Prevent simultaneous editing of comments and issues (#31053)

fixes #22907

Tested:
- [x] issue content edit
- [x] issue content change tasklist
- [x] pull request content edit
- [x] pull request change tasklist

![issue-content-edit](https://github.com/go-gitea/gitea/assets/29250154/a0828889-fb96-4bc4-8600-da92e3205812)

(cherry picked from commit aa92b13164e84c26be91153b6022220ce0a27720)

Conflicts:
	models/issues/comment.go
	 c7a389f2b2 [FEAT] allow setting the update date on issues and comments

	options/locale/locale_en-US.ini
	 trivial context conflicts

	routers/api/v1/repo/issue_comment.go
	routers/api/v1/repo/issue_comment_attachment.go
	services/issue/comments.go
	services/issue/content.go
         user blocking is implemented differently in Forgejo

	routers/web/repo/issue.go
	 trivial difference from 6a0750177f Allow to save empty comment
         user blocking is implemented differently in Forgejo

	templates/repo/issue/view_content/conversation.tmpl
	 templates changed a lot in Forgejo but the change is
	 trivially ported

	tests/integration/issue_test.go
	 other tests were added in the same region

	web_src/js/features/repo-issue-edit.js
	 the code is still web_src/js/features/repo-legacy.js
	 trivially ported

models/issues/comment.go

@@ -52,6 +52,8 @@ func (err ErrCommentNotExist) Unwrap() error {
 	return util.ErrNotExist
 }
 
+var ErrCommentAlreadyChanged = util.NewInvalidArgumentErrorf("the comment is already changed")
+
 // CommentType defines whether a comment is just a simple comment, an action (like close) or a reference.
 type CommentType int
 
@@ -262,6 +264,7 @@ type Comment struct {
 	Line            int64 // - previous line / + proposed line
 	TreePath        string
 	Content         string        `xorm:"LONGTEXT"`
+	ContentVersion  int           `xorm:"NOT NULL DEFAULT 0"`
 	RenderedContent template.HTML `xorm:"-"`
 
 	// Path represents the 4 lines of code cemented by this comment
@@ -1119,7 +1122,7 @@ func UpdateCommentInvalidate(ctx context.Context, c *Comment) error {
 }
 
 // UpdateComment updates information of comment.
-func UpdateComment(ctx context.Context, c *Comment, doer *user_model.User) error {
+func UpdateComment(ctx context.Context, c *Comment, contentVersion int, doer *user_model.User) error {
 	ctx, committer, err := db.TxContext(ctx)
 	if err != nil {
 		return err
@@ -1139,9 +1142,15 @@ func UpdateComment(ctx context.Context, c *Comment, doer *user_model.User) error
 		// see https://codeberg.org/forgejo/forgejo/pulls/764#issuecomment-1023801
 		c.UpdatedUnix = c.Issue.UpdatedUnix
 	}
-	if _, err := sess.Update(c); err != nil {
+	c.ContentVersion = contentVersion + 1
+
+	affected, err := sess.Where("content_version = ?", contentVersion).Update(c)
+	if err != nil {
 		return err
 	}
+	if affected == 0 {
+		return ErrCommentAlreadyChanged
+	}
 	if err := c.AddCrossReferences(ctx, doer, true); err != nil {
 		return err
 	}

models/issues/issue.go

@@ -94,6 +94,8 @@ func (err ErrIssueWasClosed) Error() string {
 	return fmt.Sprintf("Issue [%d] %d was already closed", err.ID, err.Index)
 }
 
+var ErrIssueAlreadyChanged = util.NewInvalidArgumentErrorf("the issue is already changed")
+
 // Issue represents an issue or pull request of repository.
 type Issue struct {
 	ID               int64                  `xorm:"pk autoincr"`
@@ -107,6 +109,7 @@ type Issue struct {
 	Title            string                 `xorm:"name"`
 	Content          string                 `xorm:"LONGTEXT"`
 	RenderedContent  template.HTML          `xorm:"-"`
+	ContentVersion   int                    `xorm:"NOT NULL DEFAULT 0"`
 	Labels           []*Label               `xorm:"-"`
 	MilestoneID      int64                  `xorm:"INDEX"`
 	Milestone        *Milestone             `xorm:"-"`

models/issues/issue_update.go

@@ -25,17 +25,18 @@ import (
 	"xorm.io/builder"
 )
 
