Move checks for pulls before merge into own function (#19271) (#19277)

Backport #19271

Fix:
* The API does ignore issue dependencies where Web does not
* The API checks if "IsSignedIfRequired" where Web does not - UI probably do but nothing will some to craft custom requests
* Default merge message is crafted a bit different between API and Web if not set on specific cases ...

7 files changed, 216 insertions, 214 deletions

diff --git a/models/pull.go b/models/pull.go
index 243d40b1fa..0511d29dfc 100644
--- a/models/pull.go
+++ b/models/pull.go
@@ -222,22 +222,19 @@ func (pr *PullRequest) loadProtectedBranch(ctx context.Context) (err error) {
 }
 
 // GetDefaultMergeMessage returns default message used when merging pull request
-func (pr *PullRequest) GetDefaultMergeMessage() string {
+func (pr *PullRequest) GetDefaultMergeMessage() (string, error) {
 	if pr.HeadRepo == nil {
 		var err error
 		pr.HeadRepo, err = repo_model.GetRepositoryByID(pr.HeadRepoID)
 		if err != nil {
-			log.Error("GetRepositoryById[%d]: %v", pr.HeadRepoID, err)
-			return ""
+			return "", fmt.Errorf("GetRepositoryById[%d]: %v", pr.HeadRepoID, err)
 		}
 	}
 	if err := pr.LoadIssue(); err != nil {
-		log.Error("Cannot load issue %d for PR id %d: Error: %v", pr.IssueID, pr.ID, err)
-		return ""
+		return "", fmt.Errorf("Cannot load issue %d for PR id %d: Error: %v", pr.IssueID, pr.ID, err)
 	}
 	if err := pr.LoadBaseRepo(); err != nil {
-		log.Error("LoadBaseRepo: %v", err)
-		return ""
+		return "", fmt.Errorf("LoadBaseRepo: %v", err)
 	}
 
 	issueReference := "#"
@@ -246,10 +243,10 @@ func (pr *PullRequest) GetDefaultMergeMessage() string {
 	}
 
 	if pr.BaseRepoID == pr.HeadRepoID {
-		return fmt.Sprintf("Merge pull request '%s' (%s%d) from %s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadBranch, pr.BaseBranch)
+		return fmt.Sprintf("Merge pull request '%s' (%s%d) from %s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadBranch, pr.BaseBranch), nil
 	}
 
-	return fmt.Sprintf("Merge pull request '%s' (%s%d) from %s:%s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseBranch)
+	return fmt.Sprintf("Merge pull request '%s' (%s%d) from %s:%s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseBranch), nil
 }
 
 // ReviewCount represents a count of Reviews
@@ -335,19 +332,17 @@ func (pr *PullRequest) getReviewedByLines(writer io.Writer) error {
 }
 
 // GetDefaultSquashMessage returns default message used when squash and merging pull request
-func (pr *PullRequest) GetDefaultSquashMessage() string {
+func (pr *PullRequest) GetDefaultSquashMessage() (string, error) {
 	if err := pr.LoadIssue(); err != nil {
-		log.Error("LoadIssue: %v", err)
-		return ""
+		return "", fmt.Errorf("LoadIssue: %v", err)
 	}
 	if err := pr.LoadBaseRepo(); err != nil {
-		log.Error("LoadBaseRepo: %v", err)
-		return ""
+		return "", fmt.Errorf("LoadBaseRepo: %v", err)
 	}
 	if pr.BaseRepo.UnitEnabled(unit.TypeExternalTracker) {
-		return fmt.Sprintf("%s (!%d)", pr.Issue.Title, pr.Issue.Index)
+		return fmt.Sprintf("%s (!%d)", pr.Issue.Title, pr.Issue.Index), nil
 	}
-	return fmt.Sprintf("%s (#%d)", pr.Issue.Title, pr.Issue.Index)
+	return fmt.Sprintf("%s (#%d)", pr.Issue.Title, pr.Issue.Index), nil
 }
 
 // GetGitRefName returns git ref for hidden pull request branch
diff --git a/models/pull_test.go b/models/pull_test.go
index 2567984cc1..9098b61161 100644
--- a/models/pull_test.go
+++ b/models/pull_test.go
@@ -261,11 +261,15 @@ func TestPullRequest_GetDefaultMergeMessage_InternalTracker(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())
 	pr := unittest.AssertExistsAndLoadBean(t, &PullRequest{ID: 2}).(*PullRequest)
 
-	assert.Equal(t, "Merge pull request 'issue3' (#3) from branch2 into master", pr.GetDefaultMergeMessage())
+	msg, err := pr.GetDefaultMergeMessage()
+	assert.NoError(t, err)
+	assert.Equal(t, "Merge pull request 'issue3' (#3) from branch2 into master", msg)
 
 	pr.BaseRepoID = 1
 	pr.HeadRepoID = 2
-	assert.Equal(t, "Merge pull request 'issue3' (#3) from user2/repo1:branch2 into master", pr.GetDefaultMergeMessage())
+	msg, err = pr.GetDefaultMergeMessage()
+	assert.NoError(t, err)
+	assert.Equal(t, "Merge pull request 'issue3' (#3) from user2/repo1:branch2 into master", msg)
 }
 
 func TestPullRequest_GetDefaultMergeMessage_ExternalTracker(t *testing.T) {
@@ -283,9 +287,13 @@ func TestPullRequest_GetDefaultMergeMessage_ExternalTracker(t *testing.T) {
 
 	pr := unittest.AssertExistsAndLoadBean(t, &PullRequest{ID: 2, BaseRepo: baseRepo}).(*PullRequest)
 
-	assert.Equal(t, "Merge pull request 'issue3' (!3) from branch2 into master", pr.GetDefaultMergeMessage())
+	msg, err := pr.GetDefaultMergeMessage()
+	assert.NoError(t, err)
+	assert.Equal(t, "Merge pull request 'issue3' (!3) from branch2 into master", msg)
 
 	pr.BaseRepoID = 1
 	pr.HeadRepoID = 2
-	assert.Equal(t, "Merge pull request 'issue3' (!3) from user2/repo1:branch2 into master", pr.GetDefaultMergeMessage())
+	msg, err = pr.GetDefaultMergeMessage()
+	assert.NoError(t, err)
+	assert.Equal(t, "Merge pull request 'issue3' (!3) from user2/repo1:branch2 into master", msg)
 }
diff --git a/routers/api/v1/repo/pull.go b/routers/api/v1/repo/pull.go
index 158abd4651..06641b3f84 100644
--- a/routers/api/v1/repo/pull.go
+++ b/routers/api/v1/repo/pull.go
@@ -95,7 +95,6 @@ func ListPullRequests(ctx *context.APIContext) {
 		Labels:      ctx.FormStrings("labels"),
 		MilestoneID: ctx.FormInt64("milestone"),
 	})
-
 	if err != nil {
 		ctx.Error(http.StatusInternalServerError, "PullRequests", err)
 		return
@@ -724,13 +723,12 @@ func MergePullRequest(ctx *context.APIContext) {
 		return
 	}
 
-	if err = pr.LoadHeadRepo(); err != nil {
+	if err := pr.LoadHeadRepo(); err != nil {
 		ctx.Error(http.StatusInternalServerError, "LoadHeadRepo", err)
 		return
 	}
 
-	err = pr.LoadIssue()
-	if err != nil {
+	if err := pr.LoadIssue(); err != nil {
 		ctx.Error(http.StatusInternalServerError, "LoadIssue", err)
 		return
 	}
@@ -744,29 +742,33 @@ func MergePullRequest(ctx *context.APIContext) {
 		}
 	}
 
-	if pr.Issue.IsClosed {
-		ctx.NotFound()
-		return
-	}
+	manuallMerge := repo_model.MergeStyle(form.Do) == repo_model.MergeStyleManuallyMerged
+	force := form.ForceMerge != nil && *form.ForceMerge
 
-	allowedMerge, err := pull_service.IsUserAllowedToMerge(pr, ctx.Repo.Permission, ctx.User)
-	if err != nil {
-		ctx.Error(http.StatusInternalServerError, "IsUSerAllowedToMerge", err)
-		return
-	}
-	if !allowedMerge {
-		ctx.Error(http.StatusMethodNotAllowed, "Merge", "User not allowed to merge PR")
-		return
-	}
-
-	if pr.HasMerged {
-		ctx.Error(http.StatusMethodNotAllowed, "PR already merged", "")
+	if err := pull_service.CheckPullMergable(ctx, ctx.User, &ctx.Repo.Permission, pr, manuallMerge, force); err != nil {
+		if errors.Is(err, pull_service.ErrIsClosed) {
+			ctx.NotFound()
+		} else if errors.Is(err, pull_service.ErrUserNotAllowedToMerge) {
+			ctx.Error(http.StatusMethodNotAllowed, "Merge", "User not allowed to merge PR")
+		} else if errors.Is(err, pull_service.ErrHasMerged) {
+			ctx.Error(http.StatusMethodNotAllowed, "PR already merged", "")
+		} else if errors.Is(err, pull_service.ErrIsWorkInProgress) {
+			ctx.Error(http.StatusMethodNotAllowed, "PR is a work in progress", "Work in progress PRs cannot be merged")
+		} else if errors.Is(err, pull_service.ErrNotMergableState) {
+			ctx.Error(http.StatusMethodNotAllowed, "PR not in mergeable state", "Please try again later")
+		} else if models.IsErrNotAllowedToMerge(err) {
+			ctx.Error(http.StatusMethodNotAllowed, "PR is not ready to be merged", err)
+		} else if asymkey_service.IsErrWontSign(err) {
+			ctx.Error(http.StatusMethodNotAllowed, fmt.Sprintf("Protected branch %s requires signed commits but this merge would not be signed", pr.BaseBranch), err)
+		} else {
+			ctx.InternalServerError(err)
+		}
 		return
 	}
 
 	// handle manually-merged mark
-	if repo_model.MergeStyle(form.Do) == repo_model.MergeStyleManuallyMerged {
-		if err = pull_service.MergedManually(pr, ctx.User, ctx.Repo.GitRepo, form.MergeCommitID); err != nil {
+	if manuallMerge {
+		if err := pull_service.MergedManually(pr, ctx.User, ctx.Repo.GitRepo, form.MergeCommitID); err != nil {
 			if models.IsErrInvalidMergeStyle(err) {
 				ctx.Error(http.StatusMethodNotAllowed, "Invalid merge style", fmt.Errorf("%s is not allowed an allowed merge style for this repository", repo_model.MergeStyle(form.Do)))
 				return
@@ -782,63 +784,13 @@ func MergePullRequest(ctx *context.APIContext) {
 		return
 	}
 
-	if !pr.CanAutoMerge() {
-		ctx.Error(http.StatusMethodNotAllowed, "PR not in mergeable state", "Please try again later")
-		return
-	}
-
-	if pr.IsWorkInProgress() {
-		ctx.Error(http.StatusMethodNotAllowed, "PR is a work in progress", "Work in progress PRs cannot be merged")
+	// set defaults to propagate needed fields
+	if err := form.SetDefaults(pr); err != nil {
+		ctx.ServerError("SetDefaults", fmt.Errorf("SetDefaults: %v", err))
 		return
 	}
 
-	if err := pull_service.CheckPRReadyToMerge(pr, false); err != nil {
-		if !models.IsErrNotAllowedToMerge(err) {
-			ctx.Error(http.StatusInternalServerError, "CheckPRReadyToMerge", err)
-			return
-		}
-		if form.ForceMerge != nil && *form.ForceMerge {
-			if isRepoAdmin, err := models.IsUserRepoAdmin(pr.BaseRepo, ctx.User); err != nil {
-				ctx.Error(http.StatusInternalServerError, "IsUserRepoAdmin", err)
-				return
-			} else if !isRepoAdmin {
-				ctx.Error(http.StatusMethodNotAllowed, "Merge", "Only repository admin can merge if not all checks are ok (force merge)")
-			}
-		} else {
-			ctx.Error(http.StatusMethodNotAllowed, "PR is not ready to be merged", err)
-			return
-		}
-	}
-
-	if _, err := pull_service.IsSignedIfRequired(pr, ctx.User); err != nil {
-		if !asymkey_service.IsErrWontSign(err) {
-			ctx.Error(http.StatusInternalServerError, "IsSignedIfRequired", err)
-			return
-		}
-		ctx.Error(http.StatusMethodNotAllowed, fmt.Sprintf("Protected branch %s requires signed commits but this merge would not be signed", pr.BaseBranch), err)
-		return
-	}
-
-	if len(form.Do) == 0 {
-		form.Do = string(repo_model.MergeStyleMerge)
-	}
-
-	message := strings.TrimSpace(form.MergeTitleField)
-	if len(message) == 0 {
-		if repo_model.MergeStyle(form.Do) == repo_model.MergeStyleMerge {
-			message = pr.GetDefaultMergeMessage()
-		}
-		if repo_model.MergeStyle(form.Do) == repo_model.MergeStyleSquash {
-			message = pr.GetDefaultSquashMessage()
-		}
-	}
-
-	form.MergeMessageField = strings.TrimSpace(form.MergeMessageField)
-	if len(form.MergeMessageField) > 0 {
-		message += "\n\n" + form.MergeMessageField
-	}
-
-	if err := pull_service.Merge(pr, ctx.User, ctx.Repo.GitRepo, repo_model.MergeStyle(form.Do), form.HeadCommitID, message); err != nil {
+	if err := pull_service.Merge(pr, ctx.User, ctx.Repo.GitRepo, repo_model.MergeStyle(form.Do), form.HeadCommitID, form.MergeTitleField); err != nil {
 		if models.IsErrInvalidMergeStyle(err) {
 			ctx.Error(http.StatusMethodNotAllowed, "Invalid merge style", fmt.Errorf("%s is not allowed an allowed merge style for this repository", repo_model.MergeStyle(form.Do)))
 			return
diff --git a/routers/web/repo/pull.go b/routers/web/repo/pull.go
index e9364ce654..cdf130abf4 100644
--- a/routers/web/repo/pull.go
+++ b/routers/web/repo/pull.go
@@ -33,6 +33,7 @@ import (
 	"code.gitea.io/gitea/modules/web"
 	"code.gitea.io/gitea/modules/web/middleware"
 	"code.gitea.io/gitea/routers/utils"
+	asymkey_service "code.gitea.io/gitea/services/asymkey"
 	"code.gitea.io/gitea/services/forms"
 	"code.gitea.io/gitea/services/gitdiff"
 	pull_service "code.gitea.io/gitea/services/pull"
@@ -48,16 +49,14 @@ const (
 	pullRequestTemplateKey = "PullRequestTemplate"
 )
 
-var (
-	pullRequestTemplateCandidates = []string{
-		"PULL_REQUEST_TEMPLATE.md",
-		"pull_request_template.md",
-		".gitea/PULL_REQUEST_TEMPLATE.md",
-		".gitea/pull_request_template.md",
-		".github/PULL_REQUEST_TEMPLATE.md",
-		".github/pull_request_template.md",
-	}
-)
+var pullRequestTemplateCandidates = []string{
+	"PULL_REQUEST_TEMPLATE.md",
+	"pull_request_template.md",
+	".gitea/PULL_REQUEST_TEMPLATE.md",
+	".gitea/pull_request_template.md",
+	".github/PULL_REQUEST_TEMPLATE.md",
+	".github/pull_request_template.md",
+}
 
 func getRepository(ctx *context.Context, repoID int64) *repo_model.Repository {
 	repo, err := repo_model.GetRepositoryByID(repoID)
@@ -125,7 +124,7 @@ func getForkRepository(ctx *context.Context) *repo_model.Repository {
 		}
 	}
 
