validateSchema: validate Input Objects self-references

src/type/__tests__/validation-test.js

@@ -728,6 +728,120 @@ describe('Type System: Input Objects must have fields', () => {
     ]);
   });
 
+  it('accepts an Input Object with breakable circular reference', () => {
+    const schema = buildSchema(`
+      type Query {
+        field(arg: SomeInputObject): String
+      }
+
+      input SomeInputObject {
+        self: SomeInputObject
+        arrayOfSelf: [SomeInputObject]
+        nonNullArrayOfSelf: [SomeInputObject]!
+        nonNullArrayOfNonNullSelf: [SomeInputObject!]!
+        intermediateSelf: AnotherInputObject
+      }
+
+      input AnotherInputObject {
+        parent: SomeInputObject
+      }
+    `);
+
+    expect(validateSchema(schema)).to.deep.equal([]);
+  });
+
+  it('rejects an Input Object with non-breakable circular reference', () => {
+    const schema = buildSchema(`
+      type Query {
+        field(arg: SomeInputObject): String
+      }
+
+      input SomeInputObject {
+        nonNullSelf: SomeInputObject!
+      }
+    `);
+
+    expect(validateSchema(schema)).to.deep.equal([
+      {
+        message:
+          'Cannot reference Input Object "SomeInputObject" within itself through a series of non-null fields: "nonNullSelf".',
+        locations: [{ line: 7, column: 9 }],
+      },
+    ]);
+  });
+
+  it('rejects Input Objects with non-breakable circular reference spread across them', () => {
+    const schema = buildSchema(`
+      type Query {
+        field(arg: SomeInputObject): String
+      }
+
+      input SomeInputObject {
+        startLoop: AnotherInputObject!
+      }
+
+      input AnotherInputObject {
+        nextInLoop: YetAnotherInputObject!
+      }
+
+      input YetAnotherInputObject {
+        closeLoop: SomeInputObject!
+      }
+    `);
+
+    expect(validateSchema(schema)).to.deep.equal([
+      {
+        message:
+          'Cannot reference Input Object "SomeInputObject" within itself through a series of non-null fields: "startLoop.nextInLoop.closeLoop".',
+        locations: [
+          { line: 7, column: 9 },
+          { line: 11, column: 9 },
+          { line: 15, column: 9 },
+        ],
+      },
+    ]);
+  });
+
+  it('rejects Input Objects with multiple non-breakable circular reference', () => {
+    const schema = buildSchema(`
+      type Query {
+        field(arg: SomeInputObject): String
+      }
+
+      input SomeInputObject {
+        startLoop: AnotherInputObject!
+      }
+
+      input AnotherInputObject {
+        closeLoop: SomeInputObject!
+        startSecondLoop: YetAnotherInputObject!
+      }
+
+      input YetAnotherInputObject {
+        closeSecondLoop: AnotherInputObject!
+        nonNullSelf: YetAnotherInputObject!
+      }
+    `);
+
+    expect(validateSchema(schema)).to.deep.equal([
+      {
+        message:
+          'Cannot reference Input Object "SomeInputObject" within itself through a series of non-null fields: "startLoop.closeLoop".',
+        locations: [{ line: 7, column: 9 }, { line: 11, column: 9 }],
+      },
+      {
+        message:
+          'Cannot reference Input Object "AnotherInputObject" within itself through a series of non-null fields: "startSecondLoop.closeSecondLoop".',
+        locations: [{ line: 12, column: 9 }, { line: 16, column: 9 }],
+      },
+      {
+        message:
+          'Cannot reference Input Object "YetAnotherInputObject" within itself through a series of non-null fields: "nonNullSelf".',
+        locations: [{ line: 17, column: 9 }],
+      },
+    ]);
+  });
+
   it('rejects an Input Object type with incorrectly typed fields', () => {
     const schema = buildSchema(`
       type Query {

src/type/validate.js

@@ -228,6 +228,9 @@ function validateName(
 }
 
 function validateTypes(context: SchemaValidationContext): void {
+  const validateInputObjectCircularRefs = createInputObjectCircularRefsValidator(
+    context,
+  );
   const typeMap = context.schema.getTypeMap();
   for (const type of objectValues(typeMap)) {
     // Ensure all provided types are in fact GraphQL type.
@@ -262,6 +265,9 @@ function validateTypes(context: SchemaValidationContext): void {
     } else if (isInputObjectType(type)) {
       // Ensure Input Object fields are valid.
       validateInputFields(context, type);
+
+      // Ensure Input Objects do not contain non-nullable circular references
+      validateInputObjectCircularRefs(type);
     }
   }
 }
@@ -554,6 +560,59 @@ function validateInputFields(
   }
 }
 
+function createInputObjectCircularRefsValidator(
+  context: SchemaValidationContext,
+) {
+  // Modified copy of algorithm from 'src/validation/rules/NoFragmentCycles.js'.
+  // Tracks already visited types to maintain O(N) and to ensure that cycles
+  // are not redundantly reported.
+  const visitedTypes = Object.create(null);
+
+  // Array of types nodes used to produce meaningful errors
+  const fieldPath = [];
+
+  // Position in the type path
+  const fieldPathIndexByTypeName = Object.create(null);
+
+  return detectCycleRecursive;
+
+  // This does a straight-forward DFS to find cycles.
+  // It does not terminate when a cycle was found but continues to explore
+  // the graph to find all possible cycles.
+  function detectCycleRecursive(inputObj: GraphQLInputObjectType) {
+    if (visitedTypes[inputObj.name]) {
+      return;
+    }
+
+    visitedTypes[inputObj.name] = true;
+    fieldPathIndexByTypeName[inputObj.name] = fieldPath.length;
+
+    const fields = objectValues(inputObj.getFields());
+    for (const field of fields) {
+      if (isNonNullType(field.type) && isInputObjectType(field.type.ofType)) {
+        const fieldType = field.type.ofType;
+        const cycleIndex = fieldPathIndexByTypeName[fieldType.name];
+
+        fieldPath.push(field);
+        if (cycleIndex === undefined) {
+          detectCycleRecursive(fieldType);
+        } else {
+          const cyclePath = fieldPath.slice(cycleIndex);
+          const fieldNames = cyclePath.map(fieldObj => fieldObj.name);
+          context.reportError(
+            `Cannot reference Input Object "${fieldType.name}" within itself ` +
+              `through a series of non-null fields: "${fieldNames.join('.')}".`,
+            cyclePath.map(fieldObj => fieldObj.astNode),
+          );
+        }
+        fieldPath.pop();
+      }
+    }
+
+    fieldPathIndexByTypeName[inputObj.name] = undefined;
+  }
+}
+
 type SDLDefinedObject<T, K> = {
   +astNode: ?T,
   +extensionASTNodes?: ?$ReadOnlyArray<K>,