-// UpdateIssueCols updates cols of issue
 func UpdateIssueCols(ctx context.Context, issue *Issue, cols ...string) error {
+	_, err := UpdateIssueColsWithCond(ctx, issue, builder.NewCond(), cols...)
+	return err
+}
+
+func UpdateIssueColsWithCond(ctx context.Context, issue *Issue, cond builder.Cond, cols ...string) (int64, error) {
 	sess := db.GetEngine(ctx).ID(issue.ID)
 	if issue.NoAutoTime {
 		cols = append(cols, []string{"updated_unix"}...)
 		sess.NoAutoTime()
 	}
-	if _, err := sess.Cols(cols...).Update(issue); err != nil {
+	return sess.Cols(cols...).Where(cond).Update(issue)
-		return err
-	}
-	return nil
 }
 
 func changeIssueStatus(ctx context.Context, issue *Issue, doer *user_model.User, isClosed, isMergePull bool) (*Comment, error) {
@@ -250,7 +251,7 @@ func UpdateIssueAttachments(ctx context.Context, issueID int64, uuids []string)
 }
 
 // ChangeIssueContent changes issue content, as the given user.
-func ChangeIssueContent(ctx context.Context, issue *Issue, doer *user_model.User, content string) (err error) {
+func ChangeIssueContent(ctx context.Context, issue *Issue, doer *user_model.User, content string, contentVersion int) (err error) {
 	ctx, committer, err := db.TxContext(ctx)
 	if err != nil {
 		return err
@@ -269,10 +270,16 @@ func ChangeIssueContent(ctx context.Context, issue *Issue, doer *user_model.User
 	}
 
 	issue.Content = content
+	issue.ContentVersion = contentVersion + 1
 
-	if err = UpdateIssueCols(ctx, issue, "content"); err != nil {
+	expectedContentVersion := builder.NewCond().And(builder.Eq{"content_version": contentVersion})
+	affected, err := UpdateIssueColsWithCond(ctx, issue, expectedContentVersion, "content", "content_version")
+	if err != nil {
 		return fmt.Errorf("UpdateIssueCols: %w", err)
 	}
+	if affected == 0 {
+		return ErrIssueAlreadyChanged
+	}
 
 	historyDate := timeutil.TimeStampNow()
 	if issue.NoAutoTime {

models/migrations/migrations.go

@@ -22,6 +22,7 @@ import (
 	"code.gitea.io/gitea/models/migrations/v1_20"
 	"code.gitea.io/gitea/models/migrations/v1_21"
 	"code.gitea.io/gitea/models/migrations/v1_22"
+	"code.gitea.io/gitea/models/migrations/v1_23"
 	"code.gitea.io/gitea/models/migrations/v1_6"
 	"code.gitea.io/gitea/models/migrations/v1_7"
 	"code.gitea.io/gitea/models/migrations/v1_8"
@@ -589,6 +590,9 @@ var migrations = []Migration{
 	NewMigration("Drop wrongly created table o_auth2_application", v1_22.DropWronglyCreatedTable),
 
 	// Gitea 1.22.0-rc1 ends at 299
+
+	// v299 -> v300
+	NewMigration("Add content version to issue and comment table", v1_23.AddContentVersionToIssueAndComment),
 }
 
 // GetCurrentDBVersion returns the current db version

models/migrations/v1_23/v299.go

@@ -0,0 +1,18 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_23 //nolint
+
+import "xorm.io/xorm"
+
+func AddContentVersionToIssueAndComment(x *xorm.Engine) error {
+	type Issue struct {
+		ContentVersion int `xorm:"NOT NULL DEFAULT 0"`
+	}
+
+	type Comment struct {
+		ContentVersion int `xorm:"NOT NULL DEFAULT 0"`
+	}
+
+	return x.Sync(new(Comment), new(Issue))
+}

