Fix compare dropdown for branches without common history (#37470)

modules/gitrepo/compare_test.go

@@ -4,9 +4,15 @@
 package gitrepo
 
 import (
+	"path/filepath"
+	"strings"
 	"testing"
 
+	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/util"
+
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 type mockRepository struct {
@@ -17,6 +23,32 @@ func (r *mockRepository) RelativePath() string {
 	return r.path
 }
 
+func TestMergeBaseNoCommonHistory(t *testing.T) {
+	repoDir := filepath.Join(t.TempDir(), "repo.git")
+	require.NoError(t, gitcmd.NewCommand("init").AddDynamicArguments(repoDir).Run(t.Context()))
+	_, _, runErr := gitcmd.NewCommand("fast-import").WithDir(repoDir).WithStdinBytes([]byte(strings.TrimSpace(`
+commit refs/heads/branch1
+committer User <user@example.com> 1714310400 +0000
+data 12
+First commit
+M 100644 inline file1.txt
+data 12
+Hello from 1
+
+commit refs/heads/branch2
+committer User <user@example.com> 1714310400 +0000
+data 13
+Second commit
+M 100644 inline file2.txt
+data 12
+Hello from 2
+`))).RunStdString(t.Context())
+	require.NoError(t, runErr)
+	mergeBase, err := MergeBase(t.Context(), &mockRepository{path: repoDir}, "branch1", "branch2")
+	assert.Empty(t, mergeBase)
+	assert.ErrorIs(t, err, util.ErrNotExist)
+}
+
 func TestRepoGetDivergingCommits(t *testing.T) {
 	repo := &mockRepository{path: "repo1_bare"}
 	do, err := GetDivergingCommits(t.Context(), repo, "master", "branch2")

modules/gitrepo/merge.go

@@ -9,13 +9,17 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/util"
 )
 
 // MergeBase checks and returns merge base of two commits.
 func MergeBase(ctx context.Context, repo Repository, baseCommitID, headCommitID string) (string, error) {
-	mergeBase, _, err := RunCmdString(ctx, repo, gitcmd.NewCommand("merge-base").
+	mergeBase, stderr, err := RunCmdString(ctx, repo, gitcmd.NewCommand("merge-base").
 		AddDashesAndList(baseCommitID, headCommitID))
 	if err != nil {
+		if gitcmd.IsErrorExitCode(err, 1) && strings.TrimSpace(stderr) == "" {
+			return "", util.NewNotExistErrorf("merge-base for %s and %s doesn't exist", baseCommitID, headCommitID)
+		}
 		return "", fmt.Errorf("get merge-base of %s and %s failed: %w", baseCommitID, headCommitID, err)
 	}
 	return strings.TrimSpace(mergeBase), nil

options/locale/locale_en-US.json

@@ -1784,6 +1784,7 @@
   "repo.pulls.review_only_possible_for_full_diff": "Review is only possible when viewing the full diff",
   "repo.pulls.filter_changes_by_commit": "Filter by commit",
   "repo.pulls.nothing_to_compare": "These branches are equal. There is no need to create a pull request.",
+  "repo.pulls.no_common_history": "These branches do not share a common merge base. Select a different base or compare branch.",
   "repo.pulls.nothing_to_compare_have_tag": "The selected branches/tags are equal.",
   "repo.pulls.nothing_to_compare_and_allow_empty_pr": "These branches are equal. This PR will be empty.",
   "repo.pulls.has_pull_request": "A pull request between these branches already exists: <a href=\"%[1]s\">%[2]s#%[3]d</a>",

routers/web/repo/compare.go

@@ -189,8 +189,8 @@ func setCsvCompareContext(ctx *context.Context) {
 	}
 }
 
-// ParseCompareInfo parse compare info between two commit for preparing comparing references
-func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
+// parseCompareInfo parse compare info between two commit for preparing comparing references
+func parseCompareInfo(ctx *context.Context) (*git_service.CompareInfo, error) {
 	baseRepo := ctx.Repo.Repository
 	fileOnly := ctx.FormBool("file-only")
 
@@ -199,47 +199,29 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 
 	// remove the check when we support compare with carets
 	if compareReq.BaseOriRefSuffix != "" {
-		ctx.HTTPError(http.StatusBadRequest, "Unsupported comparison syntax: ref with suffix")
-		return nil
+		return nil, util.NewInvalidArgumentErrorf("unsupported comparison syntax: ref with suffix")
 	}
 