-	var traverseParentRepo = forkRepo
+	traverseParentRepo := forkRepo
 	for {
 		if ctx.User.ID == traverseParentRepo.OwnerID {
 			canForkToUser = false
@@ -195,7 +194,7 @@ func ForkPost(ctx *context.Context) {
 	}
 
 	var err error
-	var traverseParentRepo = forkRepo
+	traverseParentRepo := forkRepo
 	for {
 		if ctxUser.ID == traverseParentRepo.OwnerID {
 			ctx.RenderWithErr(ctx.Tr("repo.settings.new_owner_has_same_repo"), tplFork, &form)
@@ -853,39 +852,53 @@ func MergePullRequest(ctx *context.Context) {
 	if ctx.Written() {
 		return
 	}
-	if issue.IsClosed {
-		if issue.IsPull {
-			ctx.Flash.Error(ctx.Tr("repo.pulls.is_closed"))
-			ctx.Redirect(issue.Link())
-			return
-		}
-		ctx.Flash.Error(ctx.Tr("repo.issues.closed_title"))
-		ctx.Redirect(issue.Link())
-		return
-	}
 
 	pr := issue.PullRequest
+	pr.Issue = issue
+	pr.Issue.Repo = ctx.Repo.Repository
+	manuallMerge := repo_model.MergeStyle(form.Do) == repo_model.MergeStyleManuallyMerged
+	forceMerge := form.ForceMerge != nil && *form.ForceMerge
 
-	allowedMerge, err := pull_service.IsUserAllowedToMerge(pr, ctx.Repo.Permission, ctx.User)
-	if err != nil {
-		ctx.ServerError("IsUserAllowedToMerge", err)
-		return
-	}
-	if !allowedMerge {
-		ctx.Flash.Error(ctx.Tr("repo.pulls.update_not_allowed"))
-		ctx.Redirect(issue.Link())
-		return
-	}
+	if err := pull_service.CheckPullMergable(ctx, ctx.User, &ctx.Repo.Permission, pr, manuallMerge, forceMerge); err != nil {
+		if errors.Is(err, pull_service.ErrIsClosed) {
+			if issue.IsPull {
+				ctx.Flash.Error(ctx.Tr("repo.pulls.is_closed"))
+				ctx.Redirect(issue.Link())
+			} else {
+				ctx.Flash.Error(ctx.Tr("repo.issues.closed_title"))
+				ctx.Redirect(issue.Link())
+			}
+		} else if errors.Is(err, pull_service.ErrUserNotAllowedToMerge) {
+			ctx.Flash.Error(ctx.Tr("repo.pulls.update_not_allowed"))
+			ctx.Redirect(issue.Link())
+		} else if errors.Is(err, pull_service.ErrHasMerged) {
+			ctx.Flash.Error(ctx.Tr("repo.pulls.has_merged"))
+			ctx.Redirect(issue.Link())
+		} else if errors.Is(err, pull_service.ErrIsWorkInProgress) {
+			ctx.Flash.Error(ctx.Tr("repo.pulls.no_merge_wip"))
+			ctx.Redirect(issue.Link())
+		} else if errors.Is(err, pull_service.ErrNotMergableState) {
+			ctx.Flash.Error(ctx.Tr("repo.pulls.no_merge_not_ready"))
+			ctx.Redirect(issue.Link())
+		} else if models.IsErrNotAllowedToMerge(err) {
+			ctx.Flash.Error(ctx.Tr("repo.pulls.no_merge_not_ready"))
+			ctx.Redirect(issue.Link())
+		} else if asymkey_service.IsErrWontSign(err) {
+			ctx.Flash.Error(err.Error()) // has not translation ...
+			ctx.Redirect(issue.Link())
+		} else if errors.Is(err, pull_service.ErrDependenciesLeft) {
+			ctx.Flash.Error(ctx.Tr("repo.issues.dependency.pr_close_blocked"))
+			ctx.Redirect(issue.Link())
+		} else {
+			ctx.ServerError("WebCheck", err)
+		}
 
-	if pr.HasMerged {
-		ctx.Flash.Error(ctx.Tr("repo.pulls.has_merged"))
-		ctx.Redirect(issue.Link())
 		return
 	}
 
