force a full sync when fields or templates changed without schema mod

This is not an ideal solution and will not catch repositioned fields
or templates, but is at least an improvement over the previous behaviour.

https://github.com/dae/anki/pull/349#issuecomment-547236285

anki/sync.py

@@ -22,6 +22,9 @@ HTTP_TIMEOUT = 90
 HTTP_PROXY = None
 HTTP_BUF_SIZE = 64*1024
 
+class UnexpectedSchemaChange(Exception):
+    pass
+
 # Incremental syncing
 ##########################################################################
 
@@ -95,7 +98,11 @@ class Syncer:
         # ...and small objects
         lchg = self.changes()
         rchg = self.server.applyChanges(changes=lchg)
-        self.mergeChanges(lchg, rchg)
+        try:
+            self.mergeChanges(lchg, rchg)
+        except UnexpectedSchemaChange:
+            self.server.abort()
+            return self._forceFullSync()
         # step 3: stream large tables from server
         runHook("sync", "server")
         while 1:
@@ -119,17 +126,20 @@ class Syncer:
         c = self.sanityCheck()
         ret = self.server.sanityCheck2(client=c)
         if ret['status'] != "ok":
-            # roll back and force full sync
-            self.col.rollback()
-            self.col.modSchema(False)
-            self.col.save()
-            return "sanityCheckFailed"
+            return self._forceFullSync()
         # finalize
         runHook("sync", "finalize")
         mod = self.server.finish()
         self.finish(mod)
         return "success"
 
+    def _forceFullSync(self):
+        # roll back and force full sync
+        self.col.rollback()
+        self.col.modSchema(False)
+        self.col.save()
+        return "sanityCheckFailed"
+
     def _gravesChunk(self, graves):
         lim = 250
         chunk = dict(notes=[], cards=[], decks=[])
@@ -330,6 +340,14 @@ from notes where %s""" % d)
             l = self.col.models.get(r['id'])
             # if missing locally or server is newer, update
             if not l or r['mod'] > l['mod']:
+                # This is a hack to detect when the note type has been altered
+                # in an import without a full sync being forced. A future
+                # syncing algorithm should handle this in a better way.
+                if l:
+                    if len(l['flds']) != len(r['flds']):
+                        raise UnexpectedSchemaChange()
+                    if len(l['tmpls']) != len(r['tmpls']):
+                        raise UnexpectedSchemaChange()
                 self.col.models.update(r)
 
     # Decks