Fix validation of @oneOf input objects.

libraries/apollo-ast/src/commonMain/kotlin/com/apollographql/apollo/ast/internal/SchemaValidationScope.kt

@@ -827,16 +827,209 @@ private fun ValidationScope.validateScalars() {
   }
 }
 
+internal class FieldAndNode(val field: GQLInputValueDefinition, val node: Node)
+
+internal class Node(val typeDefinition: GQLInputObjectTypeDefinition) {
+  val isOneOf = typeDefinition.directives.findOneOf()
+
+  /**
+   * Whether that node is valid (can reach a leaf node)
+   * This will be updated as we traverse the graph
+   */
+  var isValid = false
+
+  /**
+   * Whether that node is visited
+   */
+  var visited = false
+  var edgeCount = 0
+  val predecessors = mutableSetOf<Node>()
+  val sucessors = mutableSetOf<FieldAndNode>()
+
+  /**
+   * tarjan
+   */
+  var index: Int? = null
+  var lowLink: Int? = null
+  var onStack = false
+
+  override fun toString() = typeDefinition.name
+}
+
+private fun ValidationScope.reverseGraph(inputObjectTypeDefinitions: List<GQLInputObjectTypeDefinition>): MutableCollection<Node> {
+  val nodes = mutableMapOf<String, Node>()
+  inputObjectTypeDefinitions.forEach {
+    nodes.put(it.name, Node(it))
+  }
+
+  nodes.values.forEach { node ->
+    /**
+     * Track the leaf fields.
+     * - `@oneOf` are not valid by default but may become if they have one escape hatch.
+     * - other types are valid by default but may become invalid if they have one non-null reference
+     */
+    node.isValid = !node.isOneOf
+    node.typeDefinition.inputFields.forEach { field ->
+      val fieldType = field.type
+      if (node.isOneOf) {
+        if (fieldType is GQLNamedType) {
+          val fieldTypeDefinition = typeDefinitions.get(fieldType.name)
+          if (fieldTypeDefinition is GQLInputObjectTypeDefinition) {
+            val successor = nodes.get(fieldTypeDefinition.name)!!
+            successor.predecessors.add(node)
+            node.sucessors.add(FieldAndNode(field, successor))
+          } else {
+            // scalar or enum
+            node.isValid  = true
+          }
+        } else {
+          // Maybe a list
+          // Should not be non-null. If it is, other validation rules will catch it.
+          node.isValid = true
+        }
+      } else {
+        if (fieldType is GQLNonNullType) {
+          val innerType = fieldType.type
+          if (innerType is GQLNamedType) {
+            val fieldTypeDefinition = typeDefinitions.get(innerType.name)
+            if (fieldTypeDefinition is GQLInputObjectTypeDefinition) {
+              // Not a leaf field
+              node.isValid = false
+              node.edgeCount++
+              val successor = nodes.get(fieldTypeDefinition.name)!!
+              successor.predecessors.add(node)
+              node.sucessors.add(FieldAndNode(field, successor))
+            }
+          } else {
+            // List type => escape
+          }
+        } else {
+          // Nullable type => escape
+        }
+      }
+    }
+  }
+  return nodes.values
+}
+
+/**
+ * walks the reverse graph, starting with the leaf nodes to find all the valid nodes
+ */
+private fun findValid(nodes: Collection<Node>) {
+  val stack = ArrayDeque<Node>()
+  // Start with the leaf, non-oneOf types
+  stack.addAll(nodes.filter { it.isValid })
+
+  while (stack.isNotEmpty()) {
+    val node = stack.removeFirst()
+    if (node.visited) continue
+    node.visited = true
+    node.predecessors.forEach { predecessor ->
+      if (predecessor.isOneOf) {
+        predecessor.isValid = true
+        stack.addAll(predecessor.predecessors)
+      } else {
+        predecessor.edgeCount--
+        if (predecessor.edgeCount == 0) {
+          predecessor.isValid = true
+          stack.addAll(predecessor.predecessors)
+        }
+      }
+    }
+  }
+}
+
+private fun removeValid(nodes: MutableCollection<Node>) {
+  nodes.removeAll { it.isValid }
+  // At this point, there shouldn't be any edge pointing to a valid node or that would be an escape input field
+//  nodes.forEach {
+//    check(it.sucessors.none { it.node.isValid })
+//  }
+}
+
+internal class PathElement(val typename: String, val inputField: GQLInputValueDefinition)
+internal typealias Scc = Collection<Node>
+
+/**
+ * For error reporting purposes, find the longest cycle inside the SCC
+ */
+private fun findWitnessCycle(scc: Collection<Node>): List<PathElement> {
+  val start = scc.first()
+
+  val path = mutableListOf<PathElement>()
+  val visited = mutableSetOf<Node>()
+
+  fun dfs(current: Node): Boolean {
+    visited.add(current)
+    for (fieldAndNode in current.sucessors) {
+      val next = fieldAndNode.node
+      path.add(PathElement(current.typeDefinition.name, fieldAndNode.field))
+      if (next == start) return true
+      if (next !in visited && dfs(next)) return true
+      path.removeAt(path.lastIndex)
+    }
+    visited.remove(current)
+    return false
+  }
+
+  dfs(start)
+  return path
+}
+
+internal fun tarjanScc(nodes: Collection<Node>): Collection<Scc> {
+  var index = 0
+  val stack = ArrayDeque<Node>()
+  val result = mutableListOf<Scc>()
+
+  fun strongConnect(v: Node) {
+    v.index = index
+    v.lowLink = index
+    index++
+    stack.addLast(v)
+    v.onStack = true
+
+    v.sucessors.forEach {
+      val w = it.node
+      if (w.index == null) {
+        strongConnect(w)
+        v.lowLink = minOf(v.lowLink!!, w.lowLink!!)
+      } else if (w.onStack) {
+        v.lowLink = minOf(v.lowLink!!, w.index!!)
+      }
+    }
+
+    if (v.lowLink == v.index) {
+      val scc = mutableListOf<Node>()
+      while (true) {
+        val w = stack.removeLast()
+        w.onStack = false
+        scc.add(w)
+        if (w == v) break
+      }
+      result.add(scc)
+    }
+  }
+
+  nodes.forEach {
+    if (it.index == null) {
+      strongConnect(it)
+    }
+  }
+
+  return result
+}
+
 private fun ValidationScope.validateInputObjects() {
-  val traversalState = TraversalState()
+  val inputObjects = typeDefinitions.values.filterIsInstance<GQLInputObjectTypeDefinition>()
+  validateInputObjectsCycles(inputObjects)
+
   val defaultValueTraversalState = DefaultValueTraversalState()
-  typeDefinitions.values.filterIsInstance<GQLInputObjectTypeDefinition>().forEach { o ->
+  inputObjects.forEach { o ->
     if (o.inputFields.isEmpty()) {
       registerIssue("Input object must specify one or more input fields", o.sourceLocation)
     }
 
