Merge pull request juju#1008 from cderici/target-ceiling-version

juju#1008

#### Description

Currently 

1. the “target” version is hardcoded, 
2. we spam debug output for any use of pylibjuju where the target version != juju controller version. 

This reads the version from the `VERSION` file we keep at the toplevel. And removes the minor version sniffing.

We still keep the major version sniffing because of the two tracks we have in pylibjuju (2.9 and 3.x).

#### QA Steps

No added functionality, so no QA is needed other than reading the code.

If you wanna be really pedantic, set the logging to DEBUG and run the line above, and you shouldn't see any messages like 

```
"This version was tested using ...... juju version ...... may have compatibility issues"
```

#### Notes & Discussion

JUJU-5315

juju/client/connection.py

@@ -17,7 +17,7 @@
 from juju import errors, tag, utils, jasyncio
 from juju.client import client
 from juju.utils import IdQueue
-from juju.version import TARGET_JUJU_VERSION
+from juju.version import CLIENT_VERSION
 
 log = logging.getLogger('juju.client.connection')
 
@@ -963,7 +963,7 @@ def _build_facades(self, facades_from_connection):
     async def login(self):
         params = {}
         # Set the client version
-        params['client-version'] = TARGET_JUJU_VERSION
+        params['client-version'] = CLIENT_VERSION
         params['auth-tag'] = self.usertag
         if self.password:
             params['credentials'] = self.password

juju/client/connector.py

@@ -4,15 +4,17 @@
 import copy
 import logging
 
+from packaging import version
+
 import macaroonbakery.httpbakery as httpbakery
 
 from juju.client import client
 from juju.client.connection import Connection
 from juju.client.gocookies import GoCookieJar, go_to_py_cookie
 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.errors import JujuConnectionError, JujuError, JujuUnknownVersion
+from juju.version import CLIENT_VERSION
 
 log = logging.getLogger("connector")
 
@@ -86,16 +88,20 @@ 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
-        ):
+        server_version = self._connection.info["server-version"]
+        try:
+            juju_server_version = version.parse(server_version)
+        except version.InvalidVersion as err:
+            # We're only interested in the major version, so
+            # we attempt to clean up versions such as 3.4-rc1.2 as just 3.4
+            if '-' not in server_version:
+                raise JujuUnknownVersion(err)
+            juju_server_version = version.parse(server_version.split('-')[0])
+
+        # CLIENT_VERSION statically comes from the VERSION file in the repo
+        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
             )

juju/errors.py

@@ -137,3 +137,7 @@ class AbstractMethodError(Exception):
 
 class PylibjujuError(JujuError):
     pass
+
+
+class JujuUnknownVersion(PylibjujuError):
+    pass

juju/version.py

@@ -1,16 +1,19 @@
 # Copyright 2023 Canonical Ltd.
 # Licensed under the Apache V2, see LICENCE file for details.
 
+import pathlib
+import re
 
 LTS_RELEASES = ["jammy", "focal", "bionic", "xenial", "trusty", "precise"]
 
 DEFAULT_ARCHITECTURE = 'amd64'
 
-# 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'
+# 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 = pathlib.Path(__file__).parent.parent / 'VERSION'
+CLIENT_VERSION = re.search(r'\d+\.\d+\.\d+', open(VERSION_FILE_PATH).read().strip()).group()