Use only client version to check the server connection

Currently we spam debug output for any use of pylibjuju where the
"target" != juju controller version. It should be turned into a
ceiling, as it’s pretty normal to use the latest pylibjuju (currently
3.3.0.0) against any 3.x controller (e.g. 3.1). The only time we emit
a warning should be when we think the client should be upgraded to a
more recent version (e.g. running pylibjuju 3.3.0.0 against juju
controller 3.6).

juju/client/connector.py

@@ -4,6 +4,8 @@
 import copy
 import logging
 
+from packaging import version
+
 import macaroonbakery.httpbakery as httpbakery
 
 from juju.client import client
@@ -12,7 +14,7 @@
 from juju.client.jujudata import API_ENDPOINTS_KEY, FileJujuData
 from juju.client.proxy.factory import proxy_from_config
 from juju.errors import JujuConnectionError, JujuError
-from juju.version import SUPPORTED_MAJOR_VERSION, TARGET_JUJU_VERSION
+from juju.version import CLIENT_VERSION
 
 log = logging.getLogger("connector")
 
@@ -86,20 +88,19 @@ async def connect(self, **kwargs):
             self._connection = await Connection.connect(**kwargs)
 
         # Check if we support the target controller
-        juju_server_version = self._connection.info["server-version"]
-        if not juju_server_version.startswith(TARGET_JUJU_VERSION):
-            log.debug(
-                "This version was tested using {} juju version {} may have compatibility issues".format(
-                    TARGET_JUJU_VERSION, juju_server_version
-                )
-            )
-        if not self._connection.info["server-version"].startswith(
-            SUPPORTED_MAJOR_VERSION
-        ):
+        juju_server_version = version.parse(self._connection.info["server-version"])
+        client_version = version.parse(CLIENT_VERSION)
+
+        if juju_server_version.major != client_version.major:
             raise JujuConnectionError(
                 "juju server-version %s not supported" % juju_server_version
             )
 
+        if juju_server_version > client_version:
+            log.warning(
+                f"This client is tested up to the version {client_version} Juju controller. Detected a Juju controller version {juju_server_version} that's higher than the {client_version}. Some functionalities that the Juju {juju_server_version} offers may not be available. Please consider upgrading to pylibjuju {juju_server_version}."
+            )
+
     async def disconnect(self, entity):
         """Shut down the watcher task and close websockets."""
         if self._connection:

juju/version.py

@@ -7,14 +7,12 @@
 
 DEFAULT_ARCHITECTURE = 'amd64'
 
-VERSION_FILE_PATH = '../VERSION'
+# CLIENT_VERSION (that's read from the VERSION file) is the highest Juju server
+# version that this client supports.
+# Note that this is a ceiling. CLIENT_VERSION <= juju-controller-version works.
+# For CLIENT_VERSION < juju-controller-version (strictly smaller), we emit a warning
+# to update the client to the latest.
+# However, for any CLIENT_VERSION > juju-controller-version, a "client incompatible
+# with server" will be returned by the juju controller.
+VERSION_FILE_PATH = './VERSION'
 CLIENT_VERSION = re.search(r'\d+\.\d+\.\d+', open(VERSION_FILE_PATH).read().strip()).group()
-
-# Juju server version we target. Depending on this value, the Juju server
-# may stop the connecting considering us not compatible.
-TARGET_JUJU_VERSION = '3.2.0'
-
-# Used by connector to determine if we are compatible with the juju server
-SUPPORTED_MAJOR_VERSION = '3'
-
-SUPPORTED_MAJOR_MINOR_VERSION = '3.2'