     validateDirectivesInConstContext(o.directives, o)
-    validateInputFieldCycles(o, traversalState)
     validateInputObjectDefaultValue(o, defaultValueTraversalState)
 
     val isOneOfInputObject = o.directives.findOneOf()
@@ -853,65 +1046,46 @@ private fun ValidationScope.validateInputObjects() {
   }
 }
 
-private class TraversalState {
-  val visitedTypes = mutableSetOf<String>()
-  val fieldPath = mutableListOf<Pair<String, SourceLocation?>>()
-  val fieldPathIndexByTypeName = mutableMapOf<String, Int>()
-}
-
-private class DefaultValueTraversalState {
-  val visitedFields = mutableSetOf<String>()
-  val fieldPath = mutableListOf<Pair<String, SourceLocation?>>()
-  val fieldPathIndex = mutableMapOf<String, Int>()
-}
-
-
-private fun ValidationScope.validateInputFieldCycles(inputObjectTypeDefinition: GQLInputObjectTypeDefinition, state: TraversalState) {
-  if (state.visitedTypes.contains(inputObjectTypeDefinition.name)) {
-    return
-  }
-  state.visitedTypes.add(inputObjectTypeDefinition.name)
-
-  state.fieldPathIndexByTypeName[inputObjectTypeDefinition.name] = state.fieldPath.size
-
-  inputObjectTypeDefinition.inputFields.forEach {
-    val type = it.type
-    if (type is GQLNonNullType && type.type is GQLNamedType) {
-      val fieldType = typeDefinitions.get(type.type.name)
-      if (fieldType is GQLInputObjectTypeDefinition) {
-        val cycleIndex = state.fieldPathIndexByTypeName.get(fieldType.name)
-
-        state.fieldPath.add("${fieldType.name}.${it.name}" to it.sourceLocation)
-
-        if (cycleIndex == null) {
-          validateInputFieldCycles(fieldType, state)
-        } else {
-          val cyclePath = state.fieldPath.subList(cycleIndex, state.fieldPath.size)
-
-          cyclePath.forEach {
-            issues.add(
-                OtherValidationIssue(
-                    buildString {
-                      append("Invalid circular reference. The Input Object '${fieldType.name}' references itself ")
-                      if (cyclePath.size > 1) {
-                        append("via the non-null fields: ")
-                      } else {
-                        append("in the non-null field ")
-                      }
-                      append(cyclePath.map { it.first }.joinToString(", "))
-                    },
-                    it.second
-                )
-            )
+private fun ValidationScope.validateInputObjectsCycles(inputObjectTypeDefinitions: List<GQLInputObjectTypeDefinition>) {
+  val nodes = reverseGraph(inputObjectTypeDefinitions)
+  findValid(nodes)
+  removeValid(nodes)
+  tarjanScc(nodes).forEach { scc ->
+    if (scc.size == 1) {
+      val firstNode = scc.first()
+      val fieldAndNode = firstNode.sucessors.firstOrNull()
+      if (fieldAndNode != null && fieldAndNode.node.typeDefinition.name == firstNode.typeDefinition.name) {
+        registerIssue("Input object `${firstNode.typeDefinition.name}` references itself through field `${firstNode.typeDefinition.name}.${fieldAndNode.field.name}` and cannot be constructed.", fieldAndNode.field.sourceLocation)
+      } else {
+        // Trivial SCC containing a single, non self-referncing node are not an issue.
+      }
+    } else {
+      val cycle = findWitnessCycle(scc)
+      cycle.indices.forEach { i ->
+        val start = cycle.get(i)
+        val cycleAsString = buildString {
+          var j = i
+          repeat(cycle.size) {
+            val cur = cycle.get(j)
+            append("${cur.typename}.${cur.inputField.name} --> ")
+            j++
+            if (j == cycle.size) {
+              j = 0
+            }
           }
+          append(start.typename)
         }
-
-        state.fieldPath.removeLast()
+        registerIssue("Input object `${start.typename}` references itself through an unbreakable chain of input fields and cannot be constructed: $cycleAsString", start.inputField.sourceLocation)
       }
     }
   }
+}
 
