Fix directory alias handling in Nginx

[iliaross]

Hi Jamie,

This PR fixes Nginx directory aliases that were incorrectly being used as filesystem paths in rewrite rules. It converts alias destinations to internal URL paths, prevents rewrite loops, and ensures regular redirects are processed before aliases.

It also keeps alias listing and deletion working, including existing aliases.

This fixes applications with a separate public directory, such as Roundcube 1.7.

Tested on Debian 12 with different alias and redirect configurations.

[jcameron]

Wow this looks really complex! Can you add some more details of exactly what kind of fixes it is doing, and when?

[iliaross]

Wow this looks really complex! Can you add some more details of exactly what kind of fixes it is doing, and when?

Yeah, sure!

It looks complex because Virtualmin must be able to create, find, and delete the same Nginx alias safely.

So we care about the following:

• When an alias is created, it converts the directory path into a URL path Nginx can use. For example, /home/user/public_html/app/public_html becomes /app/public_html;
• It prevents the alias from rewriting the same URL again and causing a loop;
• It supports all alias options such as root paths, subdirectories, exact matches, ignored subpaths, trailing slashes, and HTTP/HTTPS limits;
• It places normal redirects before aliases. This matters because an alias can stop Nginx from checking rules below it;
• It allows Virtualmin to find and delete aliases it created, including older aliases;
• It leaves custom Nginx rewrite rules alone;
• It rejects alias directories outside the website's public_html, because they cannot be converted safely;
• For PHP websites, it avoids sending an empty PATH_INFO value after an alias is created.

And as you remember, Roundcube 1.7 exposed the problem because now its public files are inside a separate public_html directory. And, the fix applies to any application with the same directory structure.

[jcameron]

Ah so this fixes a case where an alias points to a sub-directory of the original directory?

[iliaross]

Ah so this fixes a case where an alias points to a sub-directory of the original directory?

Yeah, that is one example, but the fix is more general. The alias API receives a filesystem path, while an Nginx rewrite needs a URL path.

This change converts alias destinations under the website's document root into the correct internal URL.

It handles both root and subdirectory aliases, and the other supported alias options. It also prevents the generated rewrite from matching itself again.