Remove the first element from the Vec, not the last

.changesets/fix_garypen_batch_ordering_fix.md

@@ -0,0 +1,5 @@
+### Maintain query ordering within a batch ([Issue #4143](https://github.com/apollographql/router/issues/4143))
+
+A bug in batch manipulation meant that the last element in a batch was treated as the first element. Ordering should be maintained and there is now an updated unit test to ensure this.
+
+By [@garypen](https://github.com/garypen) in https://github.com/apollographql/router/pull/4144

apollo-router/src/services/query_batching/testdata/expected_good_response.json

@@ -3,7 +3,6 @@
     "data": {
       "topProducts": [
         {
-          "upc": "1",
           "name": "Table",
           "reviews": [
             {
@@ -29,7 +28,6 @@
           ]
         },
         {
-          "upc": "2",
           "name": "Couch",
           "reviews": [
             {
@@ -94,5 +92,50 @@
         }
       ]
     }
+  },
+  {
+    "data": {
+      "topProducts": [
+        {
+          "upc": "1",
+          "name": "Table",
+          "reviews": [
+            {
+              "id": "1",
+              "product": {
+                "name": "Table"
+              },
+              "author": {
+                "name": "Ada Lovelace"
+              }
+            },
+            {
+              "id": "4",
+              "product": {
+                "name": "Table"
+              },
+              "author": {
+                "name": "Alan Turing"
+              }
+            }
+          ]
+        },
+        {
+          "upc": "2",
+          "name": "Couch",
+          "reviews": [
+            {
+              "id": "2",
+              "product": {
+                "name": "Couch"
+              },
+              "author": {
+                "name": "Ada Lovelace"
+              }
+            }
+          ]
+        }
+      ]
+    }
   }
 ]

apollo-router/src/services/router_service.rs

@@ -402,7 +402,7 @@ impl RouterService {
         if results.len() == 1 {
             Ok(results.pop().expect("we should have at least one response"))
         } else {
-            let first = results.pop().expect("we should have at least one response");
+            let first = results.remove(0);
             let (parts, body) = first.response.into_parts();
             let context = first.context;
             let mut bytes = BytesMut::new();
@@ -601,9 +601,7 @@ impl RouterService {
 
         let mut ok_results = graphql_requests?;
         let mut results = Vec::with_capacity(ok_results.len());
-        let first = ok_results
-            .pop()
-            .expect("We must have at least one response");
+        let first = ok_results.remove(0);
         let sg = http::Request::from_parts(parts, first);
 
         if !ok_results.is_empty() {
@@ -1055,13 +1053,23 @@ mod tests {
                 .build()
                 .unwrap()
                 .supergraph_request
-                .map(|req: crate::request::Request| {
-                    // Modify the request so that it is a valid array of requests.
-                    let mut json_bytes = serde_json::to_vec(&req).unwrap();
+                .map(|req_2: crate::request::Request| {
+                    // Create clones of our standard query and update it to have 3 unique queries
+                    let mut req_1 = req_2.clone();
+                    let mut req_3 = req_2.clone();
+                    req_1.query = req_2.query.clone().map(|x| x.replace("upc\n", ""));
+                    req_3.query = req_2.query.clone().map(|x| x.replace("id name", "name"));
+
+                    // Modify the request so that it is a valid array of 3 requests.
+                    let mut json_bytes_1 = serde_json::to_vec(&req_1).unwrap();
+                    let mut json_bytes_2 = serde_json::to_vec(&req_2).unwrap();
+                    let mut json_bytes_3 = serde_json::to_vec(&req_3).unwrap();
                     let mut result = vec![b'['];
-                    result.append(&mut json_bytes.clone());
+                    result.append(&mut json_bytes_1);
                     result.push(b',');
-                    result.append(&mut json_bytes);
+                    result.append(&mut json_bytes_2);
+                    result.push(b',');
+                    result.append(&mut json_bytes_3);
                     result.push(b']');
                     hyper::Body::from(result)
                 });