options/locale/locale_en-US.ini

@@ -1474,6 +1474,7 @@ issues.new.assignees = Assignees
 issues.new.clear_assignees = Clear assignees
 issues.new.no_assignees = No assignees
 issues.new.no_reviewers = No reviewers
+issues.edit.already_changed = Unable to save changes to the issue. It appears the content has already been changed by another user. Please refresh the page and try editing again to avoid overwriting their changes
 issues.choose.get_started = Get started
 issues.choose.open_external_link = Open
 issues.choose.blank = Default
@@ -1791,6 +1792,7 @@ compare.compare_head = compare
 pulls.desc = Enable pull requests and code reviews.
 pulls.new = New pull request
 pulls.view = View pull request
+pulls.edit.already_changed = Unable to save changes to the pull request. It appears the content has already been changed by another user. Please refresh the page and try editing again to avoid overwriting their changes
 pulls.compare_changes = New pull request
 pulls.allow_edits_from_maintainers = Allow edits from maintainers
 pulls.allow_edits_from_maintainers_desc = Users with write access to the base branch can also push to this branch
@@ -1946,6 +1948,8 @@ pulls.recently_pushed_new_branches = You pushed on branch <a href="%[3]s"><stron
 
 pull.deleted_branch = (deleted):%s
 
+comments.edit.already_changed = Unable to save changes to the comment. It appears the content has already been changed by another user. Please refresh the page and try editing again to avoid overwriting their changes
+
 milestones.new = New milestone
 milestones.closed = Closed %s
 milestones.update_ago = Updated %s

routers/api/v1/repo/issue.go

@@ -816,8 +816,13 @@ func EditIssue(ctx *context.APIContext) {
 		}
 	}
 	if form.Body != nil {
-		err = issue_service.ChangeContent(ctx, issue, ctx.Doer, *form.Body)
+		err = issue_service.ChangeContent(ctx, issue, ctx.Doer, *form.Body, issue.ContentVersion)
 		if err != nil {
+			if errors.Is(err, issues_model.ErrIssueAlreadyChanged) {
+				ctx.Error(http.StatusBadRequest, "ChangeContent", err)
+				return
+			}
+
 			ctx.Error(http.StatusInternalServerError, "ChangeContent", err)
 			return
 		}

routers/api/v1/repo/issue_attachment.go

@@ -220,7 +220,7 @@ func CreateIssueAttachment(ctx *context.APIContext) {
 
 	issue.Attachments = append(issue.Attachments, attachment)
 
-	if err := issue_service.ChangeContent(ctx, issue, ctx.Doer, issue.Content); err != nil {
+	if err := issue_service.ChangeContent(ctx, issue, ctx.Doer, issue.Content, issue.ContentVersion); err != nil {
 		ctx.Error(http.StatusInternalServerError, "ChangeContent", err)
 		return
 	}

routers/api/v1/repo/issue_comment.go

@@ -586,7 +586,7 @@ func editIssueComment(ctx *context.APIContext, form api.EditIssueCommentOption)
 
 	oldContent := comment.Content
 	comment.Content = form.Body
-	if err := issue_service.UpdateComment(ctx, comment, ctx.Doer, oldContent); err != nil {
+	if err := issue_service.UpdateComment(ctx, comment, comment.ContentVersion, ctx.Doer, oldContent); err != nil {
 		ctx.Error(http.StatusInternalServerError, "UpdateComment", err)
 		return
 	}

routers/api/v1/repo/issue_comment_attachment.go

@@ -225,7 +225,7 @@ func CreateIssueCommentAttachment(ctx *context.APIContext) {
 		return
 	}
 