 	// handle manually-merged mark
-	if repo_model.MergeStyle(form.Do) == repo_model.MergeStyleManuallyMerged {
-		if err = pull_service.MergedManually(pr, ctx.User, ctx.Repo.GitRepo, form.MergeCommitID); err != nil {
+	if manuallMerge {
+		if err := pull_service.MergedManually(pr, ctx.User, ctx.Repo.GitRepo, form.MergeCommitID); err != nil {
 			if models.IsErrInvalidMergeStyle(err) {
 				ctx.Flash.Error(ctx.Tr("repo.pulls.invalid_merge_option"))
 				ctx.Redirect(issue.Link())
@@ -904,72 +917,13 @@ func MergePullRequest(ctx *context.Context) {
 		return
 	}
 
-	if !pr.CanAutoMerge() {
-		ctx.Flash.Error(ctx.Tr("repo.pulls.no_merge_not_ready"))
-		ctx.Redirect(issue.Link())
-		return
-	}
-
-	if pr.IsWorkInProgress() {
-		ctx.Flash.Error(ctx.Tr("repo.pulls.no_merge_wip"))
-		ctx.Redirect(issue.Link())
-		return
-	}
-
-	if err := pull_service.CheckPRReadyToMerge(pr, false); err != nil {
-		if !models.IsErrNotAllowedToMerge(err) {
-			ctx.ServerError("Merge PR status", err)
-			return
-		}
-		if isRepoAdmin, err := models.IsUserRepoAdmin(pr.BaseRepo, ctx.User); err != nil {
-			ctx.ServerError("IsUserRepoAdmin", err)
-			return
-		} else if !isRepoAdmin {
-			ctx.Flash.Error(ctx.Tr("repo.pulls.no_merge_not_ready"))
-			ctx.Redirect(issue.Link())
-			return
-		}
-	}
-
-	if ctx.HasError() {
-		ctx.Flash.Error(ctx.Data["ErrorMsg"].(string))
-		ctx.Redirect(issue.Link())
-		return
-	}
-
-	message := strings.TrimSpace(form.MergeTitleField)
-	if len(message) == 0 {
-		if repo_model.MergeStyle(form.Do) == repo_model.MergeStyleMerge {
-			message = pr.GetDefaultMergeMessage()
-		}
-		if repo_model.MergeStyle(form.Do) == repo_model.MergeStyleRebaseMerge {
-			message = pr.GetDefaultMergeMessage()
-		}
-		if repo_model.MergeStyle(form.Do) == repo_model.MergeStyleSquash {
-			message = pr.GetDefaultSquashMessage()
-		}
-	}
-
-	form.MergeMessageField = strings.TrimSpace(form.MergeMessageField)
-	if len(form.MergeMessageField) > 0 {
-		message += "\n\n" + form.MergeMessageField
-	}
-
-	pr.Issue = issue
-	pr.Issue.Repo = ctx.Repo.Repository
-
-	noDeps, err := models.IssueNoDependenciesLeft(issue)
-	if err != nil {
-		return
-	}
-
-	if !noDeps {
-		ctx.Flash.Error(ctx.Tr("repo.issues.dependency.pr_close_blocked"))
-		ctx.Redirect(issue.Link())
+	// set defaults to propagate needed fields
+	if err := form.SetDefaults(pr); err != nil {
+		ctx.ServerError("SetDefaults", fmt.Errorf("SetDefaults: %v", err))
 		return
 	}
 
-	if err = pull_service.Merge(pr, ctx.User, ctx.Repo.GitRepo, repo_model.MergeStyle(form.Do), form.HeadCommitID, message); err != nil {
+	if err := pull_service.Merge(pr, ctx.User, ctx.Repo.GitRepo, repo_model.MergeStyle(form.Do), form.HeadCommitID, form.MergeTitleField); err != nil {
 		if models.IsErrInvalidMergeStyle(err) {
 			ctx.Flash.Error(ctx.Tr("repo.pulls.invalid_merge_option"))
 			ctx.Redirect(issue.Link())
@@ -1079,7 +1033,6 @@ func MergePullRequest(ctx *context.Context) {
 }
 
