Fixed gh pr checkout on detached HEAD

pkg/cmd/pr/checkout/checkout.go

@@ -3,6 +3,11 @@ package checkout
 import (
 	"errors"
 	"fmt"
+	"net/http"
+	"os"
+	"os/exec"
+	"strings"
+
 	"github.com/cli/cli/api"
 	"github.com/cli/cli/context"
 	"github.com/cli/cli/git"
@@ -13,10 +18,6 @@ import (
 	"github.com/cli/cli/pkg/cmdutil"
 	"github.com/cli/cli/pkg/iostreams"
 	"github.com/spf13/cobra"
-	"net/http"
-	"os"
-	"os/exec"
-	"strings"
 )
 
 type CheckoutOptions struct {
@@ -70,11 +71,6 @@ func NewCmdCheckout(f *cmdutil.Factory, runF func(*CheckoutOptions) error) *cobr
 }
 
 func checkoutRun(opts *CheckoutOptions) error {
-	currentBranch, err := opts.Branch()
-	if err != nil {
-		return err
-	}
-
 	remotes, err := opts.Remotes()
 	if err != nil {
 		return err
@@ -133,6 +129,7 @@ func checkoutRun(opts *CheckoutOptions) error {
 		}
 	} else {
 		// no git remote for PR head
+		currentBranch, _ := opts.Branch()
 
 		defaultBranchName, err := api.RepoDefaultBranch(apiClient, baseRepo)
 		if err != nil {

pkg/cmd/pr/checkout/checkout_test.go

@@ -448,6 +448,46 @@ func TestPRCheckout_differentRepo_existingBranch(t *testing.T) {
 	eq(t, strings.Join(ranCommands[1], " "), "git checkout feature")
 }
 
+func TestPRCheckout_detachedHead(t *testing.T) {
+	http := &httpmock.Registry{}
+	defer http.Verify(t)
+
+	http.Register(httpmock.GraphQL(`query PullRequestByNumber\b`), httpmock.StringResponse(`
+	{ "data": { "repository": { "pullRequest": {
+		"number": 123,
+		"headRefName": "feature",
+		"headRepositoryOwner": {
+			"login": "hubot"
+		},
+		"headRepository": {
+			"name": "REPO"
+		},
+		"isCrossRepository": true,
+		"maintainerCanModify": true
+	} } } }
+	`))
+
+	ranCommands := [][]string{}
+	restoreCmd := run.SetPrepareCmd(func(cmd *exec.Cmd) run.Runnable {
+		switch strings.Join(cmd.Args, " ") {
+		case "git config branch.feature.merge":
+			return &test.OutputStub{Out: []byte("refs/heads/feature\n")}
+		default:
+			ranCommands = append(ranCommands, cmd.Args)
+			return &test.OutputStub{}
+		}
+	})
+	defer restoreCmd()
+
+	output, err := runCommand(http, nil, "", `123`)
+	eq(t, err, nil)
+	eq(t, output.String(), "")
+
+	eq(t, len(ranCommands), 2)
+	eq(t, strings.Join(ranCommands[0], " "), "git fetch origin refs/pull/123/head:feature")
+	eq(t, strings.Join(ranCommands[1], " "), "git checkout feature")
+}
+
 func TestPRCheckout_differentRepo_currentBranch(t *testing.T) {
 	http := &httpmock.Registry{}
 	defer http.Verify(t)