-	if err = issue_service.UpdateComment(ctx, comment, ctx.Doer, comment.Content); err != nil {
+	if err = issue_service.UpdateComment(ctx, comment, comment.ContentVersion, ctx.Doer, comment.Content); err != nil {
 		ctx.ServerError("UpdateComment", err)
 		return
 	}

routers/api/v1/repo/pull.go

@@ -609,8 +609,13 @@ func EditPullRequest(ctx *context.APIContext) {
 		}
 	}
 	if form.Body != nil {
-		err = issue_service.ChangeContent(ctx, issue, ctx.Doer, *form.Body)
+		err = issue_service.ChangeContent(ctx, issue, ctx.Doer, *form.Body, issue.ContentVersion)
 		if err != nil {
+			if errors.Is(err, issues_model.ErrIssueAlreadyChanged) {
+				ctx.Error(http.StatusBadRequest, "ChangeContent", err)
+				return
+			}
+
 			ctx.Error(http.StatusInternalServerError, "ChangeContent", err)
 			return
 		}

routers/web/repo/issue.go

@@ -2239,8 +2239,16 @@ func UpdateIssueContent(ctx *context.Context) {
 		return
 	}
 
-	if err := issue_service.ChangeContent(ctx, issue, ctx.Doer, ctx.Req.FormValue("content")); err != nil {
+	if err := issue_service.ChangeContent(ctx, issue, ctx.Doer, ctx.Req.FormValue("content"), ctx.FormInt("content_version")); err != nil {
+		if errors.Is(err, issues_model.ErrIssueAlreadyChanged) {
+			if issue.IsPull {
+				ctx.JSONError(ctx.Tr("repo.pulls.edit.already_changed"))
+			} else {
+				ctx.JSONError(ctx.Tr("repo.issues.edit.already_changed"))
+			}
+		} else {
 			ctx.ServerError("ChangeContent", err)
+		}
 		return
 	}
 
@@ -2267,6 +2275,7 @@ func UpdateIssueContent(ctx *context.Context) {
 
 	ctx.JSON(http.StatusOK, map[string]any{
 		"content":        content,
+		"contentVersion": issue.ContentVersion,
 		"attachments":    attachmentsHTML(ctx, issue.Attachments, issue.Content),
 	})
 }
@@ -3155,9 +3164,16 @@ func UpdateCommentContent(ctx *context.Context) {
 	}
 
 	oldContent := comment.Content
-	comment.Content = ctx.FormString("content")
+	newContent := ctx.FormString("content")
-	if err = issue_service.UpdateComment(ctx, comment, ctx.Doer, oldContent); err != nil {
+	contentVersion := ctx.FormInt("content_version")
+
+	comment.Content = newContent
+	if err = issue_service.UpdateComment(ctx, comment, contentVersion, ctx.Doer, oldContent); err != nil {
+		if errors.Is(err, issues_model.ErrCommentAlreadyChanged) {
+			ctx.JSONError(ctx.Tr("repo.comments.edit.already_changed"))
+		} else {
 			ctx.ServerError("UpdateComment", err)
+		}
 		return
 	}
 
@@ -3189,6 +3205,7 @@ func UpdateCommentContent(ctx *context.Context) {
 
 	ctx.JSON(http.StatusOK, map[string]any{
 		"content":        content,
+		"contentVersion": comment.ContentVersion,
 		"attachments":    attachmentsHTML(ctx, comment.Attachments, comment.Content),
 	})
 }

services/issue/comments.go

@@ -74,7 +74,7 @@ func CreateIssueComment(ctx context.Context, doer *user_model.User, repo *repo_m
 }
 
 // UpdateComment updates information of comment.
-func UpdateComment(ctx context.Context, c *issues_model.Comment, doer *user_model.User, oldContent string) error {
+func UpdateComment(ctx context.Context, c *issues_model.Comment, contentVersion int, doer *user_model.User, oldContent string) error {
 	needsContentHistory := c.Content != oldContent && c.Type.HasContentSupport()
 	if needsContentHistory {
 		hasContentHistory, err := issues_model.HasIssueContentHistory(ctx, c.IssueID, c.ID)
@@ -89,7 +89,7 @@ func UpdateComment(ctx context.Context, c *issues_model.Comment, doer *user_mode
 		}
 	}
 
-	if err := issues_model.UpdateComment(ctx, c, doer); err != nil {
+	if err := issues_model.UpdateComment(ctx, c, contentVersion, doer); err != nil {
 		return err
 	}
 

services/issue/content.go

@@ -12,10 +12,10 @@ import (
 )
 
