Fix pr create when push.default tracking and no merge ref (#10863)

* Fix pr create when push.default tracking and no merge ref

* Update pkg/cmd/pr/shared/find_refs_resolution.go

---------

Co-authored-by: Tyler McGoffin <jtmcg@github.com>

acceptance/testdata/pr/pr-create-push-default-upstream-no-merge-ref-fork.txtar

@@ -0,0 +1,50 @@
+skip 'it creates a fork owned by the user running the test'
+skip 'this never worked, but could be fixed if we fixed show-refs'
+
+# Setup environment variables used for testscript
+env REPO=${SCRIPT_NAME}-${RANDOM_STRING}
+env FORK=${REPO}-fork
+
+# Use gh as a credential helper
+exec gh auth setup-git
+
+# Get the current username for the fork owner
+exec gh api user --jq .login
+stdout2env USER
+
+# Create a repository to act as upstream with a file so it has a default branch
+exec gh repo create ${ORG}/${REPO} --add-readme --private
+
+# Defer repo cleanup of upstream
+defer gh repo delete --yes ${ORG}/${REPO}
+
+# Create a user fork of repository. This will be owned by USER.
+exec gh repo fork ${ORG}/${REPO} --fork-name ${FORK}
+sleep 5
+
+# Defer repo cleanup of fork
+defer gh repo delete --yes ${USER}/${FORK}
+
+# Retrieve fork repository information
+exec gh repo view ${USER}/${FORK} --json id --jq '.id'
+stdout2env FORK_ID
+
+# Clone the repo
+exec gh repo clone ${USER}/${FORK}
+cd ${FORK}
+
+# Configure push.default so that it should use the merge ref
+exec git config push.default upstream
+
+# But prepare a branch that doesn't have a tracking merge ref
+exec git checkout -b feature-branch
+exec git commit --allow-empty -m 'Empty Commit'
+exec git push origin feature-branch
+
+# Create the PR
+exec gh pr create --title 'Feature Title' --body 'Feature Body'
+stdout https://${GH_HOST}/${ORG}/${REPO}/pull/1
+
+# Assert that the PR was created with the correct head repository and refs
+exec gh pr view --json headRefName,headRepository,baseRefName,isCrossRepository
+stdout {"baseRefName":"main","headRefName":"feature-branch","headRepository":{"id":"${FORK_ID}","name":"${FORK}"},"isCrossRepository":true}

acceptance/testdata/pr/pr-create-push-default-upstream-no-merge-ref.txtar

@@ -0,0 +1,33 @@
+skip 'it creates a fork owned by the user running the test'
+
+# Setup environment variables used for testscript
+env REPO=${SCRIPT_NAME}-${RANDOM_STRING}
+
+# Use gh as a credential helper
+exec gh auth setup-git
+
+# Get the current username for the fork owner
+exec gh api user --jq .login
+stdout2env USER
+
+# Create a repository to act as upstream with a file so it has a default branch
+exec gh repo create ${ORG}/${REPO} --add-readme --private
+
+# Defer repo cleanup of upstream
+defer gh repo delete --yes ${ORG}/${REPO}
+
+# Clone the repo
+exec gh repo clone ${ORG}/${REPO}
+cd ${REPO}
+
+# Configure push.default so that it should use the merge ref
+exec git config push.default upstream
+
+# But prepare a branch that doesn't have a tracking merge ref
+exec git checkout -b feature-branch
+exec git commit --allow-empty -m 'Empty Commit'
+exec git push origin feature-branch
+
+# Create the PR
+exec gh pr create --title 'Feature Title' --body 'Feature Body'
+stdout https://${GH_HOST}/${ORG}/${REPO}/pull/1

pkg/cmd/pr/shared/find_refs_resolution.go

@@ -333,12 +333,12 @@ func tryDetermineDefaultPushTarget(gitClient GitConfigClient, localBranchName st
 	}
 
 	// We assume the PR's branch name is the same as whatever was provided, unless the user has specified
-	// push.default = upstream or tracking, then we use the branch name from the merge ref.
+	// push.default = upstream or tracking, then we use the branch name from the merge ref if it exists. Otherwise, we fall back to the local branch name
 	remoteBranch := localBranchName
 	if pushDefault == git.PushDefaultUpstream || pushDefault == git.PushDefaultTracking {
-		remoteBranch = strings.TrimPrefix(branchConfig.MergeRef, "refs/heads/")
-		if remoteBranch == "" {
-			return defaultPushTarget{}, fmt.Errorf("could not determine remote branch name")
+		mergeRef := strings.TrimPrefix(branchConfig.MergeRef, "refs/heads/")
+		if mergeRef != "" {
+			remoteBranch = mergeRef
 		}
 	}
 

pkg/cmd/pr/shared/find_refs_resolution_test.go

@@ -462,7 +462,7 @@ func TestTryDetermineDefaultPRHead(t *testing.T) {
 			})
 		}
 
-		t.Run("but if the merge ref is empty, error", func(t *testing.T) {
+		t.Run("but if the merge ref is empty, use the provided branch name", func(t *testing.T) {
 			t.Parallel()
 
 			repoResolvedFromPushRemoteClient := stubGitConfigClient{
@@ -474,12 +474,14 @@ func TestTryDetermineDefaultPRHead(t *testing.T) {
 				pushDefaultFn: stubPushDefault(git.PushDefaultUpstream, nil),
 			}
 
-			_, err := TryDetermineDefaultPRHead(
+			defaultPRHead, err := TryDetermineDefaultPRHead(
 				repoResolvedFromPushRemoteClient,
 				stubRemoteToRepoResolver(ghContext.Remotes{&baseRemote, &forkRemote}, nil),
 				"feature-branch",
 			)
-			require.Error(t, err)
+			require.NoError(t, err)
+
+			require.Equal(t, "feature-branch", defaultPRHead.BranchName)
 		})
 	})