Minimal repro of S488223

Reviewed By: lynnshaoyu

Differential Revision: D69484406

fbshipit-source-id: 9bf41b6908ec80f8d6daf0c1bd12dc952ac56734

compiler/crates/relay-transforms/tests/validate_unused_variables/fixtures/cycle-bug.expected

@@ -0,0 +1,54 @@
+==================================== INPUT ====================================
+# expected-to-throw
+
+fragment FragmentC on User {
+  url(relative: true, site: $varX)
+  nearest_neighbor {
+    ...FragmentD
+  }
+}
+
+fragment FragmentD on User {
+  url(relative: true, site: $varY)
+  ...FragmentC
+}
+
+query QueryA($varX: String, $varY: String) {
+  me {
+    # By visiting FragmentC first, we incorrectly end up recording that
+    # FragmentD has only reads its own variable ($varY) and omit that it
+    # transitively reads $varX via its spread of FragmentC
+
+    # When we traverse into FragmentC we initialize the cache for FragmentC with
+    # an empty set of variables. We then traverse into FragmentD which
+    # initializes the cache with an empty set of variables for FragmentD.
+    #
+    # FragmentD then records its own read of $varY, and attempts to traverse
+    # into FragmentC. Here we hit the empty initialized cache for FragmentC.  As
+    # a result we _incorrectly_ treat the used variables for FragmentD as just:
+    # $varY.
+    #
+    # We now pop back up to FragmentC which correctly records its read variables
+    # as $varX and $varY, and the dependencies for QueryA are correctly validated.
+    #
+    # BUT! We've left behind an invalid cache record for FragmentD.
+    ...FragmentC
+  }
+}
+
+query QueryB($varX: String, $varY: String) {
+  me {
+    # Now as we traverse into FragmentD we get a cache hit (the invalid one left
+    # behind by our traversal of QueryA) and incorrectly conclude FragmentD only
+    # read $varY, and _incorrectly_ report $varX as unused by QueryB.
+    ...FragmentD
+  }
+}
+==================================== ERROR ====================================
+✖︎ Variable `$varX` is never used in operation `QueryB`
+
+  cycle-bug.graphql:38:14
+   37 │ 
+   38 │ query QueryB($varX: String, $varY: String) {
+      │              ^^^^^
+   39 │   me {

compiler/crates/relay-transforms/tests/validate_unused_variables/fixtures/cycle-bug.graphql

@@ -0,0 +1,45 @@
+# expected-to-throw
+
+fragment FragmentC on User {
+  url(relative: true, site: $varX)
+  nearest_neighbor {
+    ...FragmentD
+  }
+}
+
+fragment FragmentD on User {
+  url(relative: true, site: $varY)
+  ...FragmentC
+}
+
+query QueryA($varX: String, $varY: String) {
+  me {
+    # By visiting FragmentC first, we incorrectly end up recording that
+    # FragmentD has only reads its own variable ($varY) and omit that it
+    # transitively reads $varX via its spread of FragmentC
+
+    # When we traverse into FragmentC we initialize the cache for FragmentC with
+    # an empty set of variables. We then traverse into FragmentD which
+    # initializes the cache with an empty set of variables for FragmentD.
+    #
+    # FragmentD then records its own read of $varY, and attempts to traverse
+    # into FragmentC. Here we hit the empty initialized cache for FragmentC.  As
+    # a result we _incorrectly_ treat the used variables for FragmentD as just:
+    # $varY.
+    #
+    # We now pop back up to FragmentC which correctly records its read variables
+    # as $varX and $varY, and the dependencies for QueryA are correctly validated.
+    #
+    # BUT! We've left behind an invalid cache record for FragmentD.
+    ...FragmentC
+  }
+}
+
+query QueryB($varX: String, $varY: String) {
+  me {
+    # Now as we traverse into FragmentD we get a cache hit (the invalid one left
+    # behind by our traversal of QueryA) and incorrectly conclude FragmentD only
+    # read $varY, and _incorrectly_ report $varX as unused by QueryB.
+    ...FragmentD
+  }
+}

compiler/crates/relay-transforms/tests/validate_unused_variables_test.rs

@@ -4,14 +4,21 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<2d9c78832993b2bbe4c580b22d0aeecf>>
+ * @generated SignedSource<<129140cf3a271d88276cfcac0046937b>>
  */
 
 mod validate_unused_variables;
 
 use validate_unused_variables::transform_fixture;
 use fixture_tests::test_fixture;
 
+#[tokio::test]
+async fn cycle_bug() {
+    let input = include_str!("validate_unused_variables/fixtures/cycle-bug.graphql");
+    let expected = include_str!("validate_unused_variables/fixtures/cycle-bug.expected");
+    test_fixture(transform_fixture, file!(), "cycle-bug.graphql", "validate_unused_variables/fixtures/cycle-bug.expected", input, expected).await;
+}
+
 #[tokio::test]
 async fn fragment_with_root_arguments() {
     let input = include_str!("validate_unused_variables/fixtures/fragment-with-root-arguments.graphql");