Add a directory-specific nameserver data to templates

If a template is being rendered into a directory that represents a
nameserver (e.g. "ns1"), include a nameserver-specific information in
the data - variable called "ns" which has information about the
nameserver this file belongs to.

Ensure the "ns" variable is only exposed to the template when rendered,
without affecting the environment variables (always work with a copy of
the env_vars).

bin/tests/system/isctest/template.py

@@ -13,6 +13,7 @@
 
 from dataclasses import dataclass
 from pathlib import Path
+from re import compile as Re
 from typing import Any
 
 import re
@@ -22,6 +23,8 @@ import jinja2
 from .log import debug
 from .vars import ALL
 
+NS_DIR_RE = Re(r"^(a?ns([0-9]+))/")
+
 
 class TemplateEngine:
     """
@@ -63,10 +66,16 @@ class TemplateEngine:
             raise RuntimeError('No jinja2 template found for "{output}"')
 
         if data is None:
-            data = self.env_vars
+            data = {**self.env_vars}
         else:
             data = {**self.env_vars, **data}
 
+        # directory-specific "ns" var
+        assert "ns" not in data, '"ns" variable is reserved for nameserver data'
+        match = NS_DIR_RE.search(output)
+        if match:
+            data["ns"] = Nameserver(match.group(1))
+
         debug("rendering template `%s` to file `%s`", template, output)
         stream = self.j2env.get_template(template).stream(data)
         stream.dump(output, encoding="utf-8")