Add missing query graph edge for interfaces implementing interfaces (#…

…3122)

During `addAdditionalAbstractTypeEdges()`, we add edges between abstract
types, but we specifically were skipping edges between interfaces
implementing interfaces because that function expected it to be added
already by `addAbstractTypeEdges()`. However, the latter function only
handles edges between concrete (object) and abstract types. This PR
accordingly updates `addAdditionalAbstractTypeEdges()` to stop skipping
interfaces-implementing-interfaces edges, and updates tests accordingly.

.changeset/cuddly-badgers-sit.md

@@ -0,0 +1,6 @@
+---
+"@apollo/gateway": patch
+"@apollo/query-graphs": patch
+---
+
+Avoid type explosion for inline fragments where the type condition is an interface that implements the parent type.

gateway-js/src/__tests__/executeQueryPlan.test.ts

@@ -2300,22 +2300,14 @@ describe('executeQueryPlan', () => {
 
       queryPlan = buildPlan(operation, queryPlanner);
 
-      // TODO: we're actually type-exploding in this case because currently, as soon as we need to type-explode, we do
-      // so into all the runtime types, while here it could make sense to only type-explode into the direct sub-types=
-      // (the sub-interfaces). We should fix this (but it's only sub-optimal, not incorrect).
       expect(queryPlan).toMatchInlineSnapshot(`
         QueryPlan {
           Fetch(service: "S1") {
             {
               allValues {
                 __typename
-                ... on T1 {
-                  a
-                }
-                ... on T2 {
-                  a
-                }
-                ... on T4 {
+                ... on SubInterface1 {
+                  __typename
                   a
                 }
               }

query-graphs-js/src/querygraph.ts

@@ -1611,12 +1611,6 @@ class GraphBuilderFromSchema extends GraphBuilder {
       for (let j = i; j < abstractTypesWithTheirRuntimeTypes.length; j++) {
         const t2 = abstractTypesWithTheirRuntimeTypes[j];
 
-        // We ignore the pair if both are interfaces and one implements the other. We'll already have appropriate
-        // edges if that's the case.
-        if (isInterfaceType(t1.type) && isInterfaceType(t2.type) && (t1.type.implementsInterface(t2.type) || t2.type.implementsInterface(t1.type))) {
-          continue;
-        }
-
         let addT1ToT2 = false;
         let addT2ToT1 = false;
         if (t1.type === t2.type) {