Follow @wxiaoguang's suggestion

Author: lunny

models/issues/issue.go

@@ -364,8 +364,8 @@ func (issue *Issue) LoadAttributes(ctx context.Context) (err error) {
 
 // IsPinned returns if a Issue is pinned
 func (issue *Issue) IsPinned() bool {
-	if issue.PinOrder == 0 && (!setting.IsProd || setting.IsInTesting) {
-		log.Fatal("issue's pinorder has not been loaded")
+	if issue.PinOrder == 0 {
+		setting.PanicInDevOrTesting("issue's pinorder has not been loaded")
 	}
 	return issue.PinOrder > 0
 }

models/issues/issue_pin.go

@@ -70,11 +70,16 @@ func PinIssue(ctx context.Context, issue *Issue, user *user_model.User) error {
 			return ErrIssueMaxPinReached
 		}
 
+		pinnedIssuesMaxPinOrder, err := getPinnedIssuesMaxPinOrder(ctx, issue.RepoID, issue.IsPull)
+		if err != nil {
+			return err
+		}
+
 		if _, err = db.GetEngine(ctx).Insert(&IssuePin{
 			RepoID:   issue.RepoID,
 			IssueID:  issue.ID,
 			IsPull:   issue.IsPull,
-			PinOrder: pinnedIssuesNum + 1,
+			PinOrder: pinnedIssuesMaxPinOrder + 1,
 		}); err != nil {
 			return err
 		}
@@ -119,6 +124,12 @@ func getPinnedIssuesNum(ctx context.Context, repoID int64, isPull bool) (int, er
 	return pinnedIssuesNum, err
 }
 
+func getPinnedIssuesMaxPinOrder(ctx context.Context, repoID int64, isPull bool) (int, error) {
+	var maxPinnedIssuesMaxPinOrder int
+	_, err := db.GetEngine(ctx).SQL("SELECT max(pin_order) FROM issue_pin WHERE repo_id = ? AND is_pull = ?", repoID, isPull).Get(&maxPinnedIssuesMaxPinOrder)
+	return maxPinnedIssuesMaxPinOrder, err
+}
+
 // MovePin moves a Pinned Issue to a new Position
 func MovePin(ctx context.Context, issue *Issue, newPosition int) error {
 	if newPosition < 1 {
@@ -134,9 +145,9 @@ func MovePin(ctx context.Context, issue *Issue, newPosition int) error {
 	}
 
 	return db.WithTx(ctx, func(ctx context.Context) error {
-		if issuePin.PinOrder > newPosition {
+		if issuePin.PinOrder > newPosition { // move the issue to a lower position
 			_, err = db.GetEngine(ctx).Exec("UPDATE issue_pin SET pin_order = pin_order + 1 WHERE repo_id = ? AND is_pull = ? AND pin_order >= ? AND pin_order < ?", issue.RepoID, issue.IsPull, newPosition, issuePin.PinOrder)
-		} else {
+		} else { // move the issue to a higher position
 			// Lower the Position of all Pinned Issue that came after the current Position
 			_, err = db.GetEngine(ctx).Exec("UPDATE issue_pin SET pin_order = pin_order - 1 WHERE repo_id = ? AND is_pull = ? AND pin_order > ? AND pin_order <= ?", issue.RepoID, issue.IsPull, issuePin.PinOrder, newPosition)
 		}

services/convert/pull.go

@@ -18,6 +18,7 @@ import (
 	"code.gitea.io/gitea/modules/gitrepo"
 	"code.gitea.io/gitea/modules/log"
 	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/modules/util"
 )
 
 // ToAPIPullRequest assumes following fields have been assigned with valid values:
@@ -92,7 +93,7 @@ func ToAPIPullRequest(ctx context.Context, pr *issues_model.PullRequest, doer *u
 		Deadline:       apiIssue.Deadline,
 		Created:        pr.Issue.CreatedUnix.AsTimePtr(),
 		Updated:        pr.Issue.UpdatedUnix.AsTimePtr(),
-		PinOrder:       apiIssue.PinOrder,
+		PinOrder:       util.Iif(apiIssue.PinOrder == -1, 0, apiIssue.PinOrder),
 
 		AllowMaintainerEdit: pr.AllowMaintainerEdit,
 
@@ -366,7 +367,7 @@ func ToAPIPullRequests(ctx context.Context, baseRepo *repo_model.Repository, prs
 			Deadline:       apiIssue.Deadline,
 			Created:        pr.Issue.CreatedUnix.AsTimePtr(),
 			Updated:        pr.Issue.UpdatedUnix.AsTimePtr(),
-			PinOrder:       apiIssue.PinOrder,
+			PinOrder:       util.Iif(apiIssue.PinOrder == -1, 0, apiIssue.PinOrder),
 
 			AllowMaintainerEdit: pr.AllowMaintainerEdit,