parser: update @include in templates, to work with relative paths & prevent recursive calls

[kylepritchard]

Allows calling a partial template directly especially if the partial was previously included as part of a root template. Reasoning added to discussion #21868

TLDR;

Using @include! will reference the template relative to the base template directory i.e. templates folder. Normal action is to include it relative to the template being called. Allows the partial to be called from any other template within the templates directory and its nested directories.

[spytheman]

Please add a test too. See vlib/v/tests/tmpl_test.v which uses vlib/v/tests/tmpl/base.txt and vlib/v/tests/tmpl/include.txt etc.

[kylepritchard]

I was looking into this again and have a few changes which I did not push yet. I got thrown when making additional tests and found that the test templates where in a folder which was named 'tmpl' instead of 'templates'. The logic I originally put down was to search for the 'templates' folder which is only applicable to vweb/veb programs. I was currently looking into using the p.template_paths so that any included template would be relative to it's calling template. That may remove the need to have an extra directive completely. The alternative was to have the root folder named in the @include line so it is easy to find but the syntax could start to get a little verbose @include! [tmpl] 'path/to/template' . It would be good to have it working for both vweb calls and normal $tmpl calls too.

[kylepritchard]

This PR is a rethink on the original PR. When an @include is called, it will now call a recursive function which will import the called template and scan to see if it in turn imports any more templates. Using recursion it is possible to track the current directory which will allow nested templates to call templates in a higher directory. To provide compatibility with the current implementation, it will walk back up the directory path to find the 'templates' folder if the called file is not in the same directory as the template being rendered.

Furthermore there was a bug in the code for circular references. If a template called itself or one of its children called it again then it would result in a circular reference that would crash the program. Now all called templates are entered into a dependency tree which will check if a circular reference exists and will fail with error message should one be encountered.

Finally all called templates are put in a temporary cache which will be used instead of costly IO operations if a template is reused in the code. This should benefit complex web applications were templates may represent web components and could be used many times in the app.

Note: The original idea of @include! has been removed in favour of the fallback to templates folder when finding files.

[spytheman]

Excellent work. It makes the @include directive a lot more usable, while it improves the error handling too.

@medvednikov what do you think?

[medvednikov]

Sounds good to me.

[kylepritchard]

Thanks for the consideration and advice. Hopefully I can help out more in time to come