Simplify Go Endpoints + Add Tests (#120)

This MR represents a major refactor of the Go codebase, as well as introducing tests for the handlers. The MR also introduces an endpoint to shutdown or restart the Go server, which may be useful for clients who want to refresh the state of the plugin after checking out branches. Finally, this MR adds a contributing document for users who want to make feature changes.

cmd/server.go

@@ -0,0 +1,154 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"os"
+	"time"
+)
+
+/*
+startSever starts the server and runs concurrent goroutines
+to handle potential shutdown requests and incoming HTTP requests.
+*/
+func startServer(client *Client, projectInfo *ProjectInfo) {
+
+	m, a := createRouterAndApi(client,
+		func(a *api) error {
+			a.projectInfo = projectInfo
+			return nil
+		},
+		func(a *api) error {
+			a.fileReader = attachmentReader{}
+			return nil
+		})
+
+	l := createListener()
+	server := &http.Server{Handler: m}
+
+	/* Starts the Go server */
+	go func() {
+		err := server.Serve(l)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Error starting server: %s\n", err)
+			os.Exit(1)
+		}
+	}()
+
+	port := l.Addr().(*net.TCPAddr).Port
+	err := checkServer(port)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Server did not respond: %s\n", err)
+		os.Exit(1)
+	}
+
+	/* This print is detected by the Lua code */
+	fmt.Println("Server started on port: ", port)
+
+	/* Handles shutdown requests */
+	<-a.sigCh
+	err = server.Shutdown(context.Background())
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Server could not shut down gracefully: %s\n", err)
+		os.Exit(1)
+	} else {
+		os.Exit(0)
+	}
+}
+
+/*
+The api struct contains common configuration that's accessible to all handlers, such as the gitlab
+client, the project information, and the channels for signaling error or shutdown requests
+
+The handlers for different Gitlab operations are are all methods on the api struct and interact
+with the client value, which is a go-gitlab client.
+*/
+type api struct {
+	client      ClientInterface
+	projectInfo *ProjectInfo
+	fileReader  FileReader
+	sigCh       chan os.Signal
+}
+
+type optFunc func(a *api) error
+
+/*
+createRouterAndApi wires up the router and attaches all handlers to their respective routes. It also
+iterates over all option functions to configure API fields such as the project information and default
+file reader functionality
+*/
+func createRouterAndApi(client ClientInterface, optFuncs ...optFunc) (*http.ServeMux, api) {
+	m := http.NewServeMux()
+	a := api{
+		client:      client,
+		projectInfo: &ProjectInfo{},
+		fileReader:  nil,
+		sigCh:       make(chan os.Signal, 1),
+	}
+
+	/* Mutates the API struct as necessary with configuration functions */
+	for _, optFunc := range optFuncs {
+		err := optFunc(&a)
+		if err != nil {
+			panic(err)
+		}
+	}
+
+	m.Handle("/ping", http.HandlerFunc(pingHandler))
+	m.HandleFunc("/shutdown", a.shutdownHandler)
+	m.HandleFunc("/approve", a.approveHandler)
+	m.HandleFunc("/comment", a.commentHandler)
+	m.HandleFunc("/discussions/list", a.listDiscussionsHandler)
+	m.HandleFunc("/discussions/resolve", a.discussionsResolveHandler)
+	m.HandleFunc("/info", a.infoHandler)
+	m.HandleFunc("/job", a.jobHandler)
+	m.HandleFunc("/mr/attachment", a.attachmentHandler)
+	m.HandleFunc("/mr/assignee", a.assigneesHandler)
+	m.HandleFunc("/mr/summary", a.summaryHandler)
+	m.HandleFunc("/mr/reviewer", a.reviewersHandler)
+	m.HandleFunc("/mr/revisions", a.revisionsHandler)
+	m.HandleFunc("/pipeline/", a.pipelineHandler)
+	m.HandleFunc("/project/members", a.projectMembersHandler)
+	m.HandleFunc("/reply", a.replyHandler)
+	m.HandleFunc("/revoke", a.revokeHandler)
+
+	return m, a
+}
+
+/* Used to check whether the server has started yet */
+func pingHandler(w http.ResponseWriter, _ *http.Request) {
+	w.WriteHeader(http.StatusOK)
+	fmt.Fprintln(w, "pong")
+}
+
+/* checkServer pings the server repeatedly for 1 full second after startup in order to notify the plugin that the server is ready */
+func checkServer(port int) error {
+	for i := 0; i < 10; i++ {
+		resp, err := http.Get("http://localhost:" + fmt.Sprintf("%d", port) + "/ping")
+		if resp.StatusCode == 200 && err == nil {
+			return nil
+		}
+		time.Sleep(100 * time.Microsecond)
+	}
+
+	return errors.New("Could not start server!")
+}
+
+/* Creates a TCP listener on the port specified by the user or a random port */
+func createListener() (l net.Listener) {
+	port := os.Args[2]
+	if port == "" {
+		port = "0"
+	}
+	addr := fmt.Sprintf("localhost:%s", port)
+	l, err := net.Listen("tcp", addr)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error starting server: %s\n", err)
+		os.Exit(1)
+	}
+
+	return l
+}