Only log hub / singleuser version mismatch once

If your jupyterhub and jupyterhub-singleuser instances
are running at different minor or greater versions a
warning gets logged per active server which can be a lot
when you have hundreds of active servers.

This adds a flag to that version mismatch logging logic
such that the warning is only logged once per restart
of the hub server.

Closes issue #2970

jupyterhub/_version.py

@@ -18,6 +18,15 @@ version_info = (
 
 __version__ = ".".join(map(str, version_info[:3])) + ".".join(version_info[3:])
 
+# Singleton flag to only log the major/minor mismatch warning once.
+_version_mismatch_warning_logged = False
+
+
+def reset_globals():
+    """Used to reset globals between test cases."""
+    global _version_mismatch_warning_logged
+    _version_mismatch_warning_logged = False
+
 
 def _check_version(hub_version, singleuser_version, log):
     """Compare Hub and single-user server versions"""
@@ -42,14 +51,21 @@ def _check_version(hub_version, singleuser_version, log):
         hub_major_minor = V(hub_version).version[:2]
         singleuser_major_minor = V(singleuser_version).version[:2]
         extra = ""
+        do_log = True
         if singleuser_major_minor == hub_major_minor:
             # patch-level mismatch or lower, log difference at debug-level
             # because this should be fine
             log_method = log.debug
         else:
             # log warning-level for more significant mismatch, such as 0.8 vs 0.9, etc.
+            global _version_mismatch_warning_logged
+            if _version_mismatch_warning_logged:
+                do_log = False  # We already logged this warning so don't log it again.
+            else:
                 log_method = log.warning
                 extra = " This could cause failure to authenticate and result in redirect loops!"
+                _version_mismatch_warning_logged = True
+        if do_log:
             log_method(
                 "jupyterhub version %s != jupyterhub-singleuser version %s." + extra,
                 hub_version,

jupyterhub/tests/test_version.py

@@ -4,6 +4,11 @@ import logging
 import pytest
 
 from .._version import _check_version
+from .._version import reset_globals
+
+
+def setup_function(function):
+    reset_globals()
 
 
 @pytest.mark.parametrize(
@@ -25,3 +30,17 @@ def test_check_version(hub_version, singleuser_version, log_level, msg, caplog):
     record = caplog.records[0]
     assert record.levelno == log_level
     assert msg in record.getMessage()
+
+
+def test_check_version_singleton(caplog):
+    """Tests that minor version difference logging is only logged once."""
+    # Run test_check_version twice which will assert that the warning is only logged
+    # once.
+    for x in range(2):
+        test_check_version(
+            '1.2.0',
+            '1.1.0',
+            logging.WARNING,
+            'This could cause failure to authenticate',
+            caplog,
+        )