fix(pr merge): retry merge on "Base branch was modified" error

pkg/cmd/pr/merge/merge.go

@@ -5,8 +5,11 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
+	"time"
 
 	"github.com/MakeNowJust/heredoc"
+	"github.com/cenkalti/backoff/v4"
 	"github.com/cli/cli/v2/api"
 	ghContext "github.com/cli/cli/v2/context"
 	"github.com/cli/cli/v2/git"
@@ -57,6 +60,9 @@ type MergeOptions struct {
 // ErrAlreadyInMergeQueue indicates that the pull request is already in a merge queue
 var ErrAlreadyInMergeQueue = errors.New("already in merge queue")
 
+// Allow injecting backoff interval in tests.
+var mergeRetryInterval = 2 * time.Second
+
 func NewCmdMerge(f *cmdutil.Factory, runF func(*MergeOptions) error) *cobra.Command {
 	opts := &MergeOptions{
 		IO:         f.IOStreams,
@@ -345,8 +351,14 @@ func (m *mergeContext) merge() error {
 		}
 	}
 
-	err := mergePullRequest(m.httpClient, payload)
-	if err != nil {
+	bo := backoff.NewConstantBackOff(mergeRetryInterval)
+	if err := backoff.Retry(func() error {
+		err := mergePullRequest(m.httpClient, payload)
+		if err == nil || shouldRetry(err) {
+			return err
+		}
+		return backoff.Permanent(err)
+	}, backoff.WithMaxRetries(bo, 3)); err != nil {
 		return err
 	}
 
@@ -760,6 +772,21 @@ func mergeConflictStatus(status string) bool {
 	return status == MergeStateStatusDirty
 }
 
+// The "Base branch was modified" error occurs when the base
+// branch is updated by another merge and the mergeability
+// computation hasn't completed yet.
+func shouldRetry(err error) bool {
+	var graphErr api.GraphQLError
+	if errors.As(err, &graphErr) {
+		for _, e := range graphErr.Errors {
+			if strings.Contains(e.Message, "Base branch was modified.") {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 func isImmediatelyMergeable(status string) bool {
 	switch status {
 	case MergeStateStatusClean, MergeStateStatusHasHooks, MergeStateStatusUnstable:

pkg/cmd/pr/merge/merge_test.go

@@ -2030,6 +2030,61 @@ func TestPrAddToMergeQueueAdminWithMergeStrategy(t *testing.T) {
 	assert.Equal(t, "✓ Merged pull request OWNER/REPO#1 (The title of the PR)\n", output.Stderr())
 }
 
+func TestPrMerge_retryOnBaseBranchModified(t *testing.T) {
+	orig := mergeRetryInterval
+	mergeRetryInterval = 0
+	t.Cleanup(func() { mergeRetryInterval = orig })
+
+	http := initFakeHTTP()
+	defer http.Verify(t)
+
+	shared.StubFinderForRunCommandStyleTests(t,
+		"1",
+		&api.PullRequest{
+			ID:               "THE-ID",
+			Number:           1,
+			State:            "OPEN",
+			Title:            "The title of the PR",
+			MergeStateStatus: "CLEAN",
+		},
+		baseRepo("OWNER", "REPO", "main"),
+	)
+
+	http.Register(
+		httpmock.GraphQL(`mutation PullRequestMerge\b`),
+		httpmock.StringResponse(`
+		{ "data": {},
+			"errors": [
+				{
+					"message": "Base branch was modified. Review and try the merge again."
+				}
+			]
+		}`),
+	)
+	http.Register(
+		httpmock.GraphQL(`mutation PullRequestMerge\b`),
+		httpmock.GraphQLMutation(`{}`, func(input map[string]interface{}) {
+			assert.Equal(t, "THE-ID", input["pullRequestId"].(string))
+			assert.Equal(t, "MERGE", input["mergeMethod"].(string))
+		}),
+	)
+
+	cs, cmdTeardown := run.Stub()
+	defer cmdTeardown(t)
+	cs.Register(`git rev-parse --verify refs/heads/`, 0, "")
+
+	output, err := runCommand(http, nil, "main", true, "pr merge 1 --merge")
+	if err != nil {
+		t.Fatalf("error running command `pr merge`: %v", err)
+	}
+
+	r := regexp.MustCompile(`Merged pull request OWNER/REPO#1 \(The title of the PR\)`)
+
+	if !r.MatchString(output.Stderr()) {
+		t.Fatalf("output did not match regexp /%s/\n> output\n%q\n", r, output.Stderr())
+	}
+}
+
 type testEditor struct{}
 
 func (e testEditor) Edit(filename, text string) (string, error) {