 	// 2 get repository and owner for head
 	headOwner, headRepo, err := common.GetHeadOwnerAndRepo(ctx, baseRepo, compareReq)
-	switch {
-	case errors.Is(err, util.ErrInvalidArgument):
-		ctx.HTTPError(http.StatusBadRequest, err.Error())
-		return nil
-	case errors.Is(err, util.ErrNotExist):
-		ctx.NotFound(nil)
-		return nil
-	case err != nil:
-		ctx.ServerError("GetHeadOwnerAndRepo", err)
-		return nil
+	if err != nil {
+		return nil, err
 	}
 
-	isSameRepo := baseRepo.ID == headRepo.ID
-
 	// 3 permission check
 	// base repository's code unit read permission check has been done on web.go
 	permBase := ctx.Repo.Permission
 
 	// If we're not merging from the same repo:
+	isSameRepo := baseRepo.ID == headRepo.ID
 	if !isSameRepo {
 		// Assert ctx.Doer has permission to read headRepo's codes
 		permHead, err := access_model.GetDoerRepoPermission(ctx, headRepo, ctx.Doer)
 		if err != nil {
-			ctx.ServerError("GetDoerRepoPermission", err)
-			return nil
+			return nil, err
 		}
 		if !permHead.CanRead(unit.TypeCode) {
-			if log.IsTrace() {
-				log.Trace("Permission Denied: User: %-v cannot read code in Repo: %-v\nUser in headRepo has Permissions: %-+v",
-					ctx.Doer,
-					headRepo,
-					permHead)
-			}
-			ctx.NotFound(nil)
-			return nil
+			return nil, util.NewNotExistErrorf("") // permission: no error message for end users
 		}
 		ctx.Data["CanWriteToHeadRepo"] = permHead.CanWrite(unit.TypeCode)
 	}
@@ -250,24 +232,17 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 
 	baseRef := ctx.Repo.GitRepo.UnstableGuessRefByShortName(baseRefName)
 	if baseRef == "" {
-		ctx.NotFound(nil)
-		return nil
+		return nil, util.NewNotExistErrorf("no base ref: %s", baseRefName)
 	}
-	var headGitRepo *git.Repository
-	if isSameRepo {
-		headGitRepo = ctx.Repo.GitRepo
-	} else {
-		headGitRepo, err = gitrepo.OpenRepository(ctx, headRepo)
-		if err != nil {
-			ctx.ServerError("OpenRepository", err)
-			return nil
-		}
-		defer headGitRepo.Close()
+	headGitRepo, err := gitrepo.RepositoryFromRequestContextOrOpen(ctx, headRepo)
+	if err != nil {
+		ctx.ServerError("OpenRepository", err)
+		return nil, err
 	}
+
 	headRef := headGitRepo.UnstableGuessRefByShortName(headRefName)
 	if headRef == "" {
-		ctx.NotFound(nil)
-		return nil
+		return nil, util.NewNotExistErrorf("no head ref: %s", headRefName)
 	}
 
 	ctx.Data["BaseName"] = baseRepo.OwnerName
@@ -291,12 +266,9 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 	var rootRepo *repo_model.Repository
 	if baseRepo.IsFork {
 		err = baseRepo.GetBaseRepo(ctx)
-		if err != nil {
-			if !repo_model.IsErrRepoNotExist(err) {
-				ctx.ServerError("Unable to find root repo", err)
-				return nil
-			}
-		} else {
+		if err != nil && !repo_model.IsErrRepoNotExist(err) {
+			return nil, err
+		} else if err == nil {
 			rootRepo = baseRepo.BaseRepo
 		}
 	}
@@ -313,42 +285,10 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 		}
 	}
 
-	has := headRepo != nil
-	// 3. If the base is a forked from "RootRepo" and the owner of
-	// the "RootRepo" is the :headUser - set headRepo to that
-	if !has && rootRepo != nil && rootRepo.OwnerID == headOwner.ID {
-		headRepo = rootRepo
-		has = true
-	}
-
-	// 4. If the ctx.Doer has their own fork of the baseRepo and the headUser is the ctx.Doer
-	// set the headRepo to the ownFork
-	if !has && ownForkRepo != nil && ownForkRepo.OwnerID == headOwner.ID {
-		headRepo = ownForkRepo
-		has = true
-	}
-
-	// 5. If the headOwner has a fork of the baseRepo - use that
-	if !has {
-		headRepo = repo_model.GetForkedRepo(ctx, headOwner.ID, baseRepo.ID)
-		has = headRepo != nil
-	}
-
-	// 6. If the baseRepo is a fork and the headUser has a fork of that use that
-	if !has && baseRepo.IsFork {
-		headRepo = repo_model.GetForkedRepo(ctx, headOwner.ID, baseRepo.ForkID)
-		has = headRepo != nil
-	}
-
-	// 7. Otherwise if we're not the same repo and haven't found a repo give up
-	if !isSameRepo && !has {
-		ctx.Data["PageIsComparePull"] = false
-	}
-
 	ctx.Data["HeadRepo"] = headRepo
 	ctx.Data["BaseCompareRepo"] = ctx.Repo.Repository
 
