improve jinja2 templating error report for TemplateError (#783)

jinja2.TemplateError does not include the template line number
which can be of great help when troubleshooting errors like
   'foo' has no attribute 'bar'

The line number can however be inferred from the stack trace.
This was inspired by
https://github.com/saltstack/salt/blob/master/salt/utils/templates.py

Example output:

   Jsonnet error: failed to compile test/main.jsonnet:
    RUNTIME ERROR: Jsonnet jinja2 failed to render templates/abc.j2:
     Jinja2 TemplateError: 'foo' has no attribute 'bar', at templates/abc.j2:240

Signed-off-by: David Verbeiren <[email protected]>

kapitan/utils.py

@@ -16,6 +16,7 @@
 import sys
 import re
 import tarfile
+import traceback
 from zipfile import ZipFile
 from collections import Counter, defaultdict
 from functools import lru_cache, wraps
@@ -123,6 +124,14 @@ def sha256_string(string):
     return sha256(string.encode("UTF-8")).hexdigest()
 
 
+def _jinja_error_info(trace_data):
+    """Extract jinja2 templating related frames from traceback data"""
+    try:
+        return [x for x in trace_data if x[2] in ("top-level template code", "template", "<module>")][-1]
+    except IndexError:
+        pass
+
+
 def render_jinja2_file(name, context, jinja2_filters=defaults.DEFAULT_JINJA2_FILTERS_PATH, search_paths=None):
     """Render jinja2 file name with context"""
     path, filename = os.path.split(name)
@@ -136,7 +145,12 @@ def render_jinja2_file(name, context, jinja2_filters=defaults.DEFAULT_JINJA2_FIL
     )
     load_jinja2_filters(env)
     load_jinja2_filters_from_file(env, jinja2_filters)
-    return env.get_template(filename).render(context)
+    try:
+        return env.get_template(filename).render(context)
+    except jinja2.TemplateError as e:
+        # Exception misses the line number info. Retreive it from traceback
+        err_info = _jinja_error_info(traceback.extract_tb(sys.exc_info()[2]))
+        raise CompileError(f"Jinja2 TemplateError: {e}, at {err_info[0]}:{err_info[1]}")
 
 
 def render_jinja2(path, context, jinja2_filters=defaults.DEFAULT_JINJA2_FILTERS_PATH, search_paths=None):