template: T2230: helper to generate templates

Currently the pattern is to import jinja2 and re-generate
the template in every file. Dimitriy reported a reduction
in performance (1s) when commiting.

This code provide an helper function which caches the
Environment and template renderer, and can generate
template from one line (instead of the few currently)

2 files changed, 56 insertions, 0 deletions

diff --git a/python/vyos/defaults.py b/python/vyos/defaults.py
index a2ad142bc..88894674f 100644
--- a/python/vyos/defaults.py
+++ b/python/vyos/defaults.py
@@ -21,6 +21,7 @@ directories = {
   "current": "/opt/vyatta/etc/config-migrate/current",
   "migrate": "/opt/vyatta/etc/config-migrate/migrate",
   "log": "/var/log/vyatta",
+  "templates": "/usr/share/vyos/templates/"
 }
 
 cfg_group = 'vyattacfg'
diff --git a/python/vyos/template.py b/python/vyos/template.py
new file mode 100644
index 000000000..e559120c0
--- /dev/null
+++ b/python/vyos/template.py
@@ -0,0 +1,55 @@
+# Copyright 2019 VyOS maintainers and contributors <maintainers@vyos.io>
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+
+import os
+
+from jinja2 import Environment
+from jinja2 import FileSystemLoader
+
+from vyos.defaults import directories
+
+
+# reuse the same Environment to improve performance
+_templates_env = Environment(loader=FileSystemLoader(directories['templates']))
+_templates_mem = {}
+
+def render(destination, template, content):
+    """
+    render a template from the template directory, it will raise on any errors
+    destination: the file where the rendered template must be saved
+    template: the path to the template relative to the template folder
+    content: the dictionary to use to render the template
+
+    This classes cache the renderer, so rendering the same file multiple time
+    does not cause as too much overhead. If use everywhere, it could be changed
+    and load the template from python environement variables from an import 
+    python module generated when the debian package is build 
+    (recovering the load time and overhead caused by having the file out of the code)
+    """
+
+    # Setup a renderer for the given template
+    # This is cached and re-used for performance
+    if template not in _templates_mem:
+        _templates_mem[template] = _templates_env.get_template(template)
+    template = _templates_mem[template]
+
+    # As we are opening the file with 'w', we are performing the rendering
+    # before calling open() to not accidentally erase the file if the 
+    # templating fails
+    content = template.render(content)
+
+    # Write client config file
+    with open(destination, 'w') as f:
+        f.write(content)