-	// If we have a rootRepo and it's different from:
+	// If we have a rootRepo, and it's different from:
 	// 1. the computed base
 	// 2. the computed head
 	// then get the branches of it
@@ -361,17 +301,15 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 			if !fileOnly {
 				branches, tags, err := getBranchesAndTagsForRepo(ctx, rootRepo)
 				if err != nil {
-					ctx.ServerError("GetBranchesForRepo", err)
-					return nil
+					return nil, err
 				}
-
 				ctx.Data["RootRepoBranches"] = branches
 				ctx.Data["RootRepoTags"] = tags
 			}
 		}
 	}
 
-	// If we have a ownForkRepo and it's different from:
+	// If we have a ownForkRepo, and it's different from:
 	// 1. The computed base
 	// 2. The computed head
 	// 3. The rootRepo (if we have one)
@@ -386,8 +324,7 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 			if !fileOnly {
 				branches, tags, err := getBranchesAndTagsForRepo(ctx, ownForkRepo)
 				if err != nil {
-					ctx.ServerError("GetBranchesForRepo", err)
-					return nil
+					return nil, err
 				}
 				ctx.Data["OwnForkRepoBranches"] = branches
 				ctx.Data["OwnForkRepoTags"] = tags
@@ -395,33 +332,21 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 		}
 	}
 
-	// Treat as pull request if both references are branches
-	if ctx.Data["PageIsComparePull"] == nil {
-		ctx.Data["PageIsComparePull"] = baseRef.IsBranch() && headRef.IsBranch() && permBase.CanReadIssuesOrPulls(true)
-	}
-
-	if ctx.Data["PageIsComparePull"] == true && !permBase.CanReadIssuesOrPulls(true) {
-		if log.IsTrace() {
-			log.Trace("Permission Denied: User: %-v cannot create/read pull requests in Repo: %-v\nUser in baseRepo has Permissions: %-+v",
-				ctx.Doer,
-				baseRepo,
-				permBase)
-		}
-		ctx.NotFound(nil)
-		return nil
-	}
-
 	compareInfo, err := git_service.GetCompareInfo(ctx, baseRepo, headRepo, headGitRepo, baseRef, headRef, compareReq.DirectComparison(), fileOnly)
 	if err != nil {
-		ctx.ServerError("GetCompareInfo", err)
-		return nil
+		return nil, err
 	}
+
+	// Treat as pull request if both references are branches
+	allowCreatePullRequest := baseRef.IsBranch() && headRef.IsBranch() && permBase.CanReadIssuesOrPulls(true)
+	allowCreatePullRequest = allowCreatePullRequest && compareInfo.MergeBase != ""
+	ctx.Data["PageIsComparePull"] = allowCreatePullRequest
 	if compareReq.DirectComparison() {
 		ctx.Data["BeforeCommitID"] = compareInfo.BaseCommitID
 	} else {
 		ctx.Data["BeforeCommitID"] = compareInfo.MergeBase
 	}
-	return &compareInfo
+	return &compareInfo, nil
 }
 
 func prepareNewPullRequestTitleContent(ci *git_service.CompareInfo, commits []*git_model.SignCommitWithStatuses) (title, content string) {
@@ -454,12 +379,11 @@ func prepareNewPullRequestTitleContent(ci *git_service.CompareInfo, commits []*g
 	return title, content
 }
 
-// PrepareCompareDiff renders compare diff page
-func PrepareCompareDiff(
-	ctx *context.Context,
-	ci *git_service.CompareInfo,
-	whitespaceBehavior gitcmd.TrustedCmdArgs,
-) (nothingToCompare bool) {
+// prepareCompareDiff renders compare diff page. TODO: need to refactor it and other "compare diff" related functions together
+func prepareCompareDiff(ctx *context.Context, ci *git_service.CompareInfo, whitespaceBehavior gitcmd.TrustedCmdArgs) (nothingToCompare bool) {
+	if ci.MergeBase == "" {
+		return true
+	}
 	repo := ctx.Repo.Repository
 	headCommitID := ci.HeadCommitID
 
