jinja2: try to not rewrite on update if there's no change

slapos/recipe/template/jinja2_template.py

@@ -27,6 +27,8 @@
 import errno
 import os
 import json
+import stat
+import tempfile
 import zc.buildout
 from jinja2 import Environment, StrictUndefined, \
     BaseLoader, TemplateNotFound, PrefixLoader
@@ -128,6 +130,31 @@ LOADER_TYPE_DICT = {
     'folder': (FolderLoader, getKey),
 }
 
+def get_mode(fd):
+    return stat.S_IMODE(os.fstat(fd).st_mode)
+
+def get_umask():
+    global get_umask
+    while 1:
+        p = tempfile.mktemp()
+        try:
+            fd = os.open(p, os.O_CREAT | os.O_EXCL, 0o777)
+            break
+        except OSError as e:
+            if e.errno != errno.EEXIST:
+                if os.name == 'nt' and e.errno == errno.EACCES:
+                    p = os.path.basename(p)
+                    if os.path.isdir(p) and os.access(p, os.W_OK):
+                        continue
+                raise
+    try:
+        os.unlink(p)
+        umask = get_mode(fd)
+    finally:
+        os.close(fd)
+    get_umask = lambda: umask
+    return umask
+
 class Recipe(object):
 
     def __init__(self, buildout, name, options):
@@ -200,6 +227,20 @@ class Recipe(object):
             env.compile(source, filename=template),
             env.make_globals(None), None)
         rendered = template.render(**self.context).encode(self.encoding)
+        mode = self.mode
+        mask = (0o777 if rendered.startswith(b'#!') else 0o666
+               ) if mode is None else 0
+        # Try to reuse existing file. This is particularly
+        # important to avoid excessive IO because we render on update.
+        try:
+            with open(self.rendered, 'rb') as f:
+                if f.read(len(rendered)+1) == rendered:
+                    m = get_umask() & mask if mode is None else mode
+                    if get_mode(f.fileno()) != m:
+                        os.fchmod(f.fileno(), m)
+                    return self.rendered
+        except (IOError, OSError) as e:
+            pass
         # Unlink any existing file so that umask applies.
         try:
             os.unlink(self.rendered)
@@ -209,10 +250,7 @@ class Recipe(object):
             outdir = os.path.dirname(self.rendered)
             if outdir and not os.path.exists(outdir):
                 os.makedirs(outdir)
-        mode = self.mode
-        fd = os.open(self.rendered, os.O_CREAT | os.O_EXCL | os.O_WRONLY,
-                     (0o777 if rendered.startswith(b'#!') else 0o666)
-                     if mode is None else 0)
+        fd = os.open(self.rendered, os.O_CREAT | os.O_EXCL | os.O_WRONLY, mask)
         try:
             os.write(fd, rendered)
             if mode is not None: