Introduce workaround for missing pull head sha (#340)

fix #318

test with `tea pr 58`

Co-authored-by: Norwin Roosen <git@nroo.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/340
Reviewed-by: Norwin <noerw@noreply.gitea.io>
Reviewed-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>
Co-committed-by: 6543 <6543@obermui.de>

modules/workaround/pull.go

@@ -0,0 +1,30 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package workaround
+
+import (
+	"fmt"
+
+	"code.gitea.io/sdk/gitea"
+)
+
+// FixPullHeadSha is a workaround for https://github.com/go-gitea/gitea/issues/12675
+// When no head sha is available, this is because the branch got deleted in the base repo.
+// pr.Head.Ref points in this case not to the head repo branch name, but the base repo ref,
+// which stays available to resolve the commit sha.
+func FixPullHeadSha(client *gitea.Client, pr *gitea.PullRequest) error {
+	owner := pr.Base.Repository.Owner.UserName
+	repo := pr.Base.Repository.Name
+	if pr.Head != nil && pr.Head.Sha == "" {
+		refs, _, err := client.GetRepoRefs(owner, repo, pr.Head.Ref)
+		if err != nil {
+			return err
+		} else if len(refs) == 0 {
+			return fmt.Errorf("unable to resolve PR ref '%s'", pr.Head.Ref)
+		}
+		pr.Head.Sha = refs[0].Object.SHA
+	}
+	return nil
+}