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/merge_requests_by_username.go

@@ -0,0 +1,162 @@
+package app
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"sync"
+
+	"github.com/xanzy/go-gitlab"
+)
+
+type MergeRequestListerByUsername interface {
+	ListProjectMergeRequests(pid interface{}, opt *gitlab.ListProjectMergeRequestsOptions, options ...gitlab.RequestOptionFunc) ([]*gitlab.MergeRequest, *gitlab.Response, error)
+}
+
+type mergeRequestListerByUsernameService struct {
+	data
+	client MergeRequestListerByUsername
+}
+
+type MergeRequestByUsernameRequest struct {
+	UserId   int    `json:"user_id"`
+	Username string `json:"username"`
+	State    string `json:"state,omitempty"`
+}
+
+func (a mergeRequestListerByUsernameService) handler(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	if r.Method != http.MethodPost {
+		w.Header().Set("Access-Control-Allow-Methods", http.MethodPost)
+		handleError(w, InvalidRequestError{}, "Expected POST", http.StatusMethodNotAllowed)
+		return
+	}
+
+	body, err := io.ReadAll(r.Body)
+	if err != nil {
+		handleError(w, err, "Could not read request body", http.StatusBadRequest)
+		return
+	}
+
+	defer r.Body.Close()
+	var request MergeRequestByUsernameRequest
+	err = json.Unmarshal(body, &request)
+	if err != nil {
+		handleError(w, err, "Could not read JSON from request", http.StatusBadRequest)
+		return
+	}
+
+	if request.Username == "" {
+		handleError(w, errors.New("username is a required payload field"), "username is required", http.StatusBadRequest)
+		return
+	}
+
+	if request.UserId == 0 {
+		handleError(w, errors.New("user_id is a required payload field"), "user_id is required", http.StatusBadRequest)
+		return
+	}
+
+	if request.State == "" {
+		request.State = "opened"
+	}
+
+	payloads := []gitlab.ListProjectMergeRequestsOptions{
+		{
+			AuthorUsername: gitlab.Ptr(request.Username),
+			State:          gitlab.Ptr(request.State),
+			Scope:          gitlab.Ptr("all"),
+		},
+		{
+			ReviewerUsername: gitlab.Ptr(request.Username),
+			State:            gitlab.Ptr(request.State),
+			Scope:            gitlab.Ptr("all"),
+		},
+		{
+			AssigneeID: gitlab.AssigneeID(request.UserId),
+			State:      gitlab.Ptr(request.State),
+			Scope:      gitlab.Ptr("all"),
+		},
+	}
+
+	type apiResponse struct {
+		mrs []*gitlab.MergeRequest
+		err error
+	}
+
+	mrChan := make(chan apiResponse, len(payloads))
+	wg := sync.WaitGroup{}
+	go func() {
+		wg.Wait()
+		close(mrChan)
+	}()
+
+	for _, payload := range payloads {
+		wg.Add(1)
+		go func(p gitlab.ListProjectMergeRequestsOptions) {
+			defer wg.Done()
+			mrs, err := a.getMrs(&p)
+			mrChan <- apiResponse{mrs, err}
+		}(payload)
+	}
+
+	var mergeRequests []*gitlab.MergeRequest
+	existingIds := make(map[int]bool)
+	var errs []error
+	for res := range mrChan {
+		if res.err != nil {
+			errs = append(errs, res.err)
+		} else {
+			for _, mr := range res.mrs {
+				if !existingIds[mr.ID] {
+					mergeRequests = append(mergeRequests, mr)
+					existingIds[mr.ID] = true
+				}
+			}
+		}
+	}
+
+	if len(errs) > 0 {
+		combinedErr := ""
+		for _, err := range errs {
+			combinedErr += err.Error() + "; "
+		}
+		handleError(w, errors.New(combinedErr), "An error occurred", http.StatusInternalServerError)
+		return
+	}
+
+	if len(mergeRequests) == 0 {
+		handleError(w, fmt.Errorf("%s did not have any MRs", request.Username), "No MRs found", http.StatusNotFound)
+		return
+	}
+
+	w.WriteHeader(http.StatusOK)
+	response := ListMergeRequestResponse{
+		SuccessResponse: SuccessResponse{
+			Message: fmt.Sprintf("Merge requests fetched for %s", request.Username),
+			Status:  http.StatusOK,
+		},
+		MergeRequests: mergeRequests,
+	}
+
+	err = json.NewEncoder(w).Encode(response)
+	if err != nil {
+		handleError(w, err, "Could not encode response", http.StatusInternalServerError)
+	}
+}
+
+func (a mergeRequestListerByUsernameService) getMrs(payload *gitlab.ListProjectMergeRequestsOptions) ([]*gitlab.MergeRequest, error) {
+	mrs, res, err := a.client.ListProjectMergeRequests(a.projectInfo.ProjectId, payload)
+	if err != nil {
+		return []*gitlab.MergeRequest{}, err
+	}
+
+	if res.StatusCode >= 300 {
+		return []*gitlab.MergeRequest{}, GenericError{endpoint: "/merge_requests_by_username"}
+	}
+
+	defer res.Body.Close()
+
+	return mrs, err
+}