 func stopTimerIfAvailable(user *user_model.User, issue *models.Issue) error {
-
 	if models.StopwatchExists(user.ID, issue.ID) {
 		if err := models.CreateOrStopIssueStopwatch(user, issue); err != nil {
 			return err
diff --git a/services/forms/repo_form.go b/services/forms/repo_form.go
index 19b5a37664..e78559912e 100644
--- a/services/forms/repo_form.go
+++ b/services/forms/repo_form.go
@@ -582,6 +582,31 @@ func (f *MergePullRequestForm) Validate(req *http.Request, errs binding.Errors)
 	return middleware.Validate(errs, ctx.Data, f, ctx.Locale)
 }
 
+// SetDefaults if not provided for mergestyle and commit message
+func (f *MergePullRequestForm) SetDefaults(pr *models.PullRequest) (err error) {
+	if f.Do == "" {
+		f.Do = "merge"
+	}
+
+	f.MergeTitleField = strings.TrimSpace(f.MergeTitleField)
+	if len(f.MergeTitleField) == 0 {
+		switch f.Do {
+		case "merge", "rebase-merge":
+			f.MergeTitleField, err = pr.GetDefaultMergeMessage()
+		case "squash":
+			f.MergeTitleField, err = pr.GetDefaultSquashMessage()
+		}
+	}
+
+	f.MergeMessageField = strings.TrimSpace(f.MergeMessageField)
+	if len(f.MergeMessageField) > 0 {
+		f.MergeTitleField += "\n\n" + f.MergeMessageField
+		f.MergeMessageField = ""
+	}
+
+	return
+}
+
 // CodeCommentForm form for adding code comments for PRs
 type CodeCommentForm struct {
 	Origin         string `binding:"Required;In(timeline,diff)"`
diff --git a/services/pull/check.go b/services/pull/check.go
index 363a716b28..f3ffa02656 100644
--- a/services/pull/check.go
+++ b/services/pull/check.go
@@ -7,6 +7,7 @@ package pull
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"strconv"
@@ -24,11 +25,21 @@ import (
 	"code.gitea.io/gitea/modules/queue"
 	"code.gitea.io/gitea/modules/timeutil"
 	"code.gitea.io/gitea/modules/util"
+	asymkey_service "code.gitea.io/gitea/services/asymkey"
 )
 
 // prQueue represents a queue to handle update pull request tests
 var prQueue queue.UniqueQueue
 
+var (
+	ErrIsClosed              = errors.New("pull is cosed")
+	ErrUserNotAllowedToMerge = errors.New("user not allowed to merge")
+	ErrHasMerged             = errors.New("has already been merged")
+	ErrIsWorkInProgress      = errors.New("work in progress PRs cannot be merged")
+	ErrNotMergableState      = errors.New("not in mergeable state")
+	ErrDependenciesLeft      = errors.New("is blocked by an open dependency")
+)
+
 // AddToTaskQueue adds itself to pull request test task queue.
 func AddToTaskQueue(pr *models.PullRequest) {
 	err := prQueue.PushFunc(strconv.FormatInt(pr.ID, 10), func() error {
@@ -46,6 +57,79 @@ func AddToTaskQueue(pr *models.PullRequest) {
 	}
 }
 
