Use inode number in addition to mtime (#632)

* Use inode number in addition to mtime

* Changelog

CHANGELOG.rst

@@ -26,6 +26,9 @@ Version 0.16.0
 - **Vdirsyncer is now licensed under the 3-clause BSD license**, see :gh:`610`.
 - Vdirsyncer now includes experimental support for `EteSync
   <https://www.etesync.com/>`_, see :ghpr:`614`.
+- Vdirsyncer now uses more filesystem metadata for determining whether an item
+  changed. You will notice a **possibly heavy CPU/IO spike on the first sync
+  after upgrading**.
 
 Version 0.15.0
 ==============

vdirsyncer/storage/singlefile.py

@@ -11,7 +11,7 @@ from atomicwrites import atomic_write
 
 from .base import Storage
 from .. import exceptions
-from ..utils import checkfile, expand_path
+from ..utils import checkfile, expand_path, get_etag_from_file
 from ..vobject import Item, join_collection, split_collection
 
 logger = logging.getLogger(__name__)
@@ -88,7 +88,7 @@ class SingleFileStorage(Storage):
     _read_mode = 'rb'
 
     _items = None
-    _last_mtime = None
+    _last_etag = None
 
     def __init__(self, path, encoding='utf-8', **kwargs):
         super(SingleFileStorage, self).__init__(**kwargs)
@@ -149,7 +149,7 @@ class SingleFileStorage(Storage):
         self._items = collections.OrderedDict()
 
         try:
-            self._last_mtime = os.path.getmtime(self.path)
+            self._last_etag = get_etag_from_file(self.path)
             with open(self.path, self._read_mode) as f:
                 text = f.read().decode(self.encoding)
         except OSError as e:
@@ -210,8 +210,8 @@ class SingleFileStorage(Storage):
         del self._items[href]
 
     def _write(self):
-        if self._last_mtime is not None and \
-           self._last_mtime != os.path.getmtime(self.path):
+        if self._last_etag is not None and \
+           self._last_etag != get_etag_from_file(self.path):
             raise exceptions.PreconditionFailed(
                 'Some other program modified the file {r!}. Re-run the '
                 'synchronization and make sure absolutely no other program is '
@@ -224,7 +224,7 @@ class SingleFileStorage(Storage):
                 f.write(text.encode(self.encoding))
         finally:
             self._items = None
-            self._last_mtime = None
+            self._last_etag = None
 
     @contextlib.contextmanager
     def at_once(self):

vdirsyncer/utils.py

@@ -59,10 +59,10 @@ def uniq(s):
 
 
 def get_etag_from_file(f):
-    '''Get mtime-based etag from a filepath or file-like object.
+    '''Get etag from a filepath or file-like object.
 
     This function will flush/sync the file as much as necessary to obtain a
-    correct mtime.
+    correct value.
     '''
     if hasattr(f, 'read'):
         f.flush()  # Only this is necessary on Linux
@@ -75,7 +75,7 @@ def get_etag_from_file(f):
     mtime = getattr(stat, 'st_mtime_ns', None)
     if mtime is None:
         mtime = stat.st_mtime
-    return '{:.9f}'.format(mtime)
+    return '{:.9f};{}'.format(mtime, stat.st_ino)
 
 
 def get_storage_init_specs(cls, stop_at=object):