Allow checking out PRs with deleted head branch (#341)

..by explicitly fetching `refs/pulls/:idx/head` from the base repo.

Sorry, I mixed this with a split-up of `PullCheckout()`. I can try to separate that, if preferred

Co-authored-by: Norwin Roosen <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/341
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

modules/task/pull_checkout.go

@@ -7,24 +7,25 @@ package task
 import (
 	"fmt"
 
+	"code.gitea.io/sdk/gitea"
 	"code.gitea.io/tea/modules/config"
 	local_git "code.gitea.io/tea/modules/git"
 	"code.gitea.io/tea/modules/workaround"
 
 	"github.com/go-git/go-git/v5"
+	git_config "github.com/go-git/go-git/v5/config"
 	git_plumbing "github.com/go-git/go-git/v5/plumbing"
 )
 
 // PullCheckout checkout current workdir to the head branch of specified pull request
-func PullCheckout(login *config.Login, repoOwner, repoName string, forceCreateBranch bool, index int64, callback func(string) (string, error)) error {
+func PullCheckout(
+	login *config.Login,
+	repoOwner, repoName string,
+	forceCreateBranch bool,
+	index int64,
+	callback func(string) (string, error),
+) error {
 	client := login.Client()
-
-	localRepo, err := local_git.RepoForWorkdir()
-	if err != nil {
-		return err
-	}
-
-	// fetch PR source-localRepo & -branch from gitea
 	pr, _, err := client.GetPullRequest(repoOwner, repoName, index)
 	if err != nil {
 		return err
@@ -33,21 +34,15 @@ func PullCheckout(login *config.Login, repoOwner, repoName string, forceCreateBr
 		return err
 	}
 
-	remoteDeleted := pr.Head.Ref == fmt.Sprintf("refs/pull/%d/head", pr.Index)
+	// FIXME: should use ctx.LocalRepo..?
-	if remoteDeleted {
+	localRepo, err := local_git.RepoForWorkdir()
-		return fmt.Errorf("Can't checkout: remote head branch was already deleted")
+	if err != nil {
-	}
+		return err
-
-	remoteURL := pr.Head.Repository.CloneURL
-	if len(login.SSHKey) != 0 {
-		// login.SSHKey is nonempty, if user specified a key manually or we automatically
-		// found a matching private key on this machine during login creation.
-		// this means, we are very likely to have a working ssh setup.
-		remoteURL = pr.Head.Repository.SSHURL
 	}
 
+	// find or create a matching remote
+	remoteURL := remoteURLForPR(login, pr)
 	newRemoteName := fmt.Sprintf("pulls/%v", pr.Head.Repository.Owner.UserName)
-
 	// verify related remote is in local repo, otherwise add it
 	localRemote, err := localRepo.GetOrCreateRemote(remoteURL, newRemoteName)
 	if err != nil {
@@ -55,47 +50,116 @@ func PullCheckout(login *config.Login, repoOwner, repoName string, forceCreateBr
 	}
 	localRemoteName := localRemote.Config().Name
 
-	// get auth & fetch remote via its configured protocol
+	localRemoteBranchName, err := doPRFetch(login, pr, localRepo, localRemote, callback)
-	url, err := localRepo.TeaRemoteURL(localRemoteName)
 	if err != nil {
 		return err
 	}
-	auth, err := local_git.GetAuthForURL(url, login.Token, login.SSHKey, callback)
-	if err != nil {
-		return err
-	}
-	fmt.Printf("Fetching PR %v (head %s:%s) from remote '%s'\n", index, url, pr.Head.Ref, localRemoteName)
-	err = localRemote.Fetch(&git.FetchOptions{Auth: auth})
-	if err == git.NoErrAlreadyUpToDate {
-		fmt.Println(err)
-	} else if err != nil {
-		return err
-	}
 