+// CheckPullMergable check if the pull mergable based on all conditions (branch protection, merge options, ...)
+func CheckPullMergable(ctx context.Context, doer *user_model.User, perm *models.Permission, pr *models.PullRequest, manuallMerge, force bool) error {
+	if pr.HasMerged {
+		return ErrHasMerged
+	}
+
+	if err := pr.LoadIssue(); err != nil {
+		return err
+	} else if pr.Issue.IsClosed {
+		return ErrIsClosed
+	}
+
+	if allowedMerge, err := IsUserAllowedToMerge(pr, *perm, doer); err != nil {
+		return err
+	} else if !allowedMerge {
+		return ErrUserNotAllowedToMerge
+	}
+
+	if manuallMerge {
+		// don't check rules to "auto merge", doer is going to mark this pull as merged manually
+		return nil
+	}
+
+	if pr.IsWorkInProgress() {
+		return ErrIsWorkInProgress
+	}
+
+	if !pr.CanAutoMerge() {
+		return ErrNotMergableState
+	}
+
+	if err := CheckPRReadyToMerge(pr, false); err != nil {
+		if models.IsErrNotAllowedToMerge(err) {
+			if force {
+				if isRepoAdmin, err := models.IsUserRepoAdmin(pr.BaseRepo, doer); err != nil {
+					return err
+				} else if !isRepoAdmin {
+					return ErrUserNotAllowedToMerge
+				}
+			}
+		} else {
+			return err
+		}
+	}
+
+	if _, err := isSignedIfRequired(ctx, pr, doer); err != nil {
+		return err
+	}
+
+	if noDeps, err := models.IssueNoDependenciesLeft(pr.Issue); err != nil {
+		return err
+	} else if !noDeps {
+		return ErrDependenciesLeft
+	}
+
+	return nil
+}
+
+// isSignedIfRequired check if merge will be signed if required
+func isSignedIfRequired(ctx context.Context, pr *models.PullRequest, doer *user_model.User) (bool, error) {
+	if err := pr.LoadProtectedBranch(); err != nil {
+		return false, err
+	}
+
+	if pr.ProtectedBranch == nil || !pr.ProtectedBranch.RequireSignedCommits {
+		return true, nil
+	}
+
+	sign, _, _, err := asymkey_service.SignMerge(pr, doer, pr.BaseRepo.RepoPath(), pr.BaseBranch, pr.GetGitRefName())
+
+	return sign, err
+}
+
 // checkAndUpdateStatus checks if pull request is possible to leaving checking status,
 // and set to be either conflict or mergeable.
 func checkAndUpdateStatus(pr *models.PullRequest) {
diff --git a/services/pull/merge.go b/services/pull/merge.go
index f6a6415bff..37269b0c78 100644
--- a/services/pull/merge.go
+++ b/services/pull/merge.go
@@ -164,13 +164,13 @@ func rawMerge(pr *models.PullRequest, doer *user_model.User, mergeStyle repo_mod
 	}
 
 	infoPath := filepath.Join(tmpBasePath, ".git", "info")
-	if err := os.MkdirAll(infoPath, 0700); err != nil {
+	if err := os.MkdirAll(infoPath, 0o700); err != nil {
 		log.Error("Unable to create .git/info in %s: %v", tmpBasePath, err)
 		return "", fmt.Errorf("Unable to create .git/info in tmpBasePath: %v", err)
 	}
 
 	sparseCheckoutListPath := filepath.Join(infoPath, "sparse-checkout")
-	if err := os.WriteFile(sparseCheckoutListPath, []byte(sparseCheckoutList), 0600); err != nil {
+	if err := os.WriteFile(sparseCheckoutListPath, []byte(sparseCheckoutList), 0o600); err != nil {
 		log.Error("Unable to write .git/info/sparse-checkout file in %s: %v", tmpBasePath, err)
 		return "", fmt.Errorf("Unable to write .git/info/sparse-checkout file in tmpBasePath: %v", err)
 	}
@@ -561,21 +561,6 @@ func getDiffTree(repoPath, baseBranch, headBranch string) (string, error) {
 	return out.String(), nil
 }
 
-// IsSignedIfRequired check if merge will be signed if required
-func IsSignedIfRequired(pr *models.PullRequest, doer *user_model.User) (bool, error) {
-	if err := pr.LoadProtectedBranch(); err != nil {
-		return false, err
-	}
-
-	if pr.ProtectedBranch == nil || !pr.ProtectedBranch.RequireSignedCommits {
-		return true, nil
-	}
-
-	sign, _, _, err := asymkey_service.SignMerge(pr, doer, pr.BaseRepo.RepoPath(), pr.BaseBranch, pr.GetGitRefName())
-
-	return sign, err
-}
-
 // IsUserAllowedToMerge check if user is allowed to merge PR with given permissions and branch protections
 func IsUserAllowedToMerge(pr *models.PullRequest, p models.Permission, user *user_model.User) (bool, error) {
 	if user == nil {