improve log message when client cache is out of sync with server

This fixes #142

CHANGES.rst

@@ -7,6 +7,9 @@ Changelog
 - Fix data corruption due to race between load and external invalidations.
   See `issue 155 <https://github.com/zopefoundation/ZEO/issues/155>`_.
 
+- Improve log message when client cache is out of sync with server.
+  See `issue 142 <https://github.com/zopefoundation/ZEO/issues/142>`_.
+
 5.2.2 (2020-08-11)
 ------------------
 

src/ZEO/asyncio/client.py

@@ -497,7 +497,10 @@ def verify(self, server_tid):
                 elif cache_tid > server_tid:
                     self.verify_result = "Cache newer than server"
                     logger.critical(
-                        'Client has seen newer transactions than server!')
+                        'Client cache is out of sync with the server. '
+                        'Verify that this is expected and then remove '
+                        'the cache file (usually a .zec file) '
+                        'before restarting the server.')
                     raise AssertionError("Server behind client, %r < %r, %s",
                                          server_tid, cache_tid, protocol)
                 elif cache_tid == server_tid:

src/ZEO/tests/testZEO.py

@@ -1147,7 +1147,7 @@ def client_has_newer_data_than_server():
     >>> wait_until('got enough errors', lambda:
     ...    len([x for x in handler.records
     ...         if x.levelname == 'CRITICAL' and
-    ...            'Client has seen newer transactions than server!' in x.msg
+    ...            'Client cache is out of sync with the server.' in x.msg
     ...         ]) >= 2)
 
     Note that the errors repeat because the client keeps on trying to connect.