feat: send to subgraphs the "fetch_reasons" extension (#2160)

Subgraphs now can now receive the "fetch_reasons" extension that
lists subgraphs and reasons why fields marked with
@openfed__requireFetchReasons directive were requested.
Example of the request to subgraph:

{
  "query": "{accounts {__typename ... on User {some {__typename id}}}}",
  "extensions": {
    "fetch_reasons": [
      {
        "typename": "User",
        "field": "id",
        "by_user": true,
        "by_subgraphs": ["id-2"],
        "is_requires": true
      }, {
        "typename": "User",
        "field": "some",
        "by_user": true
      }, {
        "typename": "Query",
        "field": "accounts",
        "by_user": true
      }
    ]
  }
}

This can be enabled via configuration of router:

engine:
  enable_require_fetch_reasons: true

Disabled by default. Always ignored for introspection queries.

Author: ysmolski

router-tests/go.mod

@@ -27,7 +27,7 @@ require (
 	github.com/wundergraph/cosmo/demo/pkg/subgraphs/projects v0.0.0-20250715110703-10f2e5f9c79e
 	github.com/wundergraph/cosmo/router v0.0.0-20250820135159-bf8852195d3f
 	github.com/wundergraph/cosmo/router-plugin v0.0.0-20250808194725-de123ba1c65e
-	github.com/wundergraph/graphql-go-tools/v2 v2.0.0-rc.224
+	github.com/wundergraph/graphql-go-tools/v2 v2.0.0-rc.225
 	go.opentelemetry.io/otel v1.36.0
 	go.opentelemetry.io/otel/sdk v1.36.0
 	go.opentelemetry.io/otel/sdk/metric v1.36.0

router-tests/go.sum

@@ -352,8 +352,8 @@ github.com/wundergraph/astjson v0.0.0-20250106123708-be463c97e083 h1:8/D7f8gKxTB
 github.com/wundergraph/astjson v0.0.0-20250106123708-be463c97e083/go.mod h1:eOTL6acwctsN4F3b7YE+eE2t8zcJ/doLm9sZzsxxxrE=
 github.com/wundergraph/consul/sdk v0.0.0-20250204115147-ed842a8fd301 h1:EzfKHQoTjFDDcgaECCCR2aTePqMu9QBmPbyhqIYOhV0=
 github.com/wundergraph/consul/sdk v0.0.0-20250204115147-ed842a8fd301/go.mod h1:wxI0Nak5dI5RvJuzGyiEK4nZj0O9X+Aw6U0tC1wPKq0=
-github.com/wundergraph/graphql-go-tools/v2 v2.0.0-rc.224 h1:HWxOmN7GK3iccm1JhMLlhDNVbG42EJw6QI8Qx857hhQ=
-github.com/wundergraph/graphql-go-tools/v2 v2.0.0-rc.224/go.mod h1:DnYY1alnsgzkanSwbFiFIdXKOuf8dHQWQ2P4BzTc6aI=
+github.com/wundergraph/graphql-go-tools/v2 v2.0.0-rc.225 h1:D7VbIl1P6/6Zj86HNiO7UmH05DDFnFLVA/ig31+Uy5c=
+github.com/wundergraph/graphql-go-tools/v2 v2.0.0-rc.225/go.mod h1:g1IFIylu5Fd9pKjzq0mDvpaKhEB/vkwLAIbGdX2djXU=
 github.com/xrash/smetrics v0.0.0-20240521201337-686a1a2994c1 h1:gEOO8jv9F4OT7lGCjxCBTO/36wtF6j2nSip77qHd4x4=
 github.com/xrash/smetrics v0.0.0-20240521201337-686a1a2994c1/go.mod h1:Ohn+xnUBiLI6FVj/9LpzZWtj1/D6lUovWYBkxHVV3aM=
 github.com/yosida95/uritemplate/v3 v3.0.2 h1:Ed3Oyj9yrmi9087+NczuL5BwkIc4wvTb5zIM+UJPGz4=

router-tests/integration_test.go

@@ -832,6 +832,42 @@ func TestVariablesRemapping(t *testing.T) {
 	})
 }
 
+func TestEnableRequireFetchReasons(t *testing.T) {
+	t.Parallel()
+
+	// Simple test to verify that the configuration switch works.
+	// Multi subgraphs calls are tested in the engine.
+	testenv.Run(t, &testenv.Config{
+		RouterConfigJSONTemplate: testenv.ConfigWithRequireFetchReasonsJSONTemplate,
+		ModifyEngineExecutionConfiguration: func(cfg *config.EngineExecutionConfiguration) {
+			cfg.EnableRequireFetchReasons = true
+		},
+		Subgraphs: testenv.SubgraphsConfig{
+			Employees: testenv.SubgraphConfig{
+				Middleware: func(handler http.Handler) http.Handler {
+					return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+						body, _ := io.ReadAll(r.Body)
+						var req core.GraphQLRequest
+						require.NoError(t, json.Unmarshal(body, &req))
+
+						require.Equal(t, `query($a: Int!){employee(id: $a){id}}`, req.Query)
+						require.Equal(t, `{"fetch_reasons":[{"typename":"Employee","field":"id","by_user":true},{"typename":"Query","field":"employee","by_user":true}]}`, string(req.Extensions))
+
+						w.WriteHeader(http.StatusOK)
+						_, _ = w.Write([]byte(`{"data":{"employee":{"id":1}}}`))
+					})
+				},
+			},
+		},
+	}, func(t *testing.T, xEnv *testenv.Environment) {
+		res := xEnv.MakeGraphQLRequestOK(testenv.GraphQLRequest{
+			Query:     `query ($count:Int!) { employee(id:$count) { id } }`,
+			Variables: json.RawMessage(`{"count":1}`),
+		})
+		require.JSONEq(t, `{"data":{"employee":{"id":1}}}`, res.Body)
+	})
+}
+
 func TestAnonymousQuery(t *testing.T) {
 	t.Parallel()