"RAILS_ROOT" cannot be put on a place different from "Context root" in Goldspike.

Did you try setting a context init parameter rails.root to achieve this?

<context-param>
  <param-name>rails.root</param-name>
  <param-value>/subpath</param-value>
</context-param>

RAILS_ROOT is the location on the file system of the Rails app (relative to the WAR).
Context root is the URL path where the Rails app is to be mounted.

Looking at the patch, I guess you want to be able to:
+ Set the context root to /mywar
+ Mount the Rails app at /mywar/app1
i.e. a context root extension of sorts

Is this correct?

>Looking at the patch, I guess you want to be able to:
>+ Set the context root to /mywar
>+ Mount the Rails app at /mywar/app1
>i.e. a context root extension of sorts

Yes. It is correct.
ex)
context root to /mywar
RAILS_ROOT=[context root real path]/WEB-INF/rails

--start of web.xml –
<context-param>
<param-name>rails.root</param-name>
<param-value>/WEB-INF/rails</param-value>
</context-param>
<context-param>
<param-name>files.prefix</param-name>
<param-value>/WEB-INF/rails/public</param-value>
</context-param>
~~
<servlet-mapping>
<servlet-name>files</servlet-name>
<url-pattern>/app1/*</url-pattern>
</servlet-mapping>
– end of web.xml –

In this case, when I access to
/mywar/app1
I want rails application to work.

What's the redirect for?

I don't know how getServletPath differs between application servers, so this will need to be tested on multiple servers to ensure we don't regress. At a minimum we'd need to test it on Jetty, Tomcat, WebLogic and Glassfish. I don't have time to do that testing now, however if anyone can confirm it works, I'll apply the patch. Otherwise I'll look towards getting the testing done when I can.

The redirected reason is that a wrong result case.

If the end of request path is not "/"
and relative path is included in HTML,
the result is different from expected.

For example, if
<a href="rails/info/properties"/>
included in index.html.

When the request is
http://localhost/app1/
, the result is
http://localhost/app1/rails/info/properties
.

However,
When the request is
http://localhost/app1
, the result is
http://localhost/rails/info/properties
.

GoldSpike has long been deprecated in favor of Warbler, so I'm marking this as "won't fix". There are no GoldSpike maintainers left. If you'd like to take it over yourself and apply this patch, feel free to let us know and reopen the bug.