Branches not at ref commit ID should not be listed as Merged (#9614)

Once a branch has been merged if the commit ID no longer equals that of
the pulls ref commit id don't offer to delete the branch on the pull screen
and don't list it as merged on branches.

Fix #9201

When looking at the pull page we should also get the commits from the refs/pulls/x/head

Fix #9158

10 files changed, 107 insertions, 34 deletions

diff --git a/integrations/gitea-repositories-meta/user2/repo1.git/objects/00/750edc07d6415dcc07ae0351e9397b0222b7ba b/integrations/gitea-repositories-meta/user2/repo1.git/objects/00/750edc07d6415dcc07ae0351e9397b0222b7ba
new file mode 100644
index 0000000000..d3c45d51ea
--- /dev/null
+++ b/integrations/gitea-repositories-meta/user2/repo1.git/objects/00/750edc07d6415dcc07ae0351e9397b0222b7ba
diff --git a/integrations/gitea-repositories-meta/user2/repo1.git/objects/4a/357436d925b5c974181ff12a994538ddc5a269 b/integrations/gitea-repositories-meta/user2/repo1.git/objects/4a/357436d925b5c974181ff12a994538ddc5a269
new file mode 100644
index 0000000000..bf97d00fd8
--- /dev/null
+++ b/integrations/gitea-repositories-meta/user2/repo1.git/objects/4a/357436d925b5c974181ff12a994538ddc5a269
diff --git a/integrations/gitea-repositories-meta/user2/repo1.git/objects/dc/7a8ba127fee870dd683310ce660dfe59333a1b b/integrations/gitea-repositories-meta/user2/repo1.git/objects/dc/7a8ba127fee870dd683310ce660dfe59333a1b
new file mode 100644
index 0000000000..7678d6754d
--- /dev/null
+++ b/integrations/gitea-repositories-meta/user2/repo1.git/objects/dc/7a8ba127fee870dd683310ce660dfe59333a1b
diff --git a/integrations/gitea-repositories-meta/user2/repo1.git/refs/pull/3/head b/integrations/gitea-repositories-meta/user2/repo1.git/refs/pull/3/head
new file mode 100644
index 0000000000..98593d6537
--- /dev/null
+++ b/integrations/gitea-repositories-meta/user2/repo1.git/refs/pull/3/head
@@ -0,0 +1 @@
+4a357436d925b5c974181ff12a994538ddc5a269
diff --git a/models/pull.go b/models/pull.go
index 9a8777aca3..f86892cbfc 100644
--- a/models/pull.go
+++ b/models/pull.go
@@ -122,7 +122,7 @@ func (pr *PullRequest) LoadHeadRepo() error {
 		if has, err := x.ID(pr.HeadRepoID).Get(&repo); err != nil {
 			return err
 		} else if !has {
-			return ErrRepoNotExist{ID: pr.BaseRepoID}
+			return ErrRepoNotExist{ID: pr.HeadRepoID}
 		}
 		pr.HeadRepo = &repo
 	}
diff --git a/routers/repo/branch.go b/routers/repo/branch.go
index b0a1efc5b9..df6e0bf21f 100644
--- a/routers/repo/branch.go
+++ b/routers/repo/branch.go
@@ -16,6 +16,7 @@ import (
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/repofiles"
 	"code.gitea.io/gitea/modules/util"
+	"gopkg.in/src-d/go-git.v4/plumbing"
 )
 
 const (
@@ -33,6 +34,7 @@ type Branch struct {
 	CommitsAhead      int
 	CommitsBehind     int
 	LatestPullRequest *models.PullRequest
+	MergeMovedOn      bool
 }
 
 // Branches render repository branch page
@@ -185,6 +187,12 @@ func loadBranches(ctx *context.Context) []*Branch {
 		return nil
 	}
 
