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.go

@@ -0,0 +1,86 @@
+package app
+
+import (
+	"encoding/json"
+	"errors"
+	"io"
+	"net/http"
+
+	"github.com/xanzy/go-gitlab"
+)
+
+type ListMergeRequestResponse struct {
+	SuccessResponse
+	MergeRequests []*gitlab.MergeRequest `json:"merge_requests"`
+}
+
+type MergeRequestLister interface {
+	ListProjectMergeRequests(pid interface{}, opt *gitlab.ListProjectMergeRequestsOptions, options ...gitlab.RequestOptionFunc) ([]*gitlab.MergeRequest, *gitlab.Response, error)
+}
+
+type mergeRequestListerService struct {
+	data
+	client MergeRequestLister
+}
+
+func (a mergeRequestListerService) 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 listMergeRequestRequest gitlab.ListProjectMergeRequestsOptions
+	err = json.Unmarshal(body, &listMergeRequestRequest)
+	if err != nil {
+		handleError(w, err, "Could not read JSON from request", http.StatusBadRequest)
+		return
+	}
+
+	if listMergeRequestRequest.State == nil {
+		listMergeRequestRequest.State = gitlab.Ptr("opened")
+	}
+
+	if listMergeRequestRequest.Scope == nil {
+		listMergeRequestRequest.Scope = gitlab.Ptr("all")
+	}
+
+	mergeRequests, res, err := a.client.ListProjectMergeRequests(a.projectInfo.ProjectId, &listMergeRequestRequest)
+
+	if err != nil {
+		handleError(w, err, "Failed to list merge requests", http.StatusInternalServerError)
+		return
+	}
+
+	if res.StatusCode >= 300 {
+		handleError(w, GenericError{endpoint: "/merge_requests"}, "Failed to list merge requests", res.StatusCode)
+		return
+	}
+
+	if len(mergeRequests) == 0 {
+		handleError(w, errors.New("No merge requests found"), "No merge requests found", http.StatusNotFound)
+		return
+	}
+
+	w.WriteHeader(http.StatusOK)
+	response := ListMergeRequestResponse{
+		SuccessResponse: SuccessResponse{
+			Message: "Merge requests fetched successfully",
+			Status:  http.StatusOK,
+		},
+		MergeRequests: mergeRequests,
+	}
+
+	err = json.NewEncoder(w).Encode(response)
+	if err != nil {
+		handleError(w, err, "Could not encode response", http.StatusInternalServerError)
+	}
+}