, linkClass string) tfconfig.LinkSelector {
+func (s patternLinkSelector) Admit(parent utilobject.Key, child utilobject.Key, isFromParent bool, linkClass string) (_admit bool, _nextState tfconfig.LinkSelector) {
 	for _, pattern := range s.patterns {
 		if !pattern.Matches(parent, child, isFromParent, linkClass) {
-			return nil
+			return false, s
 		}
 	}
 
-	return s
+	return true, s
 }
 
 type direction bool
@@ -189,14 +187,16 @@ func (d directedDistanceLinkSelector) Admit(
 	child utilobject.Key,
 	isFromParent bool,
 	linkClass string,
-) tfconfig.LinkSelector {
+) (_admit bool, _nextState tfconfig.LinkSelector) {
 	if isFromParent != (d.direction == directionDownwards) {
-		return d
+		return true, d
 	}
+
 	if d.distance == 0 {
-		return nil
+		return false, nil
 	}
-	return directedDistanceLinkSelector{
+
+	return true, directedDistanceLinkSelector{
 		direction: d.direction,
 		distance:  d.distance - 1,
 	}