@@ -568,9 +492,6 @@ func PrepareCompareDiff(
 	ctx.Data["CommitCount"] = len(commits)
 
 	ctx.Data["title"], ctx.Data["content"] = prepareNewPullRequestTitleContent(ci, commits)
-	ctx.Data["Username"] = ci.HeadRepo.OwnerName
-	ctx.Data["Reponame"] = ci.HeadRepo.Name
-
 	setCompareContext(ctx, beforeCommit, headCommit, ci.HeadRepo.OwnerName, repo.Name)
 
 	return false
@@ -594,16 +515,24 @@ func getBranchesAndTagsForRepo(ctx gocontext.Context, repo *repo_model.Repositor
 
 // CompareDiff show different from one commit to another commit
 func CompareDiff(ctx *context.Context) {
-	ci := ParseCompareInfo(ctx)
+	ci, err := parseCompareInfo(ctx)
 	if ctx.Written() {
 		return
 	}
+	if errors.Is(err, util.ErrNotExist) || errors.Is(err, util.ErrInvalidArgument) {
+		ctx.NotFound(nil)
+		return
+	} else if err != nil {
+		ctx.ServerError("ParseCompareInfo", err)
+		return
+	}
 
 	ctx.Data["PageIsViewCode"] = true
 	ctx.Data["PullRequestWorkInProgressPrefixes"] = setting.Repository.PullRequest.WorkInProgressPrefixes
 	ctx.Data["CompareInfo"] = ci
 
-	nothingToCompare := PrepareCompareDiff(ctx, ci, gitdiff.GetWhitespaceFlag(ctx.Data["WhitespaceBehavior"].(string)))
+	// TODO: need to refactor "prepare compare" related functions together
+	nothingToCompare := prepareCompareDiff(ctx, ci, gitdiff.GetWhitespaceFlag(ctx.Data["WhitespaceBehavior"].(string)))
 	if ctx.Written() {
 		return
 	}
@@ -621,16 +550,13 @@ func CompareDiff(ctx *context.Context) {
 		return
 	}
 
-	headBranches, err := git_model.FindBranchNames(ctx, git_model.FindBranchOptions{
-		RepoID:          ci.HeadRepo.ID,
-		ListOptions:     db.ListOptionsAll,
-		IsDeletedBranch: optional.Some(false),
-	})
+	headBranches, headTags, err := getBranchesAndTagsForRepo(ctx, ci.HeadRepo)
 	if err != nil {
-		ctx.ServerError("GetBranches", err)
+		ctx.ServerError("GetBranchesAndTagsForRepo", err)
 		return
 	}
 	ctx.Data["HeadBranches"] = headBranches
+	ctx.Data["HeadTags"] = headTags
 
 	// For compare repo branches
 	PrepareBranchList(ctx)
@@ -638,13 +564,20 @@ func CompareDiff(ctx *context.Context) {
 		return
 	}
 
-	headTags, err := repo_model.GetTagNamesByRepoID(ctx, ci.HeadRepo.ID)
-	if err != nil {
-		ctx.ServerError("GetTagNamesByRepoID", err)
-		return
+	if ci.MergeBase != "" {
+		prepareCreatePullRequestPage(ctx, ci, nothingToCompare)
+		if ctx.Written() {
+			return
+		}
+	} else {
+		ctx.Flash.Error(ctx.Tr("repo.pulls.no_common_history"), true)
+		ctx.Data["PageIsComparePull"] = false
+		ctx.Data["CommitCount"] = 0
 	}
-	ctx.Data["HeadTags"] = headTags
+	ctx.HTML(http.StatusOK, tplCompare)
+}
 