-  state.fieldPathIndexByTypeName.remove(inputObjectTypeDefinition.name)
+
+private class DefaultValueTraversalState {
+  val visitedFields = mutableSetOf<String>()
+  val fieldPath = mutableListOf<Pair<String, SourceLocation?>>()
+  val fieldPathIndex = mutableMapOf<String, Int>()
 }
 
 private fun ValidationScope.validateInputObjectDefaultValue(

libraries/apollo-ast/src/jvmTest/kotlin/com/apollographql/apollo/graphql/ast/test/TarjanTest.kt

@@ -0,0 +1,62 @@
+package com.apollographql.apollo.graphql.ast.test
+
+import com.apollographql.apollo.ast.GQLField
+import com.apollographql.apollo.ast.GQLInputObjectTypeDefinition
+import com.apollographql.apollo.ast.GQLInputValueDefinition
+import com.apollographql.apollo.ast.GQLNamedType
+import com.apollographql.apollo.ast.internal.FieldAndNode
+import com.apollographql.apollo.ast.internal.Node
+import com.apollographql.apollo.ast.internal.tarjanScc
+import kotlin.test.Test
+
+class TarjanTest {
+  fun typeDefinition(name: String) = GQLInputObjectTypeDefinition(
+      sourceLocation = null,
+      description = "",
+      name = name,
+      directives = emptyList(),
+      inputFields = emptyList()
+  )
+  val field = GQLInputValueDefinition(
+      sourceLocation = null,
+      name = "",
+      directives = emptyList(),
+      description = "",
+      type = GQLNamedType(null, ""),
+      defaultValue = null,
+  )
+
+  internal fun node(name: String) = Node(typeDefinition(name)).apply { isValid = false }
+
+  @Test
+  fun test1() {
+    val a = node("a")
+    val b = node("b")
+    val c = node("c")
+
+    a.sucessors.add(FieldAndNode(field, b))
+    b.sucessors.add(FieldAndNode(field, c))
+    c.sucessors.add(FieldAndNode(field, b))
+
+    val sccs = tarjanScc(listOf(a, b, c))
+    println(sccs)
+  }
+
+  @Test
+  fun test2() {
+    val a = node("a")
+    val b = node("b")
+    val c = node("c")
+    val d = node("d")
+
+    a.sucessors.add(FieldAndNode(field, b))
+    a.sucessors.add(FieldAndNode(field, d))
+    b.sucessors.add(FieldAndNode(field, c))
+    c.sucessors.add(FieldAndNode(field, b))
+    d.sucessors.add(FieldAndNode(field, a))
+
+    val sccs = tarjanScc(listOf(a, b, c))
+    println(sccs)
+  }
+
+}

libraries/apollo-ast/test-fixtures/validation/schema/input-cycles0.graphqls

@@ -2,7 +2,7 @@ type Query {
     field(arg: SomeInputObject): String
 }
 
-# simple cycle
+# Input object `SomeInputObject` references itself through field `SomeInputObject.nonNullSelf` and cannot be constructed.
 input SomeInputObject {
     nonNullSelf: SomeInputObject!
 }