+	repoIDToRepo := map[int64]*models.Repository{}
+	repoIDToRepo[ctx.Repo.Repository.ID] = ctx.Repo.Repository
+
+	repoIDToGitRepo := map[int64]*git.Repository{}
+	repoIDToGitRepo[ctx.Repo.Repository.ID] = ctx.Repo.GitRepo
+
 	branches := make([]*Branch, len(rawBranches))
 	for i := range rawBranches {
 		commit, err := rawBranches[i].GetCommit()
@@ -213,11 +221,46 @@ func loadBranches(ctx *context.Context) []*Branch {
 			ctx.ServerError("GetLatestPullRequestByHeadInfo", err)
 			return nil
 		}
+		headCommit := commit.ID.String()
+
+		mergeMovedOn := false
 		if pr != nil {
+			pr.HeadRepo = ctx.Repo.Repository
 			if err := pr.LoadIssue(); err != nil {
 				ctx.ServerError("pr.LoadIssue", err)
 				return nil
 			}
+			if repo, ok := repoIDToRepo[pr.BaseRepoID]; ok {
+				pr.BaseRepo = repo
+			} else if err := pr.LoadBaseRepo(); err != nil {
+				ctx.ServerError("pr.LoadBaseRepo", err)
+				return nil
+			} else {
+				repoIDToRepo[pr.BaseRepoID] = pr.BaseRepo
+			}
+
+			if pr.HasMerged {
+				baseGitRepo, ok := repoIDToGitRepo[pr.BaseRepoID]
+				if !ok {
+					baseGitRepo, err = git.OpenRepository(pr.BaseRepo.RepoPath())
+					if err != nil {
+						ctx.ServerError("OpenRepository", err)
+						return nil
+					}
+					defer baseGitRepo.Close()
+					repoIDToGitRepo[pr.BaseRepoID] = baseGitRepo
+				}
+				pullCommit, err := baseGitRepo.GetRefCommitID(pr.GetGitRefName())
+				if err != nil && err != plumbing.ErrReferenceNotFound {
+					ctx.ServerError("GetBranchCommitID", err)
+					return nil
+				}
+				if err == nil && headCommit != pullCommit {
+					// the head has moved on from the merge - we shouldn't delete
+					mergeMovedOn = true
+				}
+			}
+
 		}
 
 		isIncluded := divergence.Ahead == 0 && ctx.Repo.Repository.DefaultBranch != branchName
@@ -230,6 +273,7 @@ func loadBranches(ctx *context.Context) []*Branch {
 			CommitsAhead:      divergence.Ahead,
 			CommitsBehind:     divergence.Behind,
 			LatestPullRequest: pr,
+			MergeMovedOn:      mergeMovedOn,
 		}
 	}
 
diff --git a/routers/repo/issue.go b/routers/repo/issue.go
index 46020acb6d..0a78e06b41 100644
--- a/routers/repo/issue.go
+++ b/routers/repo/issue.go
@@ -966,7 +966,10 @@ func ViewIssue(ctx *context.Context) {
 			ctx.Data["IsBlockedByRejection"] = pull.ProtectedBranch.MergeBlockedByRejectedReview(pull)
 			ctx.Data["GrantedApprovals"] = cnt
 		}
-		ctx.Data["IsPullBranchDeletable"] = canDelete && pull.HeadRepo != nil && git.IsBranchExist(pull.HeadRepo.RepoPath(), pull.HeadBranch)
+		ctx.Data["IsPullBranchDeletable"] = canDelete &&
+			pull.HeadRepo != nil &&
+			git.IsBranchExist(pull.HeadRepo.RepoPath(), pull.HeadBranch) &&
+			(!pull.HasMerged || ctx.Data["HeadBranchCommitID"] == ctx.Data["PullHeadCommitID"])
 
 		ctx.Data["PullReviewers"], err = models.GetReviewersByIssueID(issue.ID)
 		if err != nil {
diff --git a/routers/repo/pull.go b/routers/repo/pull.go
index 418c2e9438..1a5c4a036f 100644
--- a/routers/repo/pull.go
+++ b/routers/repo/pull.go
@@ -330,25 +330,37 @@ func PrepareViewPullInfo(ctx *context.Context, issue *models.Issue) *git.Compare
 	repo := ctx.Repo.Repository
 	pull := issue.PullRequest
 