 // ChangeContent changes issue content, as the given user.
-func ChangeContent(ctx context.Context, issue *issues_model.Issue, doer *user_model.User, content string) (err error) {
+func ChangeContent(ctx context.Context, issue *issues_model.Issue, doer *user_model.User, content string, contentVersion int) (err error) {
 	oldContent := issue.Content
 
-	if err := issues_model.ChangeIssueContent(ctx, issue, doer, content); err != nil {
+	if err := issues_model.ChangeIssueContent(ctx, issue, doer, content, contentVersion); err != nil {
 		return err
 	}
 

templates/repo/diff/comments.tmpl

@@ -61,7 +61,7 @@
 			{{end}}
 			</div>
 			<div id="issuecomment-{{.ID}}-raw" class="raw-content tw-hidden">{{.Content}}</div>
-			<div class="edit-content-zone tw-hidden" data-update-url="{{$.root.RepoLink}}/comments/{{.ID}}" data-context="{{$.root.RepoLink}}" data-attachment-url="{{$.root.RepoLink}}/comments/{{.ID}}/attachments"></div>
+			<div class="edit-content-zone tw-hidden" data-update-url="{{$.root.RepoLink}}/comments/{{.ID}}" data-content-version="{{.ContentVersion}}" data-context="{{$.root.RepoLink}}" data-attachment-url="{{$.root.RepoLink}}/comments/{{.ID}}/attachments"></div>
 			{{if .Attachments}}
 				{{template "repo/issue/view_content/attachments" dict "Attachments" .Attachments "RenderedContent" .RenderedContent}}
 			{{end}}

templates/repo/issue/view_content.tmpl

@@ -60,7 +60,7 @@
 							{{end}}
 						</div>
 						<div id="issue-{{.Issue.ID}}-raw" class="raw-content tw-hidden">{{.Issue.Content}}</div>
-						<div class="edit-content-zone tw-hidden" data-update-url="{{$.RepoLink}}/issues/{{.Issue.Index}}/content" data-context="{{.RepoLink}}" data-attachment-url="{{$.RepoLink}}/issues/{{.Issue.Index}}/attachments" data-view-attachment-url="{{$.RepoLink}}/issues/{{.Issue.Index}}/view-attachments"></div>
+						<div class="edit-content-zone tw-hidden" data-update-url="{{$.RepoLink}}/issues/{{.Issue.Index}}/content" data-content-version="{{.Issue.ContentVersion}}" data-context="{{.RepoLink}}" data-attachment-url="{{$.RepoLink}}/issues/{{.Issue.Index}}/attachments" data-view-attachment-url="{{$.RepoLink}}/issues/{{.Issue.Index}}/view-attachments"></div>
 						{{if .Issue.Attachments}}
 							{{template "repo/issue/view_content/attachments" dict "Attachments" .Issue.Attachments "RenderedContent" .Issue.RenderedContent}}
 						{{end}}

templates/repo/issue/view_content/comments.tmpl

@@ -67,7 +67,7 @@
 							{{end}}
 						</div>
 						<div id="issuecomment-{{.ID}}-raw" class="raw-content tw-hidden">{{.Content}}</div>
-						<div class="edit-content-zone tw-hidden" data-update-url="{{$.RepoLink}}/comments/{{.ID}}" data-context="{{$.RepoLink}}" data-attachment-url="{{$.RepoLink}}/comments/{{.ID}}/attachments"></div>
+						<div class="edit-content-zone tw-hidden" data-update-url="{{$.RepoLink}}/comments/{{.ID}}" data-content-version="{{.ContentVersion}}" data-context="{{$.RepoLink}}" data-attachment-url="{{$.RepoLink}}/comments/{{.ID}}/attachments"></div>
 						{{if .Attachments}}
 							{{template "repo/issue/view_content/attachments" dict "Attachments" .Attachments "RenderedContent" .RenderedContent}}
 						{{end}}
@@ -441,7 +441,7 @@
 								{{end}}
 							</div>
 							<div id="issuecomment-{{.ID}}-raw" class="raw-content tw-hidden">{{.Content}}</div>
-							<div class="edit-content-zone tw-hidden" data-update-url="{{$.RepoLink}}/comments/{{.ID}}" data-context="{{$.RepoLink}}" data-attachment-url="{{$.RepoLink}}/comments/{{.ID}}/attachments"></div>
+							<div class="edit-content-zone tw-hidden" data-update-url="{{$.RepoLink}}/comments/{{.ID}}" data-content-version="{{.ContentVersion}}" data-context="{{$.RepoLink}}" data-attachment-url="{{$.RepoLink}}/comments/{{.ID}}/attachments"></div>
 							{{if .Attachments}}
 								{{template "repo/issue/view_content/attachments" dict "Attachments" .Attachments "RenderedContent" .RenderedContent}}
 							{{end}}

