Add CLI for partial_sync

docs/config.rst

@@ -101,6 +101,13 @@ Pair Section
 
   Vdirsyncer never attempts to "automatically merge" the two items.
 
+- ``partial_sync``: Assume A is read-only, B not. If you change items on B,
+  vdirsyncer can't sync the changes to A. What should happen instead?
+
+  - ``error``: An error is shown.
+  - ``ignore``: The change is ignored.
+  - ``revert`` (default): The change is reverted on next sync.
+
 - ``metadata``: Metadata keys that should be synchronized when ``vdirsyncer
   metasync`` is executed. Example::
 

tests/cli/test_sync.py

@@ -449,3 +449,64 @@ def test_conflict_resolution(tmpdir, runner, resolution, expect_foo,
 
     assert fooitem.read() == expect_foo
     assert baritem.read() == expect_bar
+
+
+@pytest.mark.parametrize('partial_sync', ['error', 'ignore', 'revert', None])
+def test_partial_sync(tmpdir, runner, partial_sync):
+    runner.write_with_general(dedent('''
+    [pair foobar]
+    a = foo
+    b = bar
+    collections = null
+    {partial_sync}
+
+    [storage foo]
+    type = filesystem
+    fileext = .txt
+    path = {base}/foo
+
+    [storage bar]
+    type = filesystem
+    read_only = true
+    fileext = .txt
+    path = {base}/bar
+    '''.format(
+        partial_sync=('partial_sync = {}\n'.format(partial_sync)
+                      if partial_sync else ''),
+        base=str(tmpdir)
+    )))
+
+    foo = tmpdir.mkdir('foo')
+    bar = tmpdir.mkdir('bar')
+
+    foo.join('other.txt').write('UID:other')
+    bar.join('other.txt').write('UID:other')
+
+    baritem = bar.join('lol.txt')
+    baritem.write('UID:lol')
+
+    r = runner.invoke(['discover'])
+    assert not r.exception
+
+    r = runner.invoke(['sync'])
+    assert not r.exception
+
+    fooitem = foo.join('lol.txt')
+    fooitem.remove()
+
+    r = runner.invoke(['sync'])
+
+    if partial_sync == 'error':
+        assert r.exception
+        assert 'Attempted change' in r.output
+    elif partial_sync == 'ignore':
+        assert baritem.exists()
+        r = runner.invoke(['sync'])
+        assert not r.exception
+        assert baritem.exists()
+    else:
+        assert baritem.exists()
+        r = runner.invoke(['sync'])
+        assert not r.exception
+        assert baritem.exists()
+        assert fooitem.exists()

vdirsyncer/cli/config.py

@@ -254,6 +254,7 @@ class PairConfig(object):
         self.options = options
 
         self._set_conflict_resolution()
+        self._set_partial_sync()
         self._set_collections()
 
     def _set_conflict_resolution(self):
@@ -278,6 +279,11 @@ class PairConfig(object):
         else:
             raise ValueError('Invalid value for `conflict_resolution`.')
 
+    def _set_partial_sync(self):
+        self.partial_sync = self.options.pop('partial_sync', 'revert')
+        if self.partial_sync not in ('ignore', 'revert', 'error'):
+            raise ValueError('Invalid value for `partial_sync`.')
+
     def _set_collections(self):
         try:
             collections = self.options['collections']

vdirsyncer/cli/tasks.py

@@ -68,8 +68,8 @@ def sync_collection(wq, collection, general, force_delete):
             a, b, status,
             conflict_resolution=pair.conflict_resolution,
             force_delete=force_delete,
-            error_callback=error_callback
-
+            error_callback=error_callback,
+            partial_sync=pair.partial_sync
         )
 
         save_status(general['status_path'], pair.name, collection.name,