generateQueryFragments error with nested selection set (#3043)

Fixes #3042 

generateQueryFragments does't handle repeated (... on B { ... on B ...
on B } } }) fragments well. Given the query planner can actually
generate these query structures, this causes this option to be
potentially dangerous, generating invalid subgraph queries.

---------

Co-authored-by: Marc-Andre Giroux <mgiroux@netflix.com>

.changeset/calm-candles-rest.md

@@ -0,0 +1,5 @@
+---
+"@apollo/federation-internals": patch
+---
+
+generateQueryFragments() could generate fragments with naming collisions when nested

internals-js/src/__tests__/operations.test.ts

@@ -34,6 +34,66 @@ function astSSet(...selections: SelectionNode[]): SelectionSetNode {
   };
 }
 
+describe('generate query fragments', () => {
+  test('generateQueryFragments handles repeated fragment spreads', () => {
+    const schema = parseSchema(`
+      type Query {
+        entities: [Entity]
+      }
+
+      union Entity = A | B
+
+      type A {
+          a: Int
+      }
+
+      type B {
+          b: Int
+      }
+    `);
+
+    const operation = parseOperation(schema, `
+      query {
+          entities {
+              ... on B {
+                  ... on B {
+                      ... on B {
+                          ... on B {
+                              b
+                          }
+                      }
+                  }
+              }
+          }
+      }
+    `);
+
+    const withGeneratedFragments = operation.generateQueryFragments();
+    console.log(withGeneratedFragments.toString());
+    expect(withGeneratedFragments.toString()).toMatchString(`
+      fragment _generated_onB1_0 on B {
+        ... on B {
+          b
+        }
+      }
+      
+      fragment _generated_onB1_1 on B {
+        ..._generated_onB1_0
+      }
+      
+      fragment _generated_onB1_2 on B {
+        ..._generated_onB1_1
+      }
+      
+      {
+        entities {
+          ..._generated_onB1_2
+        }
+      }
+    `);
+  });
+});
+
 describe('fragments optimization', () => {
   // Takes a query with fragments as inputs, expand all those fragments, and ensures that all the
   // fragments gets optimized back, and that we get back the exact same query.

internals-js/src/operations.ts

@@ -1594,16 +1594,17 @@ export class SelectionSet {
         // No match, so we need to create a new fragment. First, we minimize the
         // selection set before creating the fragment with it.
         const [minimizedSelectionSet] = selection.selectionSet.minimizeSelectionSet(namedFragments, seenSelections);
+        const updatedEquivalentSelectionSetCandidates = seenSelections.get(mockHashCode); // may have changed after previous statement
         const fragmentDefinition = new NamedFragmentDefinition(
           this.parentType.schema(),
-          `_generated_${mockHashCode}_${equivalentSelectionSetCandidates?.length ?? 0}`,
+          `_generated_${mockHashCode}_${updatedEquivalentSelectionSetCandidates?.length ?? 0}`,
           selection.element.typeCondition
         ).setSelectionSet(minimizedSelectionSet);
         namedFragments.add(fragmentDefinition);
 
         // Create a new "hash code" bucket or add to the existing one.
-        if (equivalentSelectionSetCandidates) {
-          equivalentSelectionSetCandidates.push([selection.selectionSet, fragmentDefinition]);
+        if (updatedEquivalentSelectionSetCandidates) {
+          updatedEquivalentSelectionSetCandidates.push([selection.selectionSet, fragmentDefinition]);
         } else {
             seenSelections.set(mockHashCode, [[selection.selectionSet, fragmentDefinition]]);
         }