internal/api: add unmarshalResponse

Add a useful helper.

Use it once. We must use it at least once to pass
our check for unused functions.

Change-Id: Ied063f74dc7e3bc7dc7c12d4a17b0e55680951ad
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/759840
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ethan Lee <ethanalee@google.com>

1 files changed, 28 insertions, 17 deletions

diff --git a/internal/api/api_test.go b/internal/api/api_test.go
index 50c11327..caa620b4 100644
--- a/internal/api/api_test.go
+++ b/internal/api/api_test.go
@@ -5,8 +5,10 @@
 package api
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"strconv"
@@ -206,23 +208,12 @@ func TestServePackage(t *testing.T) {
 			}
 
 			if test.want != nil {
-				switch want := test.want.(type) {
-				case *Package:
-					var got Package
-					if err := json.Unmarshal(w.Body.Bytes(), &got); err != nil {
-						t.Fatalf("json.Unmarshal Package: %v", err)
-					}
-					if diff := cmp.Diff(want, &got); diff != "" {
-						t.Errorf("mismatch (-want +got):\n%s", diff)
-					}
-				case *Error:
-					var got Error
-					if err := json.Unmarshal(w.Body.Bytes(), &got); err != nil {
-						t.Fatalf("json.Unmarshal Error: %v. Body: %s", err, w.Body.String())
-					}
-					if diff := cmp.Diff(want, &got); diff != "" {
-						t.Errorf("mismatch (-want +got):\n%s", diff)
-					}
+				got, err := unmarshalResponse[Package](w.Body.Bytes())
+				if err != nil {
+					t.Fatal(err)
+				}
+				if diff := cmp.Diff(test.want, got); diff != "" {
+					t.Errorf("mismatch (-want +got):\n%s", diff)
 				}
 			}
 		})
@@ -710,3 +701,23 @@ func TestServePackageSymbols(t *testing.T) {
 		})
 	}
 }
+
+// unmarshalResponse unmarshals an API response into either
+// a *T or an *Error.
+func unmarshalResponse[T any](data []byte) (any, error) {
+	d := json.NewDecoder(bytes.NewReader(data))
+	d.DisallowUnknownFields()
+	var t T
+	err1 := d.Decode(&t)
+	if err1 == nil {
+		return &t, nil
+	}
+	d = json.NewDecoder(bytes.NewReader(data))
+	d.DisallowUnknownFields()
+	var e Error
+	err2 := d.Decode(&e)
+	if err2 == nil {
+		return &e, nil
+	}
+	return nil, errors.Join(err1, err2)
+}