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

This make checks in one single place so they dont differ and maintainer can not forget a check in one place while adding it to the other .... ( as it's atm )

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 ...

3 files changed, 115 insertions, 21 deletions

diff --git a/services/forms/repo_form.go b/services/forms/repo_form.go
index 33c7658640..80123e9af3 100644
--- a/services/forms/repo_form.go
+++ b/services/forms/repo_form.go
@@ -599,6 +599,31 @@ 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 f920688f5d..a4bbd3bb89 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(ctx, pr, false); err != nil {
+		if models.IsErrDisallowedToMerge(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(ctx, 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 6108a7956e..fb18be27c7 100644
--- a/services/pull/merge.go
+++ b/services/pull/merge.go
@@ -660,21 +660,6 @@ func getDiffTree(ctx context.Context, repoPath, baseBranch, headBranch string) (
 	return out.String(), 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(ctx, 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 {
@@ -711,29 +696,29 @@ func CheckPRReadyToMerge(ctx context.Context, pr *models.PullRequest, skipProtec
 		return err
 	}
 	if !isPass {
-		return models.ErrNotAllowedToMerge{
+		return models.ErrDisallowedToMerge{
 			Reason: "Not all required status checks successful",
 		}
 	}
 
 	if !pr.ProtectedBranch.HasEnoughApprovals(pr) {
-		return models.ErrNotAllowedToMerge{
+		return models.ErrDisallowedToMerge{
 			Reason: "Does not have enough approvals",
 		}
 	}
 	if pr.ProtectedBranch.MergeBlockedByRejectedReview(pr) {
-		return models.ErrNotAllowedToMerge{
+		return models.ErrDisallowedToMerge{
 			Reason: "There are requested changes",
 		}
 	}
 	if pr.ProtectedBranch.MergeBlockedByOfficialReviewRequests(pr) {
-		return models.ErrNotAllowedToMerge{
+		return models.ErrDisallowedToMerge{
 			Reason: "There are official review requests",
 		}
 	}
 
 	if pr.ProtectedBranch.MergeBlockedByOutdatedBranch(pr) {
-		return models.ErrNotAllowedToMerge{
+		return models.ErrDisallowedToMerge{
 			Reason: "The head branch is behind the base branch",
 		}
 	}
@@ -743,7 +728,7 @@ func CheckPRReadyToMerge(ctx context.Context, pr *models.PullRequest, skipProtec
 	}
 
 	if pr.ProtectedBranch.MergeBlockedByProtectedFiles(pr) {
-		return models.ErrNotAllowedToMerge{
+		return models.ErrDisallowedToMerge{
 			Reason: "Changed protected files",
 		}
 	}