+	return doPRCheckout(localRepo, pr, localRemoteName, localRemoteBranchName, remoteURL, forceCreateBranch)
+}
+
+func isRemoteDeleted(pr *gitea.PullRequest) bool {
+	return pr.Head.Ref == fmt.Sprintf("refs/pull/%d/head", pr.Index)
+}
+
+func remoteURLForPR(login *config.Login, pr *gitea.PullRequest) string {
+	repo := pr.Head.Repository
+	if isRemoteDeleted(pr) {
+		repo = pr.Base.Repository
+	}
+	if len(login.SSHKey) != 0 {
+		// login.SSHKey is nonempty, if user specified a key manually or we automatically
+		// found a matching private key on this machine during login creation.
+		// this means, we are very likely to have a working ssh setup.
+		return repo.SSHURL
+	}
+	return repo.CloneURL
+}
+
+func doPRFetch(
+	login *config.Login,
+	pr *gitea.PullRequest,
+	localRepo *local_git.TeaRepo,
+	localRemote *git.Remote,
+	callback func(string) (string, error),
+) (string, error) {
+	localRemoteName := localRemote.Config().Name
+	localBranchName := pr.Head.Ref
+	// get auth & fetch remote via its configured protocol
+	url, err := localRepo.TeaRemoteURL(localRemoteName)
+	if err != nil {
+		return "", err
+	}
+	auth, err := local_git.GetAuthForURL(url, login.Token, login.SSHKey, callback)
+	if err != nil {
+		return "", err
+	}
+	fetchOpts := &git.FetchOptions{Auth: auth}
+	if isRemoteDeleted(pr) {
+		// When the head branch is already deleted, pr.Head.Ref points to
+		// `refs/pull/<idx>/head`, where the commits stay available.
+		// This ref must be fetched explicitly, and does not allow pushing, so we use it
+		// only in this case as fallback.
+		localBranchName = fmt.Sprintf("pulls/%d", pr.Index)
+		fetchOpts.RefSpecs = []git_config.RefSpec{git_config.RefSpec(fmt.Sprintf("%s:refs/remotes/%s/%s",
+			pr.Head.Ref,
+			localRemoteName,
+			localBranchName,
+		))}
+	}
+	fmt.Printf("Fetching PR %v (head %s:%s) from remote '%s'\n", pr.Index, url, pr.Head.Ref, localRemoteName)
+
+	err = localRemote.Fetch(fetchOpts)
+	if err == git.NoErrAlreadyUpToDate {
+		fmt.Println(err)
+	} else if err != nil {
+		return "", err
+	}
+	return localBranchName, nil
+}
+
+func doPRCheckout(
+	localRepo *local_git.TeaRepo,
+	pr *gitea.PullRequest,
+	localRemoteName,
+	localRemoteBranchName,
+	remoteURL string,
+	forceCreateBranch bool,
+) error {
+	// determine the ref to checkout, depending on existence of a matching commit on a local branch
 	var info string
 	var checkoutRef git_plumbing.ReferenceName
 
 	if b, _ := localRepo.TeaFindBranchBySha(pr.Head.Sha, remoteURL); b != nil {
+
 		// if a matching branch exists, use that
 		checkoutRef = git_plumbing.NewBranchReferenceName(b.Name)
 		info = fmt.Sprintf("Found matching local branch %s, checking it out", checkoutRef.Short())
+
 	} else if forceCreateBranch {
+
 		// create a branch if wanted
-		localBranchName := fmt.Sprintf("pulls/%v-%v", index, pr.Head.Ref)
+		localBranchName := fmt.Sprintf("pulls/%v", pr.Index)
+		if isRemoteDeleted(pr) {
+			localBranchName += "-" + pr.Head.Ref
+		}
 		checkoutRef = git_plumbing.NewBranchReferenceName(localBranchName)
-		if err = localRepo.TeaCreateBranch(localBranchName, pr.Head.Ref, localRemoteName); err == nil {
+		if err := localRepo.TeaCreateBranch(localBranchName, localRemoteBranchName, localRemoteName); err == nil {
 			info = fmt.Sprintf("Created branch '%s'\n", localBranchName)
 		} else if err == git.ErrBranchExists {
 			info = "There may be changes since you last checked out, run `git pull` to get them."
 		} else {
 			return err
 		}
+
 	} else {
+
 		// use the remote tracking branch
-		checkoutRef = git_plumbing.NewRemoteReferenceName(localRemoteName, pr.Head.Ref)
+		checkoutRef = git_plumbing.NewRemoteReferenceName(localRemoteName, localRemoteBranchName)
 		info = fmt.Sprintf(
 			"Checking out remote tracking branch %s. To make changes, create a new branch:\n  git checkout %s",
-			checkoutRef.String(), pr.Head.Ref)
+			checkoutRef.String(), localRemoteBranchName)
+
 	}
 
 	fmt.Println(info)