internal/middleware: change experiment query param behavior

Previously, setting an experiment with the experiment query param
required the flag to be present in the experiments table.

Feature flags can now be set with the experiment query param without
first being added to the database.

Change-Id: Ia24862f2e05414547b5f2dfb96561af49b568835
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/760279
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Andrew Bonventre <andybons@google.com>

internal/middleware/experiment.go

@@ -69,6 +69,10 @@ func (e *Experimenter) setExperimentsForRequest(r *http.Request) *http.Request {
 			set[exp.Name] = true
 		}
 	}
+	keys := r.URL.Query()[experimentQueryParamKey]
+	for _, k := range keys {
+		set[k] = true
+	}
 	return r.WithContext(experiment.NewContext(r.Context(), experiment.NewSet(set)))
 }
 
@@ -113,12 +117,6 @@ func (e *Experimenter) loadNextSnapshot(ctx context.Context) (err error) {
 // All requests from the same IP will be enrolled in the same set of
 // experiments.
 func shouldSetExperiment(r *http.Request, e *internal.Experiment) bool {
-	keys := r.URL.Query()[experimentQueryParamKey]
-	for _, k := range keys {
-		if k == e.Name {
-			return true
-		}
-	}
 	if e.Rollout == 0 {
 		return false
 	}

internal/middleware/experiment_test.go

@@ -137,26 +137,3 @@ func TestShouldSetExperiment(t *testing.T) {
 		})
 	}
 }
-
-func TestShouldSetExperimentWithQueryParam(t *testing.T) {
-	req, err := http.NewRequest("GET", "http://foo", nil)
-	if err != nil {
-		t.Fatal(err)
-	}
-	testExperiments := []string{
-		"experiment-test-1",
-		"experiment-test-2",
-	}
-	q := req.URL.Query()
-	for _, te := range testExperiments {
-		q.Add(experimentQueryParamKey, te)
-		q.Add(experimentQueryParamKey, te)
-	}
-	req.URL.RawQuery = q.Encode()
-
-	for _, te := range testExperiments {
-		if !shouldSetExperiment(req, &internal.Experiment{Name: te, Rollout: 0}) {
-			t.Errorf("shouldSetExperiment(%q) = false; want = true", te)
-		}
-	}
-}