Limit length of wiki file/directory names

Many common file systems have a limit of 255 bytes for file and
directory names, and while Git and GitLab both support paths exceeding
those limits, the presence of them makes it impossible for users on
those file systems to checkout a wiki repository locally.

To avoid this situation, we enforce these limits when editing pages
through the GitLab web interface and API:

- 245 bytes for page titles (reserving 10 bytes for the file extension)
- 255 bytes for directory names

app/models/wiki_page.rb

@@ -5,6 +5,9 @@ class WikiPage
   PageChangedError = Class.new(StandardError)
   PageRenameError = Class.new(StandardError)
 
+  MAX_TITLE_BYTES = 245
+  MAX_DIRECTORY_BYTES = 255
+
   include ActiveModel::Validations
   include ActiveModel::Conversion
   include StaticModel
@@ -51,6 +54,7 @@ class WikiPage
 
   validates :title, presence: true
   validates :content, presence: true
+  validate :validate_path_limits, if: :title_changed?
 
   # The GitLab ProjectWiki instance.
   attr_reader :wiki
@@ -262,7 +266,7 @@ class WikiPage
   end
 
   def title_changed?
-    title.present? && self.class.unhyphenize(@page.url_path) != title
+    title.present? && (@page.nil? || self.class.unhyphenize(@page.url_path) != title)
   end
 
   # Updates the current @attributes hash by merging a hash of params
@@ -324,4 +328,16 @@ class WikiPage
     set_attributes
     @persisted = errors.blank?
   end
+
+  def validate_path_limits
+    *dirnames, title = @attributes[:title].split('/')
+
+    if title.bytesize > MAX_TITLE_BYTES
+      errors.add(:title, _("exceeds the limit of %{bytes} bytes for page titles") % { bytes: MAX_TITLE_BYTES })
+    end
+
+    if dirnames.any? { |d| d.bytesize > MAX_DIRECTORY_BYTES }
+      errors.add(:title, _("exceeds the limit of %{bytes} bytes for directory names") % { bytes: MAX_DIRECTORY_BYTES })
+    end
+  end
 end

app/views/projects/wikis/_form.html.haml

@@ -17,9 +17,13 @@
         = icon('lightbulb-o')
         - if @page.persisted?
           = s_("WikiEditPageTip|Tip: You can move this page by adding the path to the beginning of the title.")
-          = link_to icon('question-circle'), help_page_path('user/project/wiki/index', anchor: 'moving-a-wiki-page'), target: '_blank'
+          = link_to icon('question-circle'), help_page_path('user/project/wiki/index', anchor: 'moving-a-wiki-page'),
+              target: '_blank', rel: 'noopener noreferrer'
         - else
           = s_("WikiNewPageTip|Tip: You can specify the full path for the new file. We will automatically create any missing directories.")
+        = succeed '.' do
+          = link_to _('Learn more'), help_page_path('user/project/wiki/index', anchor: 'creating-a-new-wiki-page'),
+              target: '_blank', rel: 'noopener noreferrer'
   .form-group.row
     .col-sm-12= f.label :format, class: 'control-label-full-width'
     .col-sm-12

changelogs/unreleased/24415-restrict-wiki-page-titles.yml

+---
+title: Limit length of wiki file/directory names
+merge_request: 24364
+author:
+type: changed

doc/administration/instance_limits.md

@@ -119,3 +119,7 @@ This limit can be configured for self hosted installations when [enabling
 Elasticsearch](../integration/elasticsearch.md#enabling-elasticsearch).
 
 NOTE: **Note:** Set the limit to `0` to disable it.
+
+## Wiki limits
+
+- [Length restrictions for file and directory names](../user/project/wiki/index.md#length-restrictions-for-file-and-directory-names).

doc/user/project/wiki/index.md

@@ -50,6 +50,8 @@ When you're ready, click the **Create page** and the new page will be created.
 
 ![New page](img/wiki_create_new_page.png)
 
+### Attachment storage
+
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/issues/33475) in GitLab 11.3.
 
 Starting with GitLab 11.3, any file that is uploaded to the wiki via GitLab's
@@ -58,6 +60,22 @@ if you clone the wiki repository locally. All uploaded files prior to GitLab
 11.3 are stored in GitLab itself. If you want them to be part of the wiki's Git
 repository, you will have to upload them again.
 
+### Length restrictions for file and directory names
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/24364) in GitLab 12.8.
+
+Many common file systems have a [limit of 255 bytes for file and directory names](https://en.wikipedia.org/wiki/Comparison_of_file_systems#Limits), and while Git and GitLab both support paths exceeding those limits, the presence of them makes it impossible for users on those file systems to checkout a wiki repository locally.
+
+To avoid this situation, these limits are enforced when editing pages through the GitLab web interface and API:
+
+- 245 bytes for page titles (reserving 10 bytes for the file extension).
+- 255 bytes for directory names.
+
+Please note that:
+
+- Non-ASCII characters take up more than one byte.
+- It's still possible to create files and directories exceeding those limits locally through Git, but this might break on other people's machines.
+
 ## Editing a wiki page
 
 NOTE: **Note:**

locale/gitlab.pot

@@ -22721,6 +22721,12 @@ msgstr ""
 msgid "estimateCommand|%{slash_command} will update the estimated time with the latest command."
 msgstr ""
 
+msgid "exceeds the limit of %{bytes} bytes for directory names"
+msgstr ""
+
+msgid "exceeds the limit of %{bytes} bytes for page titles"
+msgstr ""
+
 msgid "expired on %{milestone_due_date}"
 msgstr ""