render_markdown_path: Handle absolute template paths properly.

One disadvantage of relying on Jinja2 to load all templates is that it
only searches a finite set of pre-configured template directories.
Unfortunately, that breaks when someone tries to enable a custom
privacy or terms page and has the corresponding template in a
directory outside of Jinja2's recognized directories (for instance, it
won't find `/etc/zulip/terms.md`, the recommended path).

This commit makes it so that render_markdown_path can be more
sensible about pure Markdown files and load templates with
absolute paths directly without relying on Jinja2, if need be.

templates/zerver/privacy.html

@@ -21,7 +21,7 @@
         <div class="padded-content">
             <div class="inner-content">
                 {% if privacy_policy %}
-                    {{ render_markdown_path(privacy_policy) }}
+                    {{ render_markdown_path(privacy_policy, pure_markdown=True) }}
                 {% else %}
                     {% trans %}
                     This installation of Zulip does not have a configured privacy policy.

templates/zerver/terms.html

@@ -20,7 +20,7 @@
         <div class="padded-content">
             <div class="inner-content">
                 {% if terms_of_service %}
-                    {{ render_markdown_path(terms_of_service) }}
+                    {{ render_markdown_path(terms_of_service, pure_markdown=True) }}
                 {% else %}
                     {% trans %}
                     This installation of Zulip does not have a configured terms of service.

zerver/templatetags/app_filters.py

@@ -10,6 +10,7 @@ import markdown_include.include
 from django.conf import settings
 from django.template import Library, engines, loader
 from django.utils.safestring import mark_safe
+from jinja2.exceptions import TemplateNotFound
 
 import zerver.lib.bugdown.fenced_code
 import zerver.lib.bugdown.api_arguments_table_generator
@@ -71,7 +72,9 @@ docs_without_macros = [
 # the results when called if none of the arguments are unhashable.
 @ignore_unhashable_lru_cache(512)
 @register.filter(name='render_markdown_path', is_safe=True)
-def render_markdown_path(markdown_file_path: str, context: Optional[Dict[Any, Any]]=None) -> str:
+def render_markdown_path(markdown_file_path: str,
+                         context: Optional[Dict[Any, Any]]=None,
+                         pure_markdown: Optional[bool]=False) -> str:
     """Given a path to a markdown file, return the rendered html.
 
     Note that this assumes that any HTML in the markdown file is
@@ -114,7 +117,27 @@ def render_markdown_path(markdown_file_path: str, context: Optional[Dict[Any, An
     md_engine.reset()
 
     jinja = engines['Jinja2']
-    markdown_string = jinja.env.loader.get_source(jinja.env, markdown_file_path)[0]
-    html = md_engine.convert(markdown_string)
-    html_template = jinja.from_string(html)
-    return mark_safe(html_template.render(context))
+
+    if pure_markdown:
+        # For files such as /etc/zulip/terms.md where we don't intend
+        # to use Jinja2 template variables, we still try to load the
+        # template using Jinja2 (in case the file path isn't absolute
+        # and does happen to be in Jinja's recognized template
+        # directories), and if that fails, we try to load it directly
+        # from disk.
+        try:
+            markdown_string = jinja.env.loader.get_source(jinja.env, markdown_file_path)[0]
+        except TemplateNotFound:
+            with open(markdown_file_path) as fp:
+                markdown_string = fp.read()
+
+        rendered_html = md_engine.convert(markdown_string)
+    else:
+        # By default, we do both Jinja2 templating and markdown
+        # processing on the file, to make it easy to use both Jinja2
+        # context variables and markdown includes in the file.
+        markdown_string = jinja.env.loader.get_source(jinja.env, markdown_file_path)[0]
+        html = md_engine.convert(markdown_string)
+        rendered_html = jinja.from_string(html).render(context)
+
+    return mark_safe(rendered_html)