-	var err error
-	if err = pull.GetHeadRepo(); err != nil {
+	if err := pull.GetHeadRepo(); err != nil {
 		ctx.ServerError("GetHeadRepo", err)
 		return nil
 	}
 
+	if err := pull.GetBaseRepo(); err != nil {
+		ctx.ServerError("GetBaseRepo", err)
+		return nil
+	}
+
 	setMergeTarget(ctx, pull)
 
-	if err = pull.LoadProtectedBranch(); err != nil {
+	if err := pull.LoadProtectedBranch(); err != nil {
 		ctx.ServerError("GetLatestCommitStatus", err)
 		return nil
 	}
 	ctx.Data["EnableStatusCheck"] = pull.ProtectedBranch != nil && pull.ProtectedBranch.EnableStatusCheck
 
-	var headGitRepo *git.Repository
+	baseGitRepo, err := git.OpenRepository(pull.BaseRepo.RepoPath())
+	if err != nil {
+		ctx.ServerError("OpenRepository", err)
+		return nil
+	}
+	defer baseGitRepo.Close()
 	var headBranchExist bool
+	var headBranchSha string
 	// HeadRepo may be missing
 	if pull.HeadRepo != nil {
-		headGitRepo, err = git.OpenRepository(pull.HeadRepo.RepoPath())
+		var err error
+
+		headGitRepo, err := git.OpenRepository(pull.HeadRepo.RepoPath())
 		if err != nil {
 			ctx.ServerError("OpenRepository", err)
 			return nil
@@ -358,46 +370,53 @@ func PrepareViewPullInfo(ctx *context.Context, issue *models.Issue) *git.Compare
 		headBranchExist = headGitRepo.IsBranchExist(pull.HeadBranch)
 
 		if headBranchExist {
-			sha, err := headGitRepo.GetBranchCommitID(pull.HeadBranch)
+			headBranchSha, err = headGitRepo.GetBranchCommitID(pull.HeadBranch)
 			if err != nil {
 				ctx.ServerError("GetBranchCommitID", err)
 				return nil
 			}
+		}
+	}
 
-			commitStatuses, err := models.GetLatestCommitStatus(repo, sha, 0)
-			if err != nil {
-				ctx.ServerError("GetLatestCommitStatus", err)
-				return nil
-			}
-			if len(commitStatuses) > 0 {
-				ctx.Data["LatestCommitStatuses"] = commitStatuses
-				ctx.Data["LatestCommitStatus"] = models.CalcCommitStatus(commitStatuses)
-			}
+	sha, err := baseGitRepo.GetRefCommitID(pull.GetGitRefName())
+	if err != nil {
+		ctx.ServerError(fmt.Sprintf("GetRefCommitID(%s)", pull.GetGitRefName()), err)
+		return nil
+	}
 
-			if pull.ProtectedBranch != nil && pull.ProtectedBranch.EnableStatusCheck {
-				ctx.Data["is_context_required"] = func(context string) bool {
-					for _, c := range pull.ProtectedBranch.StatusCheckContexts {
-						if c == context {
-							return true
-						}
-					}
-					return false
+	commitStatuses, err := models.GetLatestCommitStatus(repo, sha, 0)
+	if err != nil {
+		ctx.ServerError("GetLatestCommitStatus", err)
+		return nil
+	}
+	if len(commitStatuses) > 0 {
+		ctx.Data["LatestCommitStatuses"] = commitStatuses
+		ctx.Data["LatestCommitStatus"] = models.CalcCommitStatus(commitStatuses)
+	}
+
+	if pull.ProtectedBranch != nil && pull.ProtectedBranch.EnableStatusCheck {
+		ctx.Data["is_context_required"] = func(context string) bool {
+			for _, c := range pull.ProtectedBranch.StatusCheckContexts {
+				if c == context {
+					return true
 				}
-				ctx.Data["IsRequiredStatusCheckSuccess"] = pull_service.IsCommitStatusContextSuccess(commitStatuses, pull.ProtectedBranch.StatusCheckContexts)
 			}
+			return false
 		}
+		ctx.Data["IsRequiredStatusCheckSuccess"] = pull_service.IsCommitStatusContextSuccess(commitStatuses, pull.ProtectedBranch.StatusCheckContexts)
 	}
 