templates/repo/issue/view_content/conversation.tmpl

@@ -93,7 +93,7 @@
 							{{end}}
 							</div>
 							<div id="issuecomment-{{.ID}}-raw" class="raw-content tw-hidden">{{.Content}}</div>
-							<div class="edit-content-zone tw-hidden" data-update-url="{{$.RepoLink}}/comments/{{.ID}}" data-context="{{$.RepoLink}}" data-attachment-url="{{$.RepoLink}}/comments/{{.ID}}/attachments"></div>
+							<div class="edit-content-zone tw-hidden" data-update-url="{{$.RepoLink}}/comments/{{.ID}}" data-content-version="{{.ContentVersion}}"  data-context="{{$.RepoLink}}" data-attachment-url="{{$.RepoLink}}/comments/{{.ID}}/attachments"></div>
 							{{if .Attachments}}
 								{{template "repo/issue/view_content/attachments" dict "Attachments" .Attachments "RenderedContent" .RenderedContent}}
 							{{end}}

tests/integration/issue_test.go

@@ -282,6 +282,34 @@ func TestIssueDependencies(t *testing.T) {
 	})
 }
 
+func TestEditIssue(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+	session := loginUser(t, "user2")
+	issueURL := testNewIssue(t, session, "user2", "repo1", "Title", "Description")
+
+	req := NewRequestWithValues(t, "POST", fmt.Sprintf("%s/content", issueURL), map[string]string{
+		"_csrf":   GetCSRF(t, session, issueURL),
+		"content": "modified content",
+		"context": fmt.Sprintf("/%s/%s", "user2", "repo1"),
+	})
+	session.MakeRequest(t, req, http.StatusOK)
+
+	req = NewRequestWithValues(t, "POST", fmt.Sprintf("%s/content", issueURL), map[string]string{
+		"_csrf":   GetCSRF(t, session, issueURL),
+		"content": "modified content",
+		"context": fmt.Sprintf("/%s/%s", "user2", "repo1"),
+	})
+	session.MakeRequest(t, req, http.StatusBadRequest)
+
+	req = NewRequestWithValues(t, "POST", fmt.Sprintf("%s/content", issueURL), map[string]string{
+		"_csrf":           GetCSRF(t, session, issueURL),
+		"content":         "modified content",
+		"content_version": "1",
+		"context":         fmt.Sprintf("/%s/%s", "user2", "repo1"),
+	})
+	session.MakeRequest(t, req, http.StatusOK)
+}
+
 func TestIssueCommentClose(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()
 	session := loginUser(t, "user2")
@@ -401,6 +429,7 @@ func TestIssueCommentUpdate(t *testing.T) {
 	req = NewRequestWithValues(t, "POST", fmt.Sprintf("/%s/%s/comments/%d", "user2", "repo1", commentID), map[string]string{
 		"_csrf":           GetCSRF(t, session, issueURL),
 		"content":         "",
+		"content_version": fmt.Sprintf("%d", comment.ContentVersion),
 	})
 	session.MakeRequest(t, req, http.StatusOK)
 
@@ -408,6 +437,44 @@ func TestIssueCommentUpdate(t *testing.T) {
 	assert.Equal(t, "", comment.Content)
 }
 
