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 ...

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) {