+func prepareCreatePullRequestPage(ctx *context.Context, ci *git_service.CompareInfo, nothingToCompare bool) {
 	if ctx.Data["PageIsComparePull"] == true {
 		pr, err := issues_model.GetUnmergedPullRequest(ctx, ci.HeadRepo.ID, ctx.Repo.Repository.ID, ci.HeadRef.ShortName(), ci.BaseRef.ShortName(), issues_model.PullRequestFlowGithub)
 		if err != nil {
@@ -685,7 +618,7 @@ func CompareDiff(ctx *context.Context) {
 	if content, ok := ctx.Data["content"].(string); ok && content != "" {
 		// If a template content is set, prepend the "content". In this case that's only
 		// applicable if you have one commit to compare and that commit has a message.
-		// In that case the commit message will be prepend to the template body.
+		// In that case the commit message will be prepended to the template body.
 		if templateContent, ok := ctx.Data[pullRequestTemplateKey].(string); ok && templateContent != "" {
 			// Re-use the same key as that's prioritized over the "content" key.
 			// Add two new lines between the content to ensure there's always at least
@@ -713,14 +646,8 @@ func CompareDiff(ctx *context.Context) {
 
 	ctx.Data["HasIssuesOrPullsWritePermission"] = ctx.Repo.Permission.CanWrite(unit.TypePullRequests)
 
-	if unit, err := ctx.Repo.Repository.GetUnit(ctx, unit.TypePullRequests); err == nil {
-		config := unit.PullRequestsConfig()
-		ctx.Data["AllowMaintainerEdit"] = config.DefaultAllowMaintainerEdit
-	} else {
-		ctx.Data["AllowMaintainerEdit"] = false
-	}
-
-	ctx.HTML(http.StatusOK, tplCompare)
+	prConfig := ctx.Repo.Repository.MustGetUnit(ctx, unit.TypePullRequests).PullRequestsConfig()
+	ctx.Data["AllowMaintainerEdit"] = prConfig.DefaultAllowMaintainerEdit
 }
 
 // attachCommentsToLines attaches comments to their corresponding diff lines

routers/web/repo/pull.go

@@ -1281,24 +1281,26 @@ func PullsNewRedirect(ctx *context.Context) {
 // CompareAndPullRequestPost response for creating pull request
 func CompareAndPullRequestPost(ctx *context.Context) {
 	form := web.GetForm(ctx).(*forms.CreateIssueForm)
-	ctx.Data["Title"] = ctx.Tr("repo.pulls.compare_changes")
-	ctx.Data["PageIsComparePull"] = true
-	ctx.Data["IsDiffCompare"] = true
-	ctx.Data["PullRequestWorkInProgressPrefixes"] = setting.Repository.PullRequest.WorkInProgressPrefixes
-	ctx.Data["IsAttachmentEnabled"] = setting.Attachment.Enabled
-	upload.AddUploadContext(ctx, "comment")
-	ctx.Data["HasIssuesOrPullsWritePermission"] = ctx.Repo.Permission.CanWrite(unit.TypePullRequests)
+	repo := ctx.Repo.Repository
 
-	var (
-		repo        = ctx.Repo.Repository
-		attachments []string
-	)
-
-	ci := ParseCompareInfo(ctx)
+	ci, err := parseCompareInfo(ctx)
 	if ctx.Written() {
 		return
 	}
-
+	if errors.Is(err, util.ErrNotExist) {
+		ctx.JSONErrorNotFound()
+		return
+	} else if errors.Is(err, util.ErrInvalidArgument) {
+		ctx.JSONError(err.Error())
+		return
+	} else if err != nil {
+		ctx.ServerError("ParseCompareInfo", err)
+		return
+	}
+	if ci.MergeBase == "" {
+		ctx.JSONError(ctx.Tr("repo.pulls.no_common_history"))
+		return
+	}
 	validateRet := ValidateRepoMetasForNewIssue(ctx, *form, true)
 	if ctx.Written() {
 		return
@@ -1306,6 +1308,7 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 
 	labelIDs, assigneeIDs, milestoneID, projectID := validateRet.LabelIDs, validateRet.AssigneeIDs, validateRet.MilestoneID, validateRet.ProjectID
 
+	var attachments []string
 	if setting.Attachment.Enabled {
 		attachments = form.Files
 	}

services/git/compare.go

@@ -45,6 +45,7 @@ func (ci *CompareInfo) DirectComparison() bool {
 
 // GetCompareInfo generates and returns compare information between base and head branches of repositories.
 // It does its best to fill the fields as many as it can.
+// MergeBase can be empty if the base and head are unrelated.
 func GetCompareInfo(ctx context.Context, baseRepo, headRepo *repo_model.Repository, headGitRepo *git.Repository, baseRef, headRef git.RefName, directComparison, fileOnly bool) (compareInfo CompareInfo, err error) {
 	baseCommitID, err1 := gitrepo.GetFullCommitID(ctx, baseRepo, baseRef.String())
 	headCommitID, err2 := gitrepo.GetFullCommitID(ctx, headRepo, headRef.String())
@@ -75,13 +76,17 @@ func GetCompareInfo(ctx context.Context, baseRepo, headRepo *repo_model.Reposito
 
 	if !directComparison {
 		compareInfo.MergeBase, err = gitrepo.MergeBase(ctx, headRepo, compareInfo.BaseCommitID, compareInfo.HeadCommitID)
-		if err != nil {
+		if err != nil && !errors.Is(err, util.ErrNotExist) {
 			return compareInfo, fmt.Errorf("MergeBase: %w", err)
 		}
 	} else {
 		compareInfo.MergeBase = compareInfo.BaseCommitID
 	}
 