-	if pull.HeadRepo == nil || !headBranchExist {
+	ctx.Data["HeadBranchMovedOn"] = headBranchSha != sha
+	ctx.Data["HeadBranchCommitID"] = headBranchSha
+	ctx.Data["PullHeadCommitID"] = sha
+
+	if pull.HeadRepo == nil || !headBranchExist || headBranchSha != sha {
 		ctx.Data["IsPullRequestBroken"] = true
 		ctx.Data["HeadTarget"] = "deleted"
-		ctx.Data["NumCommits"] = 0
-		ctx.Data["NumFiles"] = 0
-		return nil
 	}
 
-	compareInfo, err := headGitRepo.GetCompareInfo(models.RepoPath(repo.Owner.Name, repo.Name),
-		pull.BaseBranch, pull.HeadBranch)
+	compareInfo, err := baseGitRepo.GetCompareInfo(pull.BaseRepo.RepoPath(),
+		pull.BaseBranch, pull.GetGitRefName())
 	if err != nil {
 		if strings.Contains(err.Error(), "fatal: Not a valid object name") {
 			ctx.Data["IsPullRequestBroken"] = true
diff --git a/templates/repo/branch/list.tmpl b/templates/repo/branch/list.tmpl
index 8fb365d805..073516f25f 100644
--- a/templates/repo/branch/list.tmpl
+++ b/templates/repo/branch/list.tmpl
@@ -84,6 +84,12 @@
 												<button id="new-pull-request" class="ui compact basic button">{{$.i18n.Tr "repo.pulls.compare_changes"}}</button>
 											</a>
 											{{end}}
+										{{else if and .LatestPullRequest.HasMerged .MergeMovedOn}}
+											{{if and (not .IsDeleted) $.AllowsPulls (gt .CommitsAhead 0)}}
+											<a href="{{$.RepoLink}}/compare/{{$.DefaultBranch | EscapePound}}...{{if ne $.Repository.Owner.Name $.Owner.Name}}{{$.Owner.Name}}:{{end}}{{.Name | EscapePound}}">
+												<button id="new-pull-request" class="ui compact basic button">{{$.i18n.Tr "repo.pulls.compare_changes"}}</button>
+											</a>
+											{{end}}
 										{{else}}
 											<a href="{{$.RepoLink}}/pulls/{{.LatestPullRequest.Issue.Index}}">#{{.LatestPullRequest.Issue.Index}}</a>
 											{{if .LatestPullRequest.HasMerged}}
diff --git a/templates/repo/issue/view_content/pull.tmpl b/templates/repo/issue/view_content/pull.tmpl
index cec10a620e..2dc76dcf2e 100644
--- a/templates/repo/issue/view_content/pull.tmpl
+++ b/templates/repo/issue/view_content/pull.tmpl
@@ -72,7 +72,7 @@
 						{{$.i18n.Tr "repo.pulls.reopen_to_merge"}}
 					{{end}}
 				</div>
-				{{if .IsPullBranchDeletable}}
+				{{if and .IsPullBranchDeletable ( not .IsPullRequestBroken )}}
 					<div class="ui divider"></div>
 					<div>
 						<a class="delete-button ui red button" href="" data-url="{{.DeleteBranchLink}}">{{$.i18n.Tr "repo.branch.delete" .HeadTarget}}</a>
@@ -105,7 +105,7 @@
 				<div class="item text red">
 					<span class="octicon octicon-x"></span>
 				{{$.i18n.Tr "repo.pulls.blocked_by_rejection"}}
-				</div>			
+				</div>
 			{{else if .Issue.PullRequest.IsChecking}}
 				<div class="item text yellow">
 					<span class="octicon octicon-sync"></span>