add support for setting vary header in plugins

NEXT_CHANGELOG.md

@@ -27,6 +27,14 @@ By [@USERNAME](https://github.com/USERNAME) in https://github.com/apollographql/
 
 ## ❗ BREAKING ❗
 
+### Preserve Plugin response Vary headers([PR #1660](https://github.com/apollographql/router/issues/1297))
+
+It is now possible to set a `Vary` header in a client response from a plugin.
+
+Note: This is a breaking change because the prior behaviour provided three default Vary headers and we've had to drop those to enable this change. If, after all plugin processing, there is no Vary header, the router will add one with a value of "origin".
+
+By [@garypen](https://github.com/garypen) in https://github.com/apollographql/router/pull/1660
+
 ### Fix the supported defer specification version to 20220824 ([PR #1652](https://github.com/apollographql/router/issues/1652))
 
 Since the router will ship before the `@defer` specification is done, we
@@ -42,13 +50,12 @@ By [@Geal](https://github.com/Geal) in https://github.com/apollographql/router/p
 ## 🚀 Features
 ## 🐛 Fixes
 
-### Update our helm documentation to illustrate how to use our registry ([PR #1649](https://github.com/apollographql/router/issues/1649))
+### Update our helm documentation to illustrate how to use our registry ([#1643](https://github.com/apollographql/router/issues/1643))
 
 The helm chart never used to have a registry, so our docs were really just placeholders. I've updated them to reflect the fact that we now store the chart in our OCI registry.
 
 By [@garypen](https://github.com/garypen) in https://github.com/apollographql/router/pull/1649
 
-
 ### Update router-bridge to `query-planner` v2.1.0 ([PR #1650](https://github.com/apollographql/router/pull/1650))
 
 The 2.1.0 release of the query planner comes with fixes to fragment interpretation and reduced memory usage.
@@ -57,7 +64,7 @@ By [@Geal](https://github.com/Geal) in https://github.com/apollographql/router/p
 
 ## 🛠 Maintenance
 
-### Remove cache layer ([PR #1647](https://github.com/apollographql/router/issues/1647))
+### Remove cache layer ([PR #1647](https://github.com/apollographql/router/pull/1647))
 
 We removed ServiceBuilderExt::cache in 0.16.0. That was the only consumer of
 the cache layer. This completes the removal by deleting the cache layer.
@@ -66,7 +73,7 @@ By [@garypen](https://github.com/garypen) in https://github.com/apollographql/ro
 
 ### Refactor `SupergraphService` ([PR #1615](https://github.com/apollographql/router/issues/1615))
 
-The `SupergraphService` code became too complex, so much that `rsutfmt` could not modify it anymore.
+The `SupergraphService` code became too complex, so much that `rustfmt` could not modify it anymore.
 This breaks up the code in more manageable functions.
 
 By [@Geal](https://github.com/Geal) in https://github.com/apollographql/router/pull/1615

apollo-router/src/axum_http_server_factory.rs

@@ -29,6 +29,7 @@ use futures::stream::BoxStream;
 use futures::StreamExt;
 use http::header::CONTENT_ENCODING;
 use http::header::CONTENT_TYPE;
+use http::header::VARY;
 use http::HeaderValue;
 use http::Request;
 use http::Uri;
@@ -483,6 +484,13 @@ async fn health_check() -> impl IntoResponse {
     Json(json!({ "status": "pass" }))
 }
 
+// Process the headers to make sure that `VARY` is set correctly
+fn process_vary_header(headers: &mut HeaderMap<HeaderValue>) {
+    if headers.get(VARY).is_none() {
+        // We don't have a VARY header, add one with value "origin"
+        headers.insert(VARY, HeaderValue::from_static("origin"));
+    }
+}
 async fn run_graphql_request<RS>(
     service: RS,
     http_request: Request<graphql::Request>,
@@ -518,6 +526,8 @@ where
                 Ok(response) => {
                     let (mut parts, mut stream) = response.into_parts();
 
+                    process_vary_header(&mut parts.headers);
+
                     match stream.next().await {
                         None => {
                             tracing::error!("router service is not available to process request",);
@@ -2295,4 +2305,40 @@ Content-Type: application/json\r
 
         server.shutdown().await
     }
+
+    // Test Vary processing
+
+    #[test]
+    fn it_adds_default_with_value_origin_if_no_vary_header() {
+        let mut default_headers = HeaderMap::new();
+        process_vary_header(&mut default_headers);
+        let vary_opt = default_headers.get(VARY);
+        assert!(vary_opt.is_some());
+        let vary = vary_opt.expect("has a value");
+        assert_eq!(vary, "origin");
+    }
+
+    #[test]
+    fn it_leaves_vary_alone_if_set() {
+        let mut default_headers = HeaderMap::new();
+        default_headers.insert(VARY, HeaderValue::from_static("*"));
+        process_vary_header(&mut default_headers);
+        let vary_opt = default_headers.get(VARY);
+        assert!(vary_opt.is_some());
+        let vary = vary_opt.expect("has a value");
+        assert_eq!(vary, "*");
+    }
+
+    #[test]
+    fn it_leaves_varys_alone_if_there_are_more_than_one() {
+        let mut default_headers = HeaderMap::new();
+        default_headers.insert(VARY, HeaderValue::from_static("one"));
+        default_headers.append(VARY, HeaderValue::from_static("two"));
+        process_vary_header(&mut default_headers);
+        let vary = default_headers.get_all(VARY);
+        assert_eq!(vary.iter().count(), 2);
+        for value in vary {
+            assert!(value == "one" || value == "two");
+        }
+    }
 }

apollo-router/src/configuration/mod.rs

@@ -501,6 +501,7 @@ impl Cors {
             }))
         };
         let cors = CorsLayer::new()
+            .vary([])
             .allow_credentials(self.allow_credentials)
             .allow_headers(allow_headers)
             .expose_headers(cors::ExposeHeaders::list(

docs/source/configuration/cors.mdx

@@ -113,3 +113,7 @@ cors:
   # browser in response to a cross-origin request.
   expose_headers: []
 ```
+
+## Response `Vary` header
+
+A plugin may set a response `Vary` header. If, after all plugins are processed, there is no response `Vary` header, then the router will add one with a value of "origin".