+	if compareInfo.MergeBase == "" {
+		return compareInfo, nil
+	}
+
 	// We have a common base - therefore we know that ... should work
 	if !fileOnly {
 		// In git log/rev-list, the "..." syntax represents the symmetric difference between two references,
@@ -92,12 +97,10 @@ func GetCompareInfo(ctx context.Context, baseRepo, headRepo *repo_model.Reposito
 		if err != nil {
 			return compareInfo, fmt.Errorf("ShowPrettyFormatLogToList: %w", err)
 		}
-	} else {
-		compareInfo.Commits = []*git.Commit{}
 	}
 
 	// Count number of changed files.
-	// This probably should be removed as we need to use shortstat elsewhere
+	// TODO: This probably should be removed as we need to use shortstat elsewhere
 	// Now there is git diff --shortstat but this appears to be slower than simply iterating with --nameonly
 	compareInfo.NumFiles, err = headGitRepo.GetDiffNumChangedFiles(compareInfo.BaseCommitID, compareInfo.HeadCommitID, directComparison)
 	return compareInfo, err

templates/repo/diff/compare.tmpl

@@ -13,6 +13,7 @@
 				{{ctx.Locale.Tr "action.compare_commits_general"}}
 			{{end}}
 		</h2>
+		{{template "base/alert" .}}
 		{{$BaseCompareName := $.Repository.FullName -}}
 		{{$HeadCompareName := $.HeadRepo.FullName -}}
 		{{$OwnForkCompareName := "" -}}

tests/integration/compare_test.go

@@ -10,13 +10,16 @@ import (
 	"strings"
 	"testing"
 
+	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/git/gitcmd"
 	"code.gitea.io/gitea/modules/test"
 	repo_service "code.gitea.io/gitea/services/repository"
 	"code.gitea.io/gitea/tests"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestCompareTag(t *testing.T) {
@@ -124,6 +127,38 @@ func TestCompareBranches(t *testing.T) {
 	inspectCompare(t, htmlDoc, diffCount, diffChanges)
 }
 
+func TestCompareBranchesNoCommonMergeBase(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{Name: "user2"})
+	repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{OwnerID: user2.ID, Name: "repo1"})
+
+	repoPath := repo_model.RepoPath(user2.Name, repo1.Name)
+	_, _, runErr := gitcmd.NewCommand("fast-import").WithDir(repoPath).WithStdinBytes([]byte(strings.TrimSpace(`
+commit refs/heads/unrelated-history
+committer User <user@example.com> 1714310400 +0000
+data 13
+Second commit
+M 100644 inline file2.txt
+data 12
+Hello from 2
+`))).RunStdString(t.Context())
+	require.NoError(t, runErr)
+
+	session := loginUser(t, "user2")
+	req := NewRequest(t, "GET", "/user2/repo1/compare/master...unrelated-history")
+	resp := session.MakeRequest(t, req, http.StatusOK)
+	body := resp.Body.String()
+	htmlDoc := NewHTMLParser(t, resp.Body)
+
+	selection := htmlDoc.doc.Find(".ui.dropdown.select-branch")
+	assert.Lenf(t, selection.Nodes, 2, "The template has changed")
+	assert.Contains(t, body, "These branches do not share a common merge base")
+	assert.Equal(t, 1, htmlDoc.doc.Find(`a.item[href="/user2/repo1/compare/master...unrelated-history"]`).Length())
+	assert.Equal(t, 1, htmlDoc.doc.Find(`a.item[href="/user2/repo1/compare/master...master"]`).Length())
+	assert.Equal(t, 0, htmlDoc.doc.Find(".pullrequest-form").Length())
+}
+
 func TestCompareCodeExpand(t *testing.T) {
 	onGiteaRun(t, func(t *testing.T, u *url.URL) {
 		user1 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1})