Test the scenario where the target branch already exists

command/pr.go

@@ -302,7 +302,7 @@ func prCheckout(cmd *cobra.Command, args []string) error {
 		cmdQueue = append(cmdQueue, []string{"git", "fetch", headRemote.Name, refSpec})
 
 		// local branch already exists
-		if git.HasFile("refs", "heads", newBranchName) {
+		if git.VerifyRef("refs/heads/" + newBranchName) {
 			cmdQueue = append(cmdQueue, []string{"git", "checkout", newBranchName})
 			cmdQueue = append(cmdQueue, []string{"git", "merge", "--ff-only", fmt.Sprintf("refs/remotes/%s", remoteBranch)})
 		} else {

command/pr_checkout_test.go

@@ -40,8 +40,13 @@ func TestPRCheckout_sameRepo(t *testing.T) {
 
 	ranCommands := [][]string{}
 	restoreCmd := utils.SetPrepareCmd(func(cmd *exec.Cmd) utils.Runnable {
-		ranCommands = append(ranCommands, cmd.Args)
-		return &outputStub{}
+		switch strings.Join(cmd.Args, " ") {
+		case "git show-ref --verify --quiet refs/heads/feature":
+			return &errorStub{"exit status: 1"}
+		default:
+			ranCommands = append(ranCommands, cmd.Args)
+			return &outputStub{}
+		}
 	})
 	defer restoreCmd()
 
@@ -49,13 +54,61 @@ func TestPRCheckout_sameRepo(t *testing.T) {
 	_, err := prCheckoutCmd.ExecuteC()
 	eq(t, err, nil)
 
-	eq(t, len(ranCommands), 6)
-	eq(t, strings.Join(ranCommands[0], " "), "git rev-parse -q --git-path refs/heads/feature")
-	eq(t, strings.Join(ranCommands[1], " "), "git rev-parse -q --git-dir")
-	eq(t, strings.Join(ranCommands[2], " "), "git fetch origin +refs/heads/feature:refs/remotes/origin/feature")
-	eq(t, strings.Join(ranCommands[3], " "), "git checkout -b feature --no-track origin/feature")
-	eq(t, strings.Join(ranCommands[4], " "), "git config branch.feature.remote origin")
-	eq(t, strings.Join(ranCommands[5], " "), "git config branch.feature.merge refs/heads/feature")
+	eq(t, len(ranCommands), 4)
+	eq(t, strings.Join(ranCommands[0], " "), "git fetch origin +refs/heads/feature:refs/remotes/origin/feature")
+	eq(t, strings.Join(ranCommands[1], " "), "git checkout -b feature --no-track origin/feature")
+	eq(t, strings.Join(ranCommands[2], " "), "git config branch.feature.remote origin")
+	eq(t, strings.Join(ranCommands[3], " "), "git config branch.feature.merge refs/heads/feature")
+}
+
+func TestPRCheckout_existingBranch(t *testing.T) {
+	ctx := context.NewBlank()
+	ctx.SetBranch("master")
+	ctx.SetRemotes(map[string]string{
+		"origin": "OWNER/REPO",
+	})
+	initContext = func() context.Context {
+		return ctx
+	}
+	http := initFakeHTTP()
+
+	http.StubResponse(200, bytes.NewBufferString(`
+	{ "data": { "repository": { "pullRequest": {
+		"headRefName": "feature",
+		"headRepositoryOwner": {
+			"login": "hubot"
+		},
+		"headRepository": {
+			"name": "REPO",
+			"defaultBranchRef": {
+				"name": "master"
+			}
+		},
+		"isCrossRepository": false,
+		"maintainerCanModify": false
+	} } } }
+	`))
+
+	ranCommands := [][]string{}
+	restoreCmd := utils.SetPrepareCmd(func(cmd *exec.Cmd) utils.Runnable {
+		switch strings.Join(cmd.Args, " ") {
+		case "git show-ref --verify --quiet refs/heads/feature":
+			return &outputStub{}
+		default:
+			ranCommands = append(ranCommands, cmd.Args)
+			return &outputStub{}
+		}
+	})
+	defer restoreCmd()
+
+	RootCmd.SetArgs([]string{"pr", "checkout", "123"})
+	_, err := prCheckoutCmd.ExecuteC()
+	eq(t, err, nil)
+
+	eq(t, len(ranCommands), 3)
+	eq(t, strings.Join(ranCommands[0], " "), "git fetch origin +refs/heads/feature:refs/remotes/origin/feature")
+	eq(t, strings.Join(ranCommands[1], " "), "git checkout feature")
+	eq(t, strings.Join(ranCommands[2], " "), "git merge --ff-only refs/remotes/origin/feature")
 }
 
 func TestPRCheckout_differentRepo(t *testing.T) {

command/testing.go

@@ -1,6 +1,8 @@
 package command
 
 import (
+	"errors"
+
 	"github.com/github/gh-cli/api"
 	"github.com/github/gh-cli/context"
 )
@@ -34,3 +36,15 @@ func (s outputStub) Output() ([]byte, error) {
 func (s outputStub) Run() error {
 	return nil
 }
+
+type errorStub struct {
+	message string
+}
+
+func (s errorStub) Output() ([]byte, error) {
+	return nil, errors.New(s.message)
+}
+
+func (s errorStub) Run() error {
+	return errors.New(s.message)
+}