Multi-domain support, ROOT_URL problem, use relative URLs in templates instead of absolute URLs

[wxiaoguang]

At the moment, Gitea users must set ROOT_URL correctly in app.ini, otherwise many URLs on the web UI will be wrong, which makes the web UI doesn't work properly (CSRF tokens and cookies can not work across domains)

The ROOT_URL must be set to the full URL which the users visit, with correct scheme(eg: https://the-gitea-domain/) and correct sub path if it is used (eg: https://the-gitea-domain/my-gitea-path/).

New code should always use relative URLs for Web UI.

---

Many maintainers have answered the issues about incorrect ROOT_URL again and again.

Using HTMLURL(and other absolute URLs) in templates is the root case that Gitea can only serve web under ROOT_URL.

That's why we have to force users to set ROOT_URL correctly.

If all URLs in template can be refactored to use Relative URL correctly, then we do not need to force users set ROOT_URL correctly again and again.

The ROOT_URL is still needed because for example, some webhooks need the absolute URL for external systems, and Gitea should show absolute URL on the UI for clone link, etc.

Related:

• Wrong redirection after add time to issue #14254
• url error when using nginx reverse proxy mode #17912
• Preview of Markdown not working, getting 403 #17843
• Dissimilar URLs #19340
• URL of any link in section "Repository" includes port 3030 #19963
• Multiple ROOT_URL syntax/possible? #6397 (5 up-votes!)
• Multidomain support #8697 (19 up-votes!)
• Manifest: property 'start_url' ignored, should be same origin as document. #20091
• Repo links on dashboard wrong when using a proxy frontend #20450

Some more related:

• Cannot get passed initial configuration #20436 , after installation, Gitea may redirect user to an incorrect location.
• During new instance setup make 'Gitea Base URL' filled from window.location.origin #20089 , it may resolve the incorrect redirection problem during installation.

The PR to detect the problem:

• Show messages for users if the ROOT_URL is wrong, show JavaScript errors #18971

[silverwind]

+1, relative URLs may even enable use cases like running on multiple domains. Thought IIRC, ROOT_URL still needs to stay in the config because sometimes absolute URLs need to be built, like for PR link when pushing via SSH. But it's good nonetheless to remove dependencies on ROOT_URL where possible.

[lunny]

We need a new global variable ROOT_URL_PATH, it could be / or sub path like /gitea. For repository, we could have a new HTMLURLPath