Fix compare link in active feeds for new branch (go-gitea#19149)

When a new branch is pushed the old SHA is always listed as the empty sha and thus the compare link that is created does not work correctly. 

Therefore when creating the compare link for new branches:

1. Attempt to get the parent of the first commit and use that as the basis 
for the compare link.
2. If this is not possible make a comparison to the default branch
3. Finally if that is not possible simply do not show a compare link.

However, there are multiple broken compare links remaining therefore, in order for these to not break we will simply make the compare link redirect to the default branch.

Fix go-gitea#19144

Signed-off-by: a1012112796 <1012112796@qq.com>
Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: Andrew Thornton <art27@cantab.net>

routers/web/repo/compare.go

@@ -298,6 +298,13 @@ func ParseCompareInfo(ctx *context.Context) *CompareInfo {
 			ci.BaseBranch = baseCommit.ID.String()
 			ctx.Data["BaseBranch"] = ci.BaseBranch
 			baseIsCommit = true
+		} else if ci.BaseBranch == git.EmptySHA {
+			if isSameRepo {
+				ctx.Redirect(ctx.Repo.RepoLink + "/compare/" + util.PathEscapeSegments(ci.HeadBranch))
+			} else {
+				ctx.Redirect(ctx.Repo.RepoLink + "/compare/" + util.PathEscapeSegments(ci.HeadRepo.FullName()) + ":" + util.PathEscapeSegments(ci.HeadBranch))
+			}
+			return nil
 		} else {
 			ctx.NotFound("IsRefExist", nil)
 			return nil

services/repository/push.go

@@ -222,7 +222,34 @@ func pushUpdates(optsList []*repo_module.PushUpdateOptions) error {
 				if len(commits.Commits) > setting.UI.FeedMaxCommitNum {
 					commits.Commits = commits.Commits[:setting.UI.FeedMaxCommitNum]
 				}
-				commits.CompareURL = repo.ComposeCompareURL(opts.OldCommitID, opts.NewCommitID)
+
+				oldCommitID := opts.OldCommitID
+				if oldCommitID == git.EmptySHA && len(commits.Commits) > 0 {
+					oldCommit, err := gitRepo.GetCommit(commits.Commits[len(commits.Commits)-1].Sha1)
+					if err != nil && !git.IsErrNotExist(err) {
+						log.Error("unable to GetCommit %s from %-v: %v", oldCommitID, repo, err)
+					}
+					if oldCommit != nil {
+						for i := 0; i < oldCommit.ParentCount(); i++ {
+							commitID, _ := oldCommit.ParentID(i)
+							if !commitID.IsZero() {
+								oldCommitID = commitID.String()
+								break
+							}
+						}
+					}
+				}
+
+				if oldCommitID == git.EmptySHA && repo.DefaultBranch != branch {
+					oldCommitID = repo.DefaultBranch
+				}
+
+				if oldCommitID != git.EmptySHA {
+					commits.CompareURL = repo.ComposeCompareURL(oldCommitID, opts.NewCommitID)
+				} else {
+					commits.CompareURL = ""
+				}
+
 				notification.NotifyPushCommits(pusher, repo, opts, commits)
 
 				if err = models.RemoveDeletedBranchByName(repo.ID, branch); err != nil {