+func TestIssueCommentUpdateSimultaneously(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+	session := loginUser(t, "user2")
+	issueURL := testNewIssue(t, session, "user2", "repo1", "Title", "Description")
+	comment1 := "Test comment 1"
+	commentID := testIssueAddComment(t, session, issueURL, comment1, "")
+
+	comment := unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{ID: commentID})
+	assert.Equal(t, comment1, comment.Content)
+
+	modifiedContent := comment.Content + "MODIFIED"
+
+	req := NewRequestWithValues(t, "POST", fmt.Sprintf("/%s/%s/comments/%d", "user2", "repo1", commentID), map[string]string{
+		"_csrf":   GetCSRF(t, session, issueURL),
+		"content": modifiedContent,
+	})
+	session.MakeRequest(t, req, http.StatusOK)
+
+	modifiedContent = comment.Content + "2"
+
+	req = NewRequestWithValues(t, "POST", fmt.Sprintf("/%s/%s/comments/%d", "user2", "repo1", commentID), map[string]string{
+		"_csrf":   GetCSRF(t, session, issueURL),
+		"content": modifiedContent,
+	})
+	session.MakeRequest(t, req, http.StatusBadRequest)
+
+	req = NewRequestWithValues(t, "POST", fmt.Sprintf("/%s/%s/comments/%d", "user2", "repo1", commentID), map[string]string{
+		"_csrf":           GetCSRF(t, session, issueURL),
+		"content":         modifiedContent,
+		"content_version": "1",
+	})
+	session.MakeRequest(t, req, http.StatusOK)
+
+	comment = unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{ID: commentID})
+	assert.Equal(t, modifiedContent, comment.Content)
+	assert.Equal(t, 2, comment.ContentVersion)
+}
+
 func TestIssueReaction(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()
 	session := loginUser(t, "user2")

web_src/js/features/repo-legacy.js

@@ -16,6 +16,7 @@ import {initCitationFileCopyContent} from './citation.js';
 import {initCompLabelEdit} from './comp/LabelEdit.js';
 import {initRepoDiffConversationNav} from './repo-diff.js';
 import {createDropzone} from './dropzone.js';
+import {showErrorToast} from '../modules/toast.js';
 import {initCommentContent, initMarkupContent} from '../markup/content.js';
 import {initCompReactionSelector} from './comp/ReactionSelector.js';
 import {initRepoSettingBranches} from './repo-settings.js';
@@ -431,11 +432,17 @@ async function onEditContent(event) {
       const params = new URLSearchParams({
         content: comboMarkdownEditor.value(),
         context: editContentZone.getAttribute('data-context'),
+        content_version: editContentZone.getAttribute('data-content-version'),
       });
       for (const fileInput of dropzoneInst?.element.querySelectorAll('.files [name=files]')) params.append('files[]', fileInput.value);
 
       const response = await POST(editContentZone.getAttribute('data-update-url'), {data: params});
       const data = await response.json();
+      if (response.status === 400) {
+        showErrorToast(data.errorMessage);
+        return;
+      }
+      editContentZone.setAttribute('data-content-version', data.contentVersion);
       if (!data.content) {
         renderContent.innerHTML = document.getElementById('no-content').innerHTML;
         rawContent.textContent = '';

web_src/js/markup/tasklist.js

@@ -1,4 +1,5 @@
 import {POST} from '../modules/fetch.js';
+import {showErrorToast} from '../modules/toast.js';
 
 const preventListener = (e) => e.preventDefault();
 
@@ -54,13 +55,20 @@ export function initMarkupTasklist() {
           const editContentZone = container.querySelector('.edit-content-zone');
           const updateUrl = editContentZone.getAttribute('data-update-url');
           const context = editContentZone.getAttribute('data-context');
+          const contentVersion = editContentZone.getAttribute('data-content-version');
 
           const requestBody = new FormData();
           requestBody.append('ignore_attachments', 'true');
           requestBody.append('content', newContent);
           requestBody.append('context', context);
-          await POST(updateUrl, {data: requestBody});
+          requestBody.append('content_version', contentVersion);
-
+          const response = await POST(updateUrl, {data: requestBody});
+          const data = await response.json();
+          if (response.status === 400) {
+            showErrorToast(data.errorMessage);
+            return;
+          }
+          editContentZone.setAttribute('data-content-version', data.contentVersion);
           rawContent.textContent = newContent;
         } catch (err) {
           checkbox.checked = !checkbox.checked;