Fix MR Selection, Go Code Refactor (#358)

refactor: Refactors the Go codebase into a more modular and idiomatic approach
fix: require selection of specific MR when there are multiple targets for a given source branch
feat: Allows for the passing of Gitlab's filter options when choosing an MR, improves MR selection
feat: API to choose an MR from a list based on the provided username's involvement as an assignee/reviewer/author

This is a #MINOR release

cmd/app/assignee_test.go

@@ -0,0 +1,57 @@
+package app
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/xanzy/go-gitlab"
+)
+
+type fakeAssigneeClient struct {
+	testBase
+}
+
+func (f fakeAssigneeClient) UpdateMergeRequest(pid interface{}, mergeRequest int, opt *gitlab.UpdateMergeRequestOptions, options ...gitlab.RequestOptionFunc) (*gitlab.MergeRequest, *gitlab.Response, error) {
+	resp, err := f.handleGitlabError()
+	if err != nil {
+		return nil, nil, err
+	}
+	return &gitlab.MergeRequest{}, resp, nil
+}
+
+func TestAssigneeHandler(t *testing.T) {
+	var updatePayload = AssigneeUpdateRequest{Ids: []int{1, 2}}
+
+	t.Run("Updates assignees", func(t *testing.T) {
+		request := makeRequest(t, http.MethodPut, "/mr/assignee", updatePayload)
+		client := fakeAssigneeClient{}
+		svc := assigneesService{testProjectData, client}
+		data := getSuccessData(t, svc, request)
+		assert(t, data.Message, "Assignees updated")
+		assert(t, data.Status, http.StatusOK)
+	})
+
+	t.Run("Disallows non-PUT method", func(t *testing.T) {
+		request := makeRequest(t, http.MethodGet, "/mr/assignee", nil)
+		client := fakeAssigneeClient{}
+		svc := assigneesService{testProjectData, client}
+		data := getFailData(t, svc, request)
+		checkBadMethod(t, data, http.MethodPut)
+	})
+
+	t.Run("Handles errors from Gitlab client", func(t *testing.T) {
+		request := makeRequest(t, http.MethodPut, "/mr/approve", updatePayload)
+		client := fakeAssigneeClient{testBase{errFromGitlab: true}}
+		svc := assigneesService{testProjectData, client}
+		data := getFailData(t, svc, request)
+		checkErrorFromGitlab(t, data, "Could not modify merge request assignees")
+	})
+
+	t.Run("Handles non-200s from Gitlab client", func(t *testing.T) {
+		request := makeRequest(t, http.MethodPut, "/mr/approve", updatePayload)
+		client := fakeAssigneeClient{testBase{status: http.StatusSeeOther}}
+		svc := assigneesService{testProjectData, client}
+		data := getFailData(t, svc, request)
+		checkNon200(t, data, "Could not modify merge request assignees", "/